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Subject: Re: [PATCH 1/4] glibc: Perform rseq(2) registration at C startup and
 thread creation (v7)
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Carlos O'Donell <carlos@redhat.com>
Cc:     Florian Weimer <fweimer@redhat.com>,
        Joseph Myers <joseph@codesourcery.com>,
        Szabolcs Nagy <szabolcs.nagy@arm.com>,
        libc-alpha@sourceware.org, Thomas Gleixner <tglx@linutronix.de>,
        Ben Maurer <bmaurer@fb.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Will Deacon <will.deacon@arm.com>,
        Dave Watson <davejwatson@fb.com>, Paul Turner <pjt@google.com>,
        Rich Felker <dalias@libc.org>, linux-kernel@vger.kernel.org,
        linux-api@vger.kernel.org
References: <20190212194253.1951-1-mathieu.desnoyers@efficios.com>
 <20190212194253.1951-2-mathieu.desnoyers@efficios.com>
From:   Carlos O'Donell <codonell@redhat.com>
Message-ID: <5166fbe9-cfe0-8554-abc7-4fc844cf2765@redhat.com>
Date:   Fri, 22 Mar 2019 16:09:42 -0400
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On 2/12/19 2:42 PM, Mathieu Desnoyers wrote:
> Register rseq(2) TLS for each thread (including main), and unregister
> for each thread (excluding main). "rseq" stands for Restartable
> Sequences.

Thanks, the implementation is looking good, before this goes in it needs
some procedural cleanup and the following:

(a) I would like to see a final position from Florian, either accept,
     object (objection recorded), or sustained objection (blocks this patch),
     on the issue of the registration protocol.

     I suggested a possible way forward to remove the registration protocol
     down the line, leaving only the __rseq_lib_abi as the residual artifact
     once we deprecate coordination with other libraries. Applications would
     have to be written with this in mind, and coordination still remains
     with libc for the time being. Perhaps Mathieu has another suggestion.

(b) I would like to see a final position from Rich Felker, either accept,
     object (objection recorded), or sustained objection (blocks this patch),
     on the patch set as a whole. I trust Rich's opinion as an alternative
     libc maintainer in this matter and as a independent implementor of
     similar standards. Rich will have to implement this for musl, and it
     would be good to have consensus from him.

So while I reviewed this patch, and provided nit-picky feedback below,
I think we need a whole new thread, to finalize the way forward for the
registration process.

Next steps:

- Start a new thread to talk *only* about options for removing the
   refcounted registration.

> See the rseq(2) man page proposed here:
>    https://lkml.org/lkml/2018/9/19/647

Thank you for having a man page, that makes for a very easy to follow
description of the structures, syscall and use cases.

> 
> This patch is based on glibc-2.29. The rseq(2) system call was merged
> into Linux 4.18.
> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> CC: Carlos O'Donell <carlos@redhat.com>
> CC: Florian Weimer <fweimer@redhat.com>
> CC: Joseph Myers <joseph@codesourcery.com>
> CC: Szabolcs Nagy <szabolcs.nagy@arm.com>
> CC: Thomas Gleixner <tglx@linutronix.de>
> CC: Ben Maurer <bmaurer@fb.com>
> CC: Peter Zijlstra <peterz@infradead.org>
> CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> CC: Boqun Feng <boqun.feng@gmail.com>
> CC: Will Deacon <will.deacon@arm.com>
> CC: Dave Watson <davejwatson@fb.com>
> CC: Paul Turner <pjt@google.com>
> CC: Rich Felker <dalias@libc.org>
> CC: libc-alpha@sourceware.org
> CC: linux-kernel@vger.kernel.org
> CC: linux-api@vger.kernel.org
> ---
> Changes since v1:
> - Move __rseq_refcount to an extra field at the end of __rseq_abi to
>    eliminate one symbol.
> 
>    All libraries/programs which try to register rseq (glibc,
>    early-adopter applications, early-adopter libraries) should use the
>    rseq refcount. It becomes part of the ABI within a user-space
>    process, but it's not part of the ABI shared with the kernel per se.
> 
> - Restructure how this code is organized so glibc keeps building on
>    non-Linux targets.
> 
> - Use non-weak symbol for __rseq_abi.
> 
> - Move rseq registration/unregistration implementation into its own
>    nptl/rseq.c compile unit.
> 
> - Move __rseq_abi symbol under GLIBC_2.29.
> 
> Changes since v2:
> - Move __rseq_refcount to its own symbol, which is less ugly than
>    trying to play tricks with the rseq uapi.
> - Move __rseq_abi from nptl to csu (C start up), so it can be used
>    across glibc, including memory allocator and sched_getcpu(). The
>    __rseq_refcount symbol is kept in nptl, because there is no reason
>    to use it elsewhere in glibc.
> 
> Changes since v3:
> - Set __rseq_refcount TLS to 1 on register/set to 0 on unregister
>    because glibc is the first/last user.
> - Unconditionally register/unregister rseq at thread start/exit, because
>    glibc is the first/last user.
> - Add missing abilist items.
> - Rebase on glibc master commit a502c5294.
> - Add NEWS entry.
> 
> Changes since v4:
> - Do not use "weak" symbols for __rseq_abi and __rseq_refcount. Based on
>    "System V Application Binary Interface", weak only affects the link
>    editor, not the dynamic linker.
> - Install a new sys/rseq.h system header on Linux, which contains the
>    RSEQ_SIG definition, __rseq_abi declaration and __rseq_refcount
>    declaration. Move those definition/declarations from rseq-internal.h
>    to the installed sys/rseq.h header.
> - Considering that rseq is only available on Linux, move csu/rseq.c to
>    sysdeps/unix/sysv/linux/rseq-sym.c.
> - Move __rseq_refcount from nptl/rseq.c to
>    sysdeps/unix/sysv/linux/rseq-sym.c, so it is only defined on Linux.
> - Move both ABI definitions for __rseq_abi and __rseq_refcount to
>    sysdeps/unix/sysv/linux/Versions, so they only appear on Linux.
> - Document __rseq_abi and __rseq_refcount volatile.
> - Document the RSEQ_SIG signature define.
> - Move registration functions from rseq.c to rseq-internal.h static
>    inline functions. Introduce empty stubs in misc/rseq-internal.h,
>    which can be overridden by architecture code in
>    sysdeps/unix/sysv/linux/rseq-internal.h.
> - Rename __rseq_register_current_thread and __rseq_unregister_current_thread
>    to rseq_register_current_thread and rseq_unregister_current_thread,
>    now that those are only visible as internal static inline functions.
> - Invoke rseq_register_current_thread() from libc-start.c LIBC_START_MAIN
>    rather than nptl init, so applications not linked against
>    libpthread.so have rseq registered for their main() thread. Note that
>    it is invoked separately for SHARED and !SHARED builds.
> 
> Changes since v5:
> - Replace __rseq_refcount by __rseq_lib_abi, which contains two
>    uint32_t: register_state and refcount. The "register_state" field
>    allows inhibiting rseq registration from signal handlers nested on top
>    of glibc registration and occuring after rseq unregistration by glibc.
> - Introduce enum rseq_register_state, which contains the states allowed
>    for the struct rseq_lib_abi register_state field.
> 
> Changes since v6:
> - Introduce bits/rseq.h to define RSEQ_SIG for each architecture.
>    The generic bits/rseq.h does not define RSEQ_SIG, meaning that each
>    architecture implementing rseq needs to implement bits/rseq.h.
> - Rename enum item RSEQ_REGISTER_NESTED to RSEQ_REGISTER_ONGOING.
> - Port to glibc-2.29.
> ---
>   NEWS                                          | 11 +++
>   csu/libc-start.c                              | 12 ++-
>   misc/Makefile                                 |  3 +-
>   misc/rseq-internal.h                          | 34 +++++++
>   nptl/pthread_create.c                         |  9 ++
>   sysdeps/unix/sysv/linux/Makefile              |  4 +-
>   sysdeps/unix/sysv/linux/Versions              |  4 +
>   sysdeps/unix/sysv/linux/aarch64/bits/rseq.h   | 24 +++++
>   sysdeps/unix/sysv/linux/aarch64/libc.abilist  |  2 +
>   sysdeps/unix/sysv/linux/alpha/libc.abilist    |  2 +
>   sysdeps/unix/sysv/linux/arm/bits/rseq.h       | 24 +++++
>   sysdeps/unix/sysv/linux/arm/libc.abilist      |  2 +
>   sysdeps/unix/sysv/linux/bits/rseq.h           | 24 +++++
>   sysdeps/unix/sysv/linux/hppa/libc.abilist     |  2 +
>   sysdeps/unix/sysv/linux/i386/libc.abilist     |  2 +
>   sysdeps/unix/sysv/linux/ia64/libc.abilist     |  2 +
>   .../sysv/linux/m68k/coldfire/libc.abilist     |  2 +
>   .../unix/sysv/linux/m68k/m680x0/libc.abilist  |  2 +
>   .../unix/sysv/linux/microblaze/libc.abilist   |  2 +
>   sysdeps/unix/sysv/linux/mips/bits/rseq.h      | 24 +++++
>   .../sysv/linux/mips/mips32/fpu/libc.abilist   |  2 +
>   .../sysv/linux/mips/mips32/nofpu/libc.abilist |  2 +
>   .../sysv/linux/mips/mips64/n32/libc.abilist   |  2 +
>   .../sysv/linux/mips/mips64/n64/libc.abilist   |  2 +
>   sysdeps/unix/sysv/linux/nios2/libc.abilist    |  2 +
>   sysdeps/unix/sysv/linux/powerpc/bits/rseq.h   | 24 +++++
>   .../linux/powerpc/powerpc32/fpu/libc.abilist  |  2 +
>   .../powerpc/powerpc32/nofpu/libc.abilist      |  2 +
>   .../linux/powerpc/powerpc64/be/libc.abilist   |  2 +
>   .../linux/powerpc/powerpc64/le/libc.abilist   |  2 +
>   .../unix/sysv/linux/riscv/rv64/libc.abilist   |  2 +
>   sysdeps/unix/sysv/linux/rseq-internal.h       | 91 +++++++++++++++++++
>   sysdeps/unix/sysv/linux/rseq-sym.c            | 54 +++++++++++
>   sysdeps/unix/sysv/linux/s390/bits/rseq.h      | 24 +++++
>   .../unix/sysv/linux/s390/s390-32/libc.abilist |  2 +
>   .../unix/sysv/linux/s390/s390-64/libc.abilist |  2 +
>   sysdeps/unix/sysv/linux/sh/libc.abilist       |  2 +
>   .../sysv/linux/sparc/sparc32/libc.abilist     |  2 +
>   .../sysv/linux/sparc/sparc64/libc.abilist     |  2 +
>   sysdeps/unix/sysv/linux/sys/rseq.h            | 65 +++++++++++++
>   sysdeps/unix/sysv/linux/x86/bits/rseq.h       | 24 +++++
>   .../unix/sysv/linux/x86_64/64/libc.abilist    |  2 +
>   .../unix/sysv/linux/x86_64/x32/libc.abilist   |  2 +
>   43 files changed, 501 insertions(+), 6 deletions(-)
>   create mode 100644 misc/rseq-internal.h
>   create mode 100644 sysdeps/unix/sysv/linux/aarch64/bits/rseq.h
>   create mode 100644 sysdeps/unix/sysv/linux/arm/bits/rseq.h
>   create mode 100644 sysdeps/unix/sysv/linux/bits/rseq.h
>   create mode 100644 sysdeps/unix/sysv/linux/mips/bits/rseq.h
>   create mode 100644 sysdeps/unix/sysv/linux/powerpc/bits/rseq.h
>   create mode 100644 sysdeps/unix/sysv/linux/rseq-internal.h
>   create mode 100644 sysdeps/unix/sysv/linux/rseq-sym.c
>   create mode 100644 sysdeps/unix/sysv/linux/s390/bits/rseq.h
>   create mode 100644 sysdeps/unix/sysv/linux/sys/rseq.h
>   create mode 100644 sysdeps/unix/sysv/linux/x86/bits/rseq.h
> 
> diff --git a/NEWS b/NEWS
> index 912a9bdc0f..0608c60f7d 100644
> --- a/NEWS
> +++ b/NEWS
> @@ -5,6 +5,17 @@ See the end for copying conditions.
>   Please send GNU C library bug reports via <https://sourceware.org/bugzilla/>
>   using `glibc' in the "product" field.
>   
> +Version 2.30
> +
> +Major new features:
> +
> +* Support for automatically registering threads with the Linux rseq(2)
> +  system call has been added. This system call is implemented starting
> +  from Linux 4.18. In order to be activated, it requires that glibc is built
> +  against kernel headers that include this system call, and that glibc
> +  detects availability of that system call at runtime.

What benefit does the feature have for users? Can you talk about that please?
Why would a user want to use it. Please feel free to link to an external reference.

> +
> +
>   Version 2.29
>   
>   Major new features:
> diff --git a/csu/libc-start.c b/csu/libc-start.c
> index 5d9c3675fa..8680afc0ef 100644
> --- a/csu/libc-start.c
> +++ b/csu/libc-start.c
> @@ -22,6 +22,7 @@
>   #include <ldsodefs.h>
>   #include <exit-thread.h>
>   #include <libc-internal.h>
> +#include <rseq-internal.h>

OK.

>   
>   #include <elf/dl-tunables.h>
>   
> @@ -140,7 +141,10 @@ LIBC_START_MAIN (int (*main) (int, char **, char ** MAIN_AUXVEC_DECL),
>   
>     __libc_multiple_libcs = &_dl_starting_up && !_dl_starting_up;
>   
> -#ifndef SHARED
> +#ifdef SHARED
> +  /* Register rseq ABI to the kernel. */
> +  (void) rseq_register_current_thread ();

OK, early registration of main thread in shared case.

> +#else
>     _dl_relocate_static_pie ();
>   
>     char **ev = &argv[argc + 1];
> @@ -218,6 +222,9 @@ LIBC_START_MAIN (int (*main) (int, char **, char ** MAIN_AUXVEC_DECL),
>       }
>   # endif
>   
> +  /* Register rseq ABI to the kernel. */
> +  (void) rseq_register_current_thread ();

OK, early registration of main thread in static case.

> +
>     /* Initialize libpthread if linked in.  */
>     if (__pthread_initialize_minimal != NULL)
>       __pthread_initialize_minimal ();
> @@ -230,8 +237,7 @@ LIBC_START_MAIN (int (*main) (int, char **, char ** MAIN_AUXVEC_DECL),
>   # else
>     __pointer_chk_guard_local = pointer_chk_guard;
>   # endif
> -
> -#endif /* !SHARED  */
> +#endif

OK.

>   
>     /* Register the destructor of the dynamic linker if there is any.  */
>     if (__glibc_likely (rtld_fini != NULL))
> diff --git a/misc/Makefile b/misc/Makefile
> index cf0daa1161..0ae1dbaf80 100644
> --- a/misc/Makefile
> +++ b/misc/Makefile
> @@ -36,7 +36,8 @@ headers	:= sys/uio.h bits/uio-ext.h bits/uio_lim.h \
>   	   syslog.h sys/syslog.h \
>   	   bits/syslog.h bits/syslog-ldbl.h bits/syslog-path.h bits/error.h \
>   	   bits/select2.h bits/hwcap.h sys/auxv.h \
> -	   sys/sysmacros.h bits/sysmacros.h bits/types/struct_iovec.h
> +	   sys/sysmacros.h bits/sysmacros.h bits/types/struct_iovec.h \
> +	   rseq-internal.h

OK.

>   
>   routines := brk sbrk sstk ioctl \
>   	    readv writev preadv preadv64 pwritev pwritev64 \
> diff --git a/misc/rseq-internal.h b/misc/rseq-internal.h
> new file mode 100644
> index 0000000000..915122e4bf
> --- /dev/null
> +++ b/misc/rseq-internal.h
> @@ -0,0 +1,34 @@
> +/* Copyright (C) 2018 Free Software Foundation, Inc.

Add a one line description before copyright which describes the file.
Is it a stub file. Is it the Linux implementation. etc.

Please add a one sentence first line short description for all new files
you create.

> +   This file is part of the GNU C Library.
> +   Contributed by Mathieu Desnoyers <mathieu.desnoyers@efficios.com>, 2018.

We don't use "contributed by" markers anymore, we let the git commit author
carry that information, along with git blame information.

Please remove this contributed by line and all subsequent such lines.

> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <http://www.gnu.org/licenses/>.  */
> +
> +#ifndef RSEQ_INTERNAL_H
> +#define RSEQ_INTERNAL_H
> +
> +static inline int
> +rseq_register_current_thread (void)
> +{
> +  return -1;
> +}
> +
> +static inline int
> +rseq_unregister_current_thread (void)
> +{
> +  return -1;
> +}
> +

OK.

> +#endif /* rseq-internal.h */
> diff --git a/nptl/pthread_create.c b/nptl/pthread_create.c
> index 2bd2b10727..90b3419390 100644
> --- a/nptl/pthread_create.c
> +++ b/nptl/pthread_create.c
> @@ -33,6 +33,7 @@
>   #include <default-sched.h>
>   #include <futex-internal.h>
>   #include <tls-setup.h>
> +#include <rseq-internal.h>

Ok.

>   #include "libioP.h"
>   
>   #include <shlib-compat.h>
> @@ -378,6 +379,7 @@ __free_tcb (struct pthread *pd)
>   START_THREAD_DEFN
>   {
>     struct pthread *pd = START_THREAD_SELF;
> +  bool has_rseq = false;

OK.

>   
>   #if HP_TIMING_AVAIL
>     /* Remember the time when the thread was started.  */
> @@ -396,6 +398,9 @@ START_THREAD_DEFN
>     if (__glibc_unlikely (atomic_exchange_acq (&pd->setxid_futex, 0) == -2))
>       futex_wake (&pd->setxid_futex, 1, FUTEX_PRIVATE);
>   
> +  /* Register rseq TLS to the kernel. */
> +  has_rseq = !rseq_register_current_thread ();

OK, initialize rseq after enabling setxid but before setting up roubust list,
no particularly reason it should be right here that I can see, that's OK though.

> +
>   #ifdef __NR_set_robust_list
>   # ifndef __ASSUME_SET_ROBUST_LIST
>     if (__set_robust_list_avail >= 0)
> @@ -573,6 +578,10 @@ START_THREAD_DEFN
>       }
>   #endif
>   
> +  /* Unregister rseq TLS from kernel. */
> +  if (has_rseq && rseq_unregister_current_thread ())
> +    abort();

OK. Calling abort() seems harsh because it kills the whole process, but I'm OK
with failing catastrophically if that's what we want here. It seems unrecoverable.

> +
>     advise_stack_range (pd->stackblock, pd->stackblock_size, (uintptr_t) pd,
>   		      pd->guardsize);
>   
> diff --git a/sysdeps/unix/sysv/linux/Makefile b/sysdeps/unix/sysv/linux/Makefile
> index 5f8c2c7c7d..5b541469ec 100644
> --- a/sysdeps/unix/sysv/linux/Makefile
> +++ b/sysdeps/unix/sysv/linux/Makefile
> @@ -1,5 +1,5 @@
>   ifeq ($(subdir),csu)
> -sysdep_routines += errno-loc
> +sysdep_routines += errno-loc rseq-sym

OK.

>   endif
>   
>   ifeq ($(subdir),assert)
> @@ -48,7 +48,7 @@ sysdep_headers += sys/mount.h sys/acct.h sys/sysctl.h \
>   		  bits/termios-c_iflag.h bits/termios-c_oflag.h \
>   		  bits/termios-baud.h bits/termios-c_cflag.h \
>   		  bits/termios-c_lflag.h bits/termios-tcflow.h \
> -		  bits/termios-misc.h
> +		  bits/termios-misc.h sys/rseq.h bits/rseq.h

OK.

>   
>   tests += tst-clone tst-clone2 tst-clone3 tst-fanotify tst-personality \
>   	 tst-quota tst-sync_file_range tst-sysconf-iov_max tst-ttyname \
> diff --git a/sysdeps/unix/sysv/linux/Versions b/sysdeps/unix/sysv/linux/Versions
> index f1e12d9c69..ad88c2b7ff 100644
> --- a/sysdeps/unix/sysv/linux/Versions
> +++ b/sysdeps/unix/sysv/linux/Versions
> @@ -174,6 +174,10 @@ libc {
>     GLIBC_2.29 {
>       getcpu;
>     }
> +  GLIBC_2.30 {
> +    __rseq_abi;
> +    __rseq_lib_abi;
> +  }

OK, good using GLIBC_2.30 for master.

>     GLIBC_PRIVATE {
>       # functions used in other libraries
>       __syscall_rt_sigqueueinfo;
> diff --git a/sysdeps/unix/sysv/linux/aarch64/bits/rseq.h b/sysdeps/unix/sysv/linux/aarch64/bits/rseq.h
> new file mode 100644
> index 0000000000..543bc5388a
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/aarch64/bits/rseq.h
> @@ -0,0 +1,24 @@

Needs a one sentence description.

> +/* Copyright (C) 2019 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +   Contributed by Mathieu Desnoyers <mathieu.desnoyers@efficios.com>, 2019.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <http://www.gnu.org/licenses/>.  */
> +
> +#ifndef _SYS_RSEQ_H
> +# error "Never use <bits/rseq.h> directly; include <sys/rseq.h> instead."
> +#endif
> +
> +/* Signature required before each abort handler code.  */
> +#define RSEQ_SIG 0xd428bc00	/* BRK #0x45E0.  */

OK.

> diff --git a/sysdeps/unix/sysv/linux/aarch64/libc.abilist b/sysdeps/unix/sysv/linux/aarch64/libc.abilist
> index 9c330f325e..bc937f585d 100644
> --- a/sysdeps/unix/sysv/linux/aarch64/libc.abilist
> +++ b/sysdeps/unix/sysv/linux/aarch64/libc.abilist
> @@ -2141,3 +2141,5 @@ GLIBC_2.28 thrd_yield F
>   GLIBC_2.29 getcpu F
>   GLIBC_2.29 posix_spawn_file_actions_addchdir_np F
>   GLIBC_2.29 posix_spawn_file_actions_addfchdir_np F
> +GLIBC_2.30 __rseq_abi T 0x20
> +GLIBC_2.30 __rseq_lib_abi T 0x8

OK.

> diff --git a/sysdeps/unix/sysv/linux/alpha/libc.abilist b/sysdeps/unix/sysv/linux/alpha/libc.abilist
> index f630fa4c6f..89cc8b1cfb 100644
> --- a/sysdeps/unix/sysv/linux/alpha/libc.abilist
> +++ b/sysdeps/unix/sysv/linux/alpha/libc.abilist
> @@ -2036,6 +2036,8 @@ GLIBC_2.28 thrd_yield F
>   GLIBC_2.29 getcpu F
>   GLIBC_2.29 posix_spawn_file_actions_addchdir_np F
>   GLIBC_2.29 posix_spawn_file_actions_addfchdir_np F
> +GLIBC_2.30 __rseq_abi T 0x20
> +GLIBC_2.30 __rseq_lib_abi T 0x8

OK.

>   GLIBC_2.3 __ctype_b_loc F
>   GLIBC_2.3 __ctype_tolower_loc F
>   GLIBC_2.3 __ctype_toupper_loc F
> diff --git a/sysdeps/unix/sysv/linux/arm/bits/rseq.h b/sysdeps/unix/sysv/linux/arm/bits/rseq.h
> new file mode 100644
> index 0000000000..19d3755837
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/arm/bits/rseq.h
> @@ -0,0 +1,24 @@

Needs a one sentence description.

> +/* Copyright (C) 2019 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +   Contributed by Mathieu Desnoyers <mathieu.desnoyers@efficios.com>, 2019.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <http://www.gnu.org/licenses/>.  */
> +
> +#ifndef _SYS_RSEQ_H
> +# error "Never use <bits/rseq.h> directly; include <sys/rseq.h> instead."
> +#endif
> +
> +/* Signature required before each abort handler code.  */
> +#define RSEQ_SIG 0x53053053

Why isn't this an arm specific op code? Does the user have to mark this
up as part of a constant pool when placing it in front of the abort handler
to avoid disassembling the constant as code? This was at one point required
to get gdb to work properly.

> diff --git a/sysdeps/unix/sysv/linux/arm/libc.abilist b/sysdeps/unix/sysv/linux/arm/libc.abilist
> index b96f45590f..e5055f2d4e 100644
> --- a/sysdeps/unix/sysv/linux/arm/libc.abilist
> +++ b/sysdeps/unix/sysv/linux/arm/libc.abilist
> @@ -126,6 +126,8 @@ GLIBC_2.28 thrd_yield F
>   GLIBC_2.29 getcpu F
>   GLIBC_2.29 posix_spawn_file_actions_addchdir_np F
>   GLIBC_2.29 posix_spawn_file_actions_addfchdir_np F
> +GLIBC_2.30 __rseq_abi T 0x20
> +GLIBC_2.30 __rseq_lib_abi T 0x8

OK.

>   GLIBC_2.4 _Exit F
>   GLIBC_2.4 _IO_2_1_stderr_ D 0xa0
>   GLIBC_2.4 _IO_2_1_stdin_ D 0xa0
> diff --git a/sysdeps/unix/sysv/linux/bits/rseq.h b/sysdeps/unix/sysv/linux/bits/rseq.h
> new file mode 100644
> index 0000000000..d60f02cfeb
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/bits/rseq.h
> @@ -0,0 +1,24 @@
> +/* Copyright (C) 2019 Free Software Foundation, Inc.

Needs a one sentence description.

> +   This file is part of the GNU C Library.
> +   Contributed by Mathieu Desnoyers <mathieu.desnoyers@efficios.com>, 2019.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <http://www.gnu.org/licenses/>.  */
> +
> +#ifndef _SYS_RSEQ_H
> +# error "Never use <bits/rseq.h> directly; include <sys/rseq.h> instead."
> +#endif
> +
> +/* Each architecture supporting rseq should define RSEQ_SIG as a 32-bit
> +   signature inserted before each rseq abort label in the code section.  */

Needs a huge explanation about RSEQ_SIG and the reasons why it's per-arch.

Basically cut-and-paste what you wrote to libc-alpha about this.

> diff --git a/sysdeps/unix/sysv/linux/hppa/libc.abilist b/sysdeps/unix/sysv/linux/hppa/libc.abilist
> index 088a8ee369..546d073cdb 100644
> --- a/sysdeps/unix/sysv/linux/hppa/libc.abilist
> +++ b/sysdeps/unix/sysv/linux/hppa/libc.abilist
> @@ -1883,6 +1883,8 @@ GLIBC_2.28 thrd_yield F
>   GLIBC_2.29 getcpu F
>   GLIBC_2.29 posix_spawn_file_actions_addchdir_np F
>   GLIBC_2.29 posix_spawn_file_actions_addfchdir_np F
> +GLIBC_2.30 __rseq_abi T 0x20
> +GLIBC_2.30 __rseq_lib_abi T 0x8

OK.

>   GLIBC_2.3 __ctype_b_loc F
>   GLIBC_2.3 __ctype_tolower_loc F
>   GLIBC_2.3 __ctype_toupper_loc F
> diff --git a/sysdeps/unix/sysv/linux/i386/libc.abilist b/sysdeps/unix/sysv/linux/i386/libc.abilist
> index f7ff2c57b9..ac1de6e4b3 100644
> --- a/sysdeps/unix/sysv/linux/i386/libc.abilist
> +++ b/sysdeps/unix/sysv/linux/i386/libc.abilist
> @@ -2048,6 +2048,8 @@ GLIBC_2.28 thrd_yield F
>   GLIBC_2.29 getcpu F
>   GLIBC_2.29 posix_spawn_file_actions_addchdir_np F
>   GLIBC_2.29 posix_spawn_file_actions_addfchdir_np F
> +GLIBC_2.30 __rseq_abi T 0x20
> +GLIBC_2.30 __rseq_lib_abi T 0x8

OK.

>   GLIBC_2.3 __ctype_b_loc F
>   GLIBC_2.3 __ctype_tolower_loc F
>   GLIBC_2.3 __ctype_toupper_loc F
> diff --git a/sysdeps/unix/sysv/linux/ia64/libc.abilist b/sysdeps/unix/sysv/linux/ia64/libc.abilist
> index becd8b1033..cc3445b958 100644
> --- a/sysdeps/unix/sysv/linux/ia64/libc.abilist
> +++ b/sysdeps/unix/sysv/linux/ia64/libc.abilist
> @@ -1917,6 +1917,8 @@ GLIBC_2.28 thrd_yield F
>   GLIBC_2.29 getcpu F
>   GLIBC_2.29 posix_spawn_file_actions_addchdir_np F
>   GLIBC_2.29 posix_spawn_file_actions_addfchdir_np F
> +GLIBC_2.30 __rseq_abi T 0x20
> +GLIBC_2.30 __rseq_lib_abi T 0x8

OK.

>   GLIBC_2.3 __ctype_b_loc F
>   GLIBC_2.3 __ctype_tolower_loc F
>   GLIBC_2.3 __ctype_toupper_loc F
> diff --git a/sysdeps/unix/sysv/linux/m68k/coldfire/libc.abilist b/sysdeps/unix/sysv/linux/m68k/coldfire/libc.abilist
> index 74e42a5209..f7e28bd5a0 100644
> --- a/sysdeps/unix/sysv/linux/m68k/coldfire/libc.abilist
> +++ b/sysdeps/unix/sysv/linux/m68k/coldfire/libc.abilist
> @@ -127,6 +127,8 @@ GLIBC_2.28 thrd_yield F
>   GLIBC_2.29 getcpu F
>   GLIBC_2.29 posix_spawn_file_actions_addchdir_np F
>   GLIBC_2.29 posix_spawn_file_actions_addfchdir_np F
> +GLIBC_2.30 __rseq_abi T 0x20
> +GLIBC_2.30 __rseq_lib_abi T 0x8

OK.

>   GLIBC_2.4 _Exit F
>   GLIBC_2.4 _IO_2_1_stderr_ D 0x98
>   GLIBC_2.4 _IO_2_1_stdin_ D 0x98
> diff --git a/sysdeps/unix/sysv/linux/m68k/m680x0/libc.abilist b/sysdeps/unix/sysv/linux/m68k/m680x0/libc.abilist
> index 4af5a74e8a..b8f00f6111 100644
> --- a/sysdeps/unix/sysv/linux/m68k/m680x0/libc.abilist
> +++ b/sysdeps/unix/sysv/linux/m68k/m680x0/libc.abilist
> @@ -1992,6 +1992,8 @@ GLIBC_2.28 thrd_yield F
>   GLIBC_2.29 getcpu F
>   GLIBC_2.29 posix_spawn_file_actions_addchdir_np F
>   GLIBC_2.29 posix_spawn_file_actions_addfchdir_np F
> +GLIBC_2.30 __rseq_abi T 0x20
> +GLIBC_2.30 __rseq_lib_abi T 0x8

OK.

>   GLIBC_2.3 __ctype_b_loc F
>   GLIBC_2.3 __ctype_tolower_loc F
>   GLIBC_2.3 __ctype_toupper_loc F
> diff --git a/sysdeps/unix/sysv/linux/microblaze/libc.abilist b/sysdeps/unix/sysv/linux/microblaze/libc.abilist
> index ccef673fd2..19f191434f 100644
> --- a/sysdeps/unix/sysv/linux/microblaze/libc.abilist
> +++ b/sysdeps/unix/sysv/linux/microblaze/libc.abilist
> @@ -2133,3 +2133,5 @@ GLIBC_2.28 thrd_yield F
>   GLIBC_2.29 getcpu F
>   GLIBC_2.29 posix_spawn_file_actions_addchdir_np F
>   GLIBC_2.29 posix_spawn_file_actions_addfchdir_np F
> +GLIBC_2.30 __rseq_abi T 0x20
> +GLIBC_2.30 __rseq_lib_abi T 0x8

OK.

> diff --git a/sysdeps/unix/sysv/linux/mips/bits/rseq.h b/sysdeps/unix/sysv/linux/mips/bits/rseq.h
> new file mode 100644
> index 0000000000..19d3755837
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/mips/bits/rseq.h
> @@ -0,0 +1,24 @@
> +/* Copyright (C) 2019 Free Software Foundation, Inc.

Needs a one sentence description.

> +   This file is part of the GNU C Library.
> +   Contributed by Mathieu Desnoyers <mathieu.desnoyers@efficios.com>, 2019.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <http://www.gnu.org/licenses/>.  */
> +
> +#ifndef _SYS_RSEQ_H
> +# error "Never use <bits/rseq.h> directly; include <sys/rseq.h> instead."
> +#endif
> +
> +/* Signature required before each abort handler code.  */
> +#define RSEQ_SIG 0x53053053

Why isn't this a mips-specific op code?

> diff --git a/sysdeps/unix/sysv/linux/mips/mips32/fpu/libc.abilist b/sysdeps/unix/sysv/linux/mips/mips32/fpu/libc.abilist
> index 1054bb599e..fe43507f55 100644
> --- a/sysdeps/unix/sysv/linux/mips/mips32/fpu/libc.abilist
> +++ b/sysdeps/unix/sysv/linux/mips/mips32/fpu/libc.abilist
> @@ -1970,6 +1970,8 @@ GLIBC_2.28 thrd_yield F
>   GLIBC_2.29 getcpu F
>   GLIBC_2.29 posix_spawn_file_actions_addchdir_np F
>   GLIBC_2.29 posix_spawn_file_actions_addfchdir_np F
> +GLIBC_2.30 __rseq_abi T 0x20
> +GLIBC_2.30 __rseq_lib_abi T 0x8

OK.

>   GLIBC_2.3 __ctype_b_loc F
>   GLIBC_2.3 __ctype_tolower_loc F
>   GLIBC_2.3 __ctype_toupper_loc F
> diff --git a/sysdeps/unix/sysv/linux/mips/mips32/nofpu/libc.abilist b/sysdeps/unix/sysv/linux/mips/mips32/nofpu/libc.abilist
> index 4f5b5ffebf..b247c6ea9b 100644
> --- a/sysdeps/unix/sysv/linux/mips/mips32/nofpu/libc.abilist
> +++ b/sysdeps/unix/sysv/linux/mips/mips32/nofpu/libc.abilist
> @@ -1968,6 +1968,8 @@ GLIBC_2.28 thrd_yield F
>   GLIBC_2.29 getcpu F
>   GLIBC_2.29 posix_spawn_file_actions_addchdir_np F
>   GLIBC_2.29 posix_spawn_file_actions_addfchdir_np F
> +GLIBC_2.30 __rseq_abi T 0x20
> +GLIBC_2.30 __rseq_lib_abi T 0x8

OK.

>   GLIBC_2.3 __ctype_b_loc F
>   GLIBC_2.3 __ctype_tolower_loc F
>   GLIBC_2.3 __ctype_toupper_loc F
> diff --git a/sysdeps/unix/sysv/linux/mips/mips64/n32/libc.abilist b/sysdeps/unix/sysv/linux/mips/mips64/n32/libc.abilist
> index 943aee58d4..5339ca52b6 100644
> --- a/sysdeps/unix/sysv/linux/mips/mips64/n32/libc.abilist
> +++ b/sysdeps/unix/sysv/linux/mips/mips64/n32/libc.abilist
> @@ -1976,6 +1976,8 @@ GLIBC_2.28 thrd_yield F
>   GLIBC_2.29 getcpu F
>   GLIBC_2.29 posix_spawn_file_actions_addchdir_np F
>   GLIBC_2.29 posix_spawn_file_actions_addfchdir_np F
> +GLIBC_2.30 __rseq_abi T 0x20
> +GLIBC_2.30 __rseq_lib_abi T 0x8

OK.

>   GLIBC_2.3 __ctype_b_loc F
>   GLIBC_2.3 __ctype_tolower_loc F
>   GLIBC_2.3 __ctype_toupper_loc F
> diff --git a/sysdeps/unix/sysv/linux/mips/mips64/n64/libc.abilist b/sysdeps/unix/sysv/linux/mips/mips64/n64/libc.abilist
> index 17a5d17ef9..11f24eb440 100644
> --- a/sysdeps/unix/sysv/linux/mips/mips64/n64/libc.abilist
> +++ b/sysdeps/unix/sysv/linux/mips/mips64/n64/libc.abilist
> @@ -1971,6 +1971,8 @@ GLIBC_2.28 thrd_yield F
>   GLIBC_2.29 getcpu F
>   GLIBC_2.29 posix_spawn_file_actions_addchdir_np F
>   GLIBC_2.29 posix_spawn_file_actions_addfchdir_np F
> +GLIBC_2.30 __rseq_abi T 0x20
> +GLIBC_2.30 __rseq_lib_abi T 0x8

OK.

>   GLIBC_2.3 __ctype_b_loc F
>   GLIBC_2.3 __ctype_tolower_loc F
>   GLIBC_2.3 __ctype_toupper_loc F
> diff --git a/sysdeps/unix/sysv/linux/nios2/libc.abilist b/sysdeps/unix/sysv/linux/nios2/libc.abilist
> index 4d62a540fd..fd223bfc44 100644
> --- a/sysdeps/unix/sysv/linux/nios2/libc.abilist
> +++ b/sysdeps/unix/sysv/linux/nios2/libc.abilist
> @@ -2174,3 +2174,5 @@ GLIBC_2.28 thrd_yield F
>   GLIBC_2.29 getcpu F
>   GLIBC_2.29 posix_spawn_file_actions_addchdir_np F
>   GLIBC_2.29 posix_spawn_file_actions_addfchdir_np F
> +GLIBC_2.30 __rseq_abi T 0x20
> +GLIBC_2.30 __rseq_lib_abi T 0x8

OK.

> diff --git a/sysdeps/unix/sysv/linux/powerpc/bits/rseq.h b/sysdeps/unix/sysv/linux/powerpc/bits/rseq.h
> new file mode 100644
> index 0000000000..19d3755837
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/powerpc/bits/rseq.h
> @@ -0,0 +1,24 @@
> +/* Copyright (C) 2019 Free Software Foundation, Inc.

Needs a one sentence description.

> +   This file is part of the GNU C Library.
> +   Contributed by Mathieu Desnoyers <mathieu.desnoyers@efficios.com>, 2019.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <http://www.gnu.org/licenses/>.  */
> +
> +#ifndef _SYS_RSEQ_H
> +# error "Never use <bits/rseq.h> directly; include <sys/rseq.h> instead."
> +#endif
> +
> +/* Signature required before each abort handler code.  */
> +#define RSEQ_SIG 0x53053053

Why isn't this an opcode specific to power?

> diff --git a/sysdeps/unix/sysv/linux/powerpc/powerpc32/fpu/libc.abilist b/sysdeps/unix/sysv/linux/powerpc/powerpc32/fpu/libc.abilist
> index ecc2d6fa13..cc53178e81 100644
> --- a/sysdeps/unix/sysv/linux/powerpc/powerpc32/fpu/libc.abilist
> +++ b/sysdeps/unix/sysv/linux/powerpc/powerpc32/fpu/libc.abilist
> @@ -1996,6 +1996,8 @@ GLIBC_2.28 thrd_yield F
>   GLIBC_2.29 getcpu F
>   GLIBC_2.29 posix_spawn_file_actions_addchdir_np F
>   GLIBC_2.29 posix_spawn_file_actions_addfchdir_np F
> +GLIBC_2.30 __rseq_abi T 0x20
> +GLIBC_2.30 __rseq_lib_abi T 0x8

OK.

>   GLIBC_2.3 __ctype_b_loc F
>   GLIBC_2.3 __ctype_tolower_loc F
>   GLIBC_2.3 __ctype_toupper_loc F
> diff --git a/sysdeps/unix/sysv/linux/powerpc/powerpc32/nofpu/libc.abilist b/sysdeps/unix/sysv/linux/powerpc/powerpc32/nofpu/libc.abilist
> index f5830f9c33..2de3134bc7 100644
> --- a/sysdeps/unix/sysv/linux/powerpc/powerpc32/nofpu/libc.abilist
> +++ b/sysdeps/unix/sysv/linux/powerpc/powerpc32/nofpu/libc.abilist
> @@ -2000,6 +2000,8 @@ GLIBC_2.28 thrd_yield F
>   GLIBC_2.29 getcpu F
>   GLIBC_2.29 posix_spawn_file_actions_addchdir_np F
>   GLIBC_2.29 posix_spawn_file_actions_addfchdir_np F
> +GLIBC_2.30 __rseq_abi T 0x20
> +GLIBC_2.30 __rseq_lib_abi T 0x8

OK.

>   GLIBC_2.3 __ctype_b_loc F
>   GLIBC_2.3 __ctype_tolower_loc F
>   GLIBC_2.3 __ctype_toupper_loc F
> diff --git a/sysdeps/unix/sysv/linux/powerpc/powerpc64/be/libc.abilist b/sysdeps/unix/sysv/linux/powerpc/powerpc64/be/libc.abilist
> index 633d8f4792..aae3def700 100644
> --- a/sysdeps/unix/sysv/linux/powerpc/powerpc64/be/libc.abilist
> +++ b/sysdeps/unix/sysv/linux/powerpc/powerpc64/be/libc.abilist
> @@ -126,6 +126,8 @@ GLIBC_2.28 thrd_yield F
>   GLIBC_2.29 getcpu F
>   GLIBC_2.29 posix_spawn_file_actions_addchdir_np F
>   GLIBC_2.29 posix_spawn_file_actions_addfchdir_np F
> +GLIBC_2.30 __rseq_abi T 0x20
> +GLIBC_2.30 __rseq_lib_abi T 0x8

OK.

>   GLIBC_2.3 _Exit F
>   GLIBC_2.3 _IO_2_1_stderr_ D 0xe0
>   GLIBC_2.3 _IO_2_1_stdin_ D 0xe0
> diff --git a/sysdeps/unix/sysv/linux/powerpc/powerpc64/le/libc.abilist b/sysdeps/unix/sysv/linux/powerpc/powerpc64/le/libc.abilist
> index 2c712636ef..8d582a3a9b 100644
> --- a/sysdeps/unix/sysv/linux/powerpc/powerpc64/le/libc.abilist
> +++ b/sysdeps/unix/sysv/linux/powerpc/powerpc64/le/libc.abilist
> @@ -2231,3 +2231,5 @@ GLIBC_2.28 thrd_yield F
>   GLIBC_2.29 getcpu F
>   GLIBC_2.29 posix_spawn_file_actions_addchdir_np F
>   GLIBC_2.29 posix_spawn_file_actions_addfchdir_np F
> +GLIBC_2.30 __rseq_abi T 0x20
> +GLIBC_2.30 __rseq_lib_abi T 0x8

OK.

> diff --git a/sysdeps/unix/sysv/linux/riscv/rv64/libc.abilist b/sysdeps/unix/sysv/linux/riscv/rv64/libc.abilist
> index 195bc8b2cf..155953f6cf 100644
> --- a/sysdeps/unix/sysv/linux/riscv/rv64/libc.abilist
> +++ b/sysdeps/unix/sysv/linux/riscv/rv64/libc.abilist
> @@ -2103,3 +2103,5 @@ GLIBC_2.28 thrd_yield F
>   GLIBC_2.29 getcpu F
>   GLIBC_2.29 posix_spawn_file_actions_addchdir_np F
>   GLIBC_2.29 posix_spawn_file_actions_addfchdir_np F
> +GLIBC_2.30 __rseq_abi T 0x20
> +GLIBC_2.30 __rseq_lib_abi T 0x8

OK.

> diff --git a/sysdeps/unix/sysv/linux/rseq-internal.h b/sysdeps/unix/sysv/linux/rseq-internal.h
> new file mode 100644
> index 0000000000..d676da3701
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/rseq-internal.h
> @@ -0,0 +1,91 @@
> +/* Copyright (C) 2018 Free Software Foundation, Inc.

Needs a one sentence description.

> +   This file is part of the GNU C Library.
> +   Contributed by Mathieu Desnoyers <mathieu.desnoyers@efficios.com>, 2018.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <http://www.gnu.org/licenses/>.  */
> +
> +#ifndef RSEQ_INTERNAL_H
> +#define RSEQ_INTERNAL_H
> +
> +#include <sysdep.h>
> +
> +#ifdef __NR_rseq
> +
> +#include <errno.h>
> +#include <sys/rseq.h>
> +

This needs a giant comment describing the reference count algorithm
and how it is intended to work.

> +static inline int
> +rseq_register_current_thread (void)
> +{
> +  int rc, ret = 0;
> +  INTERNAL_SYSCALL_DECL (err);
> +
> +  if (__rseq_abi.cpu_id == RSEQ_CPU_ID_REGISTRATION_FAILED)
> +    return -1;
> +  /* Temporarily prevent nested signal handlers from registering rseq.  */
> +  __rseq_lib_abi.register_state = RSEQ_REGISTER_ONGOING;

OK.

> +  if (__rseq_lib_abi.refcount == UINT_MAX)
> +    {
> +      ret = -1;
> +      goto end;
> +    }
> +  if (__rseq_lib_abi.refcount++)
> +    goto end;

OK.

> +  rc = INTERNAL_SYSCALL_CALL (rseq, err, &__rseq_abi, sizeof (struct rseq),
> +                              0, RSEQ_SIG);
> +  if (!rc)
> +    goto end;
> +  if (INTERNAL_SYSCALL_ERRNO (rc, err) != EBUSY)
> +    __rseq_abi.cpu_id = RSEQ_CPU_ID_REGISTRATION_FAILED;
> +  ret = -1;
> +end:
> +  __rseq_lib_abi.register_state = RSEQ_REGISTER_ALLOWED;

OK.

> +  return ret;
> +}
> +
> +static inline int
> +rseq_unregister_current_thread (void)
> +{
> +  int rc, ret = 0;
> +  INTERNAL_SYSCALL_DECL (err);
> +
> +  /* Setting __rseq_register_state = RSEQ_REGISTER_EXITING for the rest of the
> +     thread lifetime. Ensures signal handlers nesting just before thread exit
> +     don't try to register rseq.  */
> +  __rseq_lib_abi.register_state = RSEQ_REGISTER_EXITING;
> +  __rseq_lib_abi.refcount = 0;
> +  rc = INTERNAL_SYSCALL_CALL (rseq, err, &__rseq_abi, sizeof (struct rseq),
> +                              RSEQ_FLAG_UNREGISTER, RSEQ_SIG);

OK.

> +  if (!rc)
> +    goto end;
> +  ret = -1;
> +end:
> +  return ret;
> +}
> +#else
> +static inline int
> +rseq_register_current_thread (void)
> +{
> +  return -1;
> +}
> +
> +static inline int
> +rseq_unregister_current_thread (void)
> +{
> +  return -1;
> +}

OK.

> +#endif
> +
> +#endif /* rseq-internal.h */
> diff --git a/sysdeps/unix/sysv/linux/rseq-sym.c b/sysdeps/unix/sysv/linux/rseq-sym.c
> new file mode 100644
> index 0000000000..99b277e9d6
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/rseq-sym.c
> @@ -0,0 +1,54 @@
> +/* Copyright (C) 2018 Free Software Foundation, Inc.

Needs a one sentence description.

> +   This file is part of the GNU C Library.
> +   Contributed by Mathieu Desnoyers <mathieu.desnoyers@efficios.com>, 2018.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <http://www.gnu.org/licenses/>.  */
> +
> +#include <sys/syscall.h>
> +#include <stdint.h>
> +
> +#ifdef __NR_rseq
> +#include <sys/rseq.h>
> +#else
> +
> +enum rseq_cpu_id_state {
> +  RSEQ_CPU_ID_UNINITIALIZED = -1,
> +  RSEQ_CPU_ID_REGISTRATION_FAILED = -2,

OK.

> +};
> +
> +/* linux/rseq.h defines struct rseq as aligned on 32 bytes. The kernel ABI
> +   size is 20 bytes.  */
> +struct rseq {
> +  uint32_t cpu_id_start;
> +  uint32_t cpu_id;
> +  uint64_t rseq_cs;
> +  uint32_t flags;
> +} __attribute__ ((aligned(4 * sizeof(uint64_t))));

OK, this is the kernel abi.

> +
> +struct rseq_lib_abi
> +{
> +  uint32_t register_state;
> +  uint32_t refcount;

OK. This is the refcount coordination structure for registration.

> +};
> +
> +#endif
> +
> +/* volatile because fields can be read/updated by the kernel.  */
> +__thread volatile struct rseq __rseq_abi = {
> +  .cpu_id = RSEQ_CPU_ID_UNINITIALIZED,

OK.

> +};
> +
> +/* volatile because fields can be read/updated by signal handlers.  */
> +__thread volatile struct rseq_lib_abi __rseq_lib_abi;

OK.

> diff --git a/sysdeps/unix/sysv/linux/s390/bits/rseq.h b/sysdeps/unix/sysv/linux/s390/bits/rseq.h
> new file mode 100644
> index 0000000000..19d3755837
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/s390/bits/rseq.h
> @@ -0,0 +1,24 @@
> +/* Copyright (C) 2019 Free Software Foundation, Inc.

Needs a one sentence description.

> +   This file is part of the GNU C Library.
> +   Contributed by Mathieu Desnoyers <mathieu.desnoyers@efficios.com>, 2019.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <http://www.gnu.org/licenses/>.  */
> +
> +#ifndef _SYS_RSEQ_H
> +# error "Never use <bits/rseq.h> directly; include <sys/rseq.h> instead."
> +#endif
> +
> +/* Signature required before each abort handler code.  */
> +#define RSEQ_SIG 0x53053053

Why not a s390 specific value here?

> diff --git a/sysdeps/unix/sysv/linux/s390/s390-32/libc.abilist b/sysdeps/unix/sysv/linux/s390/s390-32/libc.abilist
> index 334def033c..42316d8666 100644
> --- a/sysdeps/unix/sysv/linux/s390/s390-32/libc.abilist
> +++ b/sysdeps/unix/sysv/linux/s390/s390-32/libc.abilist
> @@ -2005,6 +2005,8 @@ GLIBC_2.28 thrd_yield F
>   GLIBC_2.29 getcpu F
>   GLIBC_2.29 posix_spawn_file_actions_addchdir_np F
>   GLIBC_2.29 posix_spawn_file_actions_addfchdir_np F
> +GLIBC_2.30 __rseq_abi T 0x20
> +GLIBC_2.30 __rseq_lib_abi T 0x8

OK.

>   GLIBC_2.3 __ctype_b_loc F
>   GLIBC_2.3 __ctype_tolower_loc F
>   GLIBC_2.3 __ctype_toupper_loc F
> diff --git a/sysdeps/unix/sysv/linux/s390/s390-64/libc.abilist b/sysdeps/unix/sysv/linux/s390/s390-64/libc.abilist
> index 536f4c4ced..c6c4e55a77 100644
> --- a/sysdeps/unix/sysv/linux/s390/s390-64/libc.abilist
> +++ b/sysdeps/unix/sysv/linux/s390/s390-64/libc.abilist
> @@ -1911,6 +1911,8 @@ GLIBC_2.29 __fentry__ F
>   GLIBC_2.29 getcpu F
>   GLIBC_2.29 posix_spawn_file_actions_addchdir_np F
>   GLIBC_2.29 posix_spawn_file_actions_addfchdir_np F
> +GLIBC_2.30 __rseq_abi T 0x20
> +GLIBC_2.30 __rseq_lib_abi T 0x8

OK.

>   GLIBC_2.3 __ctype_b_loc F
>   GLIBC_2.3 __ctype_tolower_loc F
>   GLIBC_2.3 __ctype_toupper_loc F
> diff --git a/sysdeps/unix/sysv/linux/sh/libc.abilist b/sysdeps/unix/sysv/linux/sh/libc.abilist
> index 30ae3b6ebb..8652dfea59 100644
> --- a/sysdeps/unix/sysv/linux/sh/libc.abilist
> +++ b/sysdeps/unix/sysv/linux/sh/libc.abilist
> @@ -1887,6 +1887,8 @@ GLIBC_2.28 thrd_yield F
>   GLIBC_2.29 getcpu F
>   GLIBC_2.29 posix_spawn_file_actions_addchdir_np F
>   GLIBC_2.29 posix_spawn_file_actions_addfchdir_np F
> +GLIBC_2.30 __rseq_abi T 0x20
> +GLIBC_2.30 __rseq_lib_abi T 0x8

OK.

>   GLIBC_2.3 __ctype_b_loc F
>   GLIBC_2.3 __ctype_tolower_loc F
>   GLIBC_2.3 __ctype_toupper_loc F
> diff --git a/sysdeps/unix/sysv/linux/sparc/sparc32/libc.abilist b/sysdeps/unix/sysv/linux/sparc/sparc32/libc.abilist
> index 68b107d080..95b58dfa67 100644
> --- a/sysdeps/unix/sysv/linux/sparc/sparc32/libc.abilist
> +++ b/sysdeps/unix/sysv/linux/sparc/sparc32/libc.abilist
> @@ -1999,6 +1999,8 @@ GLIBC_2.28 thrd_yield F
>   GLIBC_2.29 getcpu F
>   GLIBC_2.29 posix_spawn_file_actions_addchdir_np F
>   GLIBC_2.29 posix_spawn_file_actions_addfchdir_np F
> +GLIBC_2.30 __rseq_abi T 0x20
> +GLIBC_2.30 __rseq_lib_abi T 0x8

OK.

>   GLIBC_2.3 __ctype_b_loc F
>   GLIBC_2.3 __ctype_tolower_loc F
>   GLIBC_2.3 __ctype_toupper_loc F
> diff --git a/sysdeps/unix/sysv/linux/sparc/sparc64/libc.abilist b/sysdeps/unix/sysv/linux/sparc/sparc64/libc.abilist
> index e5b6a4da50..bfd24f9d1c 100644
> --- a/sysdeps/unix/sysv/linux/sparc/sparc64/libc.abilist
> +++ b/sysdeps/unix/sysv/linux/sparc/sparc64/libc.abilist
> @@ -1940,6 +1940,8 @@ GLIBC_2.28 thrd_yield F
>   GLIBC_2.29 getcpu F
>   GLIBC_2.29 posix_spawn_file_actions_addchdir_np F
>   GLIBC_2.29 posix_spawn_file_actions_addfchdir_np F
> +GLIBC_2.30 __rseq_abi T 0x20
> +GLIBC_2.30 __rseq_lib_abi T 0x8

OK.

>   GLIBC_2.3 __ctype_b_loc F
>   GLIBC_2.3 __ctype_tolower_loc F
>   GLIBC_2.3 __ctype_toupper_loc F
> diff --git a/sysdeps/unix/sysv/linux/sys/rseq.h b/sysdeps/unix/sysv/linux/sys/rseq.h
> new file mode 100644
> index 0000000000..83c8976f50
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/sys/rseq.h
> @@ -0,0 +1,65 @@

Needs a one sentence description.

> +/* Copyright (C) 2019 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +   Contributed by Mathieu Desnoyers <mathieu.desnoyers@efficios.com>, 2019.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <http://www.gnu.org/licenses/>.  */
> +
> +#ifndef _SYS_RSEQ_H
> +#define _SYS_RSEQ_H	1
> +
> +/* We use the structures declarations from the kernel headers.  */
> +#include <linux/rseq.h>
> +/* Architecture-specific rseq signature.  */
> +#include <bits/rseq.h>
> +#include <stdint.h>

OK.

> +
> +enum rseq_register_state
> +{
> +  /* Value RSEQ_REGISTER_ALLOWED means it is allowed to update
> +     the refcount field and to register/unregister rseq.  */
> +  RSEQ_REGISTER_ALLOWED = 0,
> +  /* Value RSEQ_REGISTER_ONGOING means a rseq registration is in progress,
> +     so it is temporarily forbidden to update the refcount field or to
> +     register/unregister rseq for this thread or signal handlers nested
> +     on this thread.  */
> +  RSEQ_REGISTER_ONGOING = 1,
> +  /* Value RSEQ_REGISTER_EXITING means it is forbidden to update the
> +     refcount field or to register/unregister rseq for the rest of the
> +     thread's lifetime.  */
> +  RSEQ_REGISTER_EXITING = 2,

OK.

> +};
> +
> +struct rseq_lib_abi
> +{
> +  uint32_t register_state; /* enum rseq_register_state.  */
> +  /* The refcount field keeps track of rseq users, so early adopters
> +     of rseq can cooperate amongst each other and with glibc to
> +     share rseq thread registration. The refcount field can only be
> +     updated when allowed by the value of field register_state.
> +     Registering rseq should be performed when incrementing refcount
> +     from 0 to 1, and unregistering rseq should be performed when
> +     decrementing refcount from 1 to 0.  */

OK.

> +  uint32_t refcount;
> +};
> +
> +/* volatile because fields can be read/updated by the kernel.  */
> +extern __thread volatile struct rseq __rseq_abi
> +__attribute__ ((tls_model ("initial-exec")));

OK.

> +
> +/* volatile because fields can be read/updated by signal handlers.  */
> +extern __thread volatile struct rseq_lib_abi __rseq_lib_abi
> +__attribute__ ((tls_model ("initial-exec")));

OK.

> +
> +#endif /* sys/rseq.h */
> diff --git a/sysdeps/unix/sysv/linux/x86/bits/rseq.h b/sysdeps/unix/sysv/linux/x86/bits/rseq.h
> new file mode 100644
> index 0000000000..19d3755837
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/x86/bits/rseq.h
> @@ -0,0 +1,24 @@
> +/* Copyright (C) 2019 Free Software Foundation, Inc.

Add one line short description.

> +   This file is part of the GNU C Library.
> +   Contributed by Mathieu Desnoyers <mathieu.desnoyers@efficios.com>, 2019.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <http://www.gnu.org/licenses/>.  */
> +
> +#ifndef _SYS_RSEQ_H
> +# error "Never use <bits/rseq.h> directly; include <sys/rseq.h> instead."
> +#endif
> +
> +/* Signature required before each abort handler code.  */
> +#define RSEQ_SIG 0x53053053

Why not an x86-specific op code?

> diff --git a/sysdeps/unix/sysv/linux/x86_64/64/libc.abilist b/sysdeps/unix/sysv/linux/x86_64/64/libc.abilist
> index 86dfb0c94d..e9f8411fb2 100644
> --- a/sysdeps/unix/sysv/linux/x86_64/64/libc.abilist
> +++ b/sysdeps/unix/sysv/linux/x86_64/64/libc.abilist
> @@ -1898,6 +1898,8 @@ GLIBC_2.28 thrd_yield F
>   GLIBC_2.29 getcpu F
>   GLIBC_2.29 posix_spawn_file_actions_addchdir_np F
>   GLIBC_2.29 posix_spawn_file_actions_addfchdir_np F
> +GLIBC_2.30 __rseq_abi T 0x20
> +GLIBC_2.30 __rseq_lib_abi T 0x8

OK.

>   GLIBC_2.3 __ctype_b_loc F
>   GLIBC_2.3 __ctype_tolower_loc F
>   GLIBC_2.3 __ctype_toupper_loc F
> diff --git a/sysdeps/unix/sysv/linux/x86_64/x32/libc.abilist b/sysdeps/unix/sysv/linux/x86_64/x32/libc.abilist
> index dd688263aa..f9432d07f1 100644
> --- a/sysdeps/unix/sysv/linux/x86_64/x32/libc.abilist
> +++ b/sysdeps/unix/sysv/linux/x86_64/x32/libc.abilist
> @@ -2149,3 +2149,5 @@ GLIBC_2.28 thrd_yield F
>   GLIBC_2.29 getcpu F
>   GLIBC_2.29 posix_spawn_file_actions_addchdir_np F
>   GLIBC_2.29 posix_spawn_file_actions_addfchdir_np F
> +GLIBC_2.30 __rseq_abi T 0x20
> +GLIBC_2.30 __rseq_lib_abi T 0x8

OK.

> 


-- 
Cheers,
Carlos.