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Date:   Wed, 29 May 2019 11:45:10 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Florian Weimer <fweimer@redhat.com>
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Subject: Re: [PATCH 1/5] glibc: Perform rseq(2) registration at C startup
 and thread creation (v10)
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Thread-Topic: glibc: Perform rseq(2) registration at C startup and thread creation (v10)
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----- On May 27, 2019, at 3:27 PM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote:

> ----- On May 27, 2019, at 7:19 AM, Florian Weimer fweimer@redhat.com wrote:
> 

[...]

>> 
>> Furthermore, the reference to ELF constructors is misleading.  I believe
>> the code you added to __libc_start_main to initialize __rseq_handled and
>> register __seq_abi with the kernel runs *after* ELF constructors have
>> executed (and not at all if the main program is written in Go, alas).
>> All initialization activity for the shared case needs to happen in
>> elf/rtld.c or called from there, probably as part of the security
>> initialization code or thereabouts.
> 
> in elf/rtld.c:dl_main() we have the following code:
> 
>  /* We do not initialize any of the TLS functionality unless any of the
>     initial modules uses TLS.  This makes dynamic loading of modules with
>     TLS impossible, but to support it requires either eagerly doing setup
>     now or lazily doing it later.  Doing it now makes us incompatible with
>     an old kernel that can't perform TLS_INIT_TP, even if no TLS is ever
>     used.  Trying to do it lazily is too hairy to try when there could be
>     multiple threads (from a non-TLS-using libpthread).  */
>  bool was_tls_init_tp_called = tls_init_tp_called;
>  if (tcbp == NULL)
>    tcbp = init_tls ();
> 
> If I understand your point correctly, I should move the rseq_init() and
> rseq_register_current_thread() for the SHARED case just after this
> initialization, otherwise calling those from LIBC_START_MAIN() is too
> late and it runs after initial modules constructors (or not at all for
> Go). However, this means glibc will start using TLS internally. I'm
> concerned that this is not quite in line with the above comment which
> states that TLS is not initialized if no initial modules use TLS.
> 
> For the !SHARED use-case, if my understanding is correct, I should keep
> rseq_init() and rseq_register_current_thread() calls within LIBC_START_MAIN().

I've moved the rseq initialization for SHARED case to the very end of
elf/rtld.c:init_tls(), and get the following error on make check:

Generating locale am_ET.UTF-8: this might take a while...
Inconsistency detected by ld.so: get-dynamic-info.h: 143: elf_get_dynamic_info: Assertion `info[DT_FLAGS] == NULL || (info[DT_FLAGS]->d_un.d_val & ~DF_BIND_NOW) == 0' failed!
Charmap: "UTF-8" Inputfile: "am_ET" Outputdir: "am_ET.UTF-8" failed
/bin/sh: 4: cannot create /home/efficios/git/glibc-build/localedata/am_ET.UTF-8/LC_CTYPE.test-result: Directory nonexistent

This error goes away if I comment out the call to rseq_register_current_thread (),
which touches the __rseq_abi __thread variable and issues a system call.

Currently, the __rseq_abi __thread variable is within
sysdeps/unix/sysv/linux/rseq-sym.c, which is added to the
sysdep_routines within sysdeps/unix/sysv/linux/Makefile. I
suspect it may need to be moved elsewhere.

Any thoughts on how to solve this ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com