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From:   Jisheng Zhang <Jisheng.Zhang@synaptics.com>
To:     Masami Hiramatsu <mhiramat@kernel.org>
CC:     Mark Rutland <mark.rutland@arm.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Catalin Marinas <catalin.marinas@arm.com>,
        "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>,
        Ingo Molnar <mingo@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>,
        Will Deacon <will@kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v7 3/3] arm64: implement KPROBES_ON_FTRACE
Thread-Topic: [PATCH v7 3/3] arm64: implement KPROBES_ON_FTRACE
Thread-Index: AQHVuwfioeAAP6kOPkCZLC/0/SnpxKfLukGAgAAB3gCAABLQAA==
Date:   Thu, 26 Dec 2019 04:25:24 +0000
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On Thu, 26 Dec 2019 03:18:07 +0000 Jisheng Zhang wrote:


>=20
>=20
> Hi
>=20
> On Thu, 26 Dec 2019 11:57:07 +0900 Masami Hiramatsu wrote:
>=20
> >
> > Hi Jisheng,
> >
> > On Wed, 25 Dec 2019 09:44:21 +0000
> > Jisheng Zhang <Jisheng.Zhang@synaptics.com> wrote:
> > =20
> > > KPROBES_ON_FTRACE avoids much of the overhead with regular kprobes as=
 it
> > > eliminates the need for a trap, as well as the need to emulate or
> > > single-step instructions.
> > >
> > > Tested on berlin arm64 platform.
> > >
> > > ~ # mount -t debugfs debugfs /sys/kernel/debug/
> > > ~ # cd /sys/kernel/debug/
> > > /sys/kernel/debug # echo 'p _do_fork' > tracing/kprobe_events
> > >
> > > before the patch:
> > >
> > > /sys/kernel/debug # cat kprobes/list
> > > ffffff801009fe28  k  _do_fork+0x0    [DISABLED]
> > >
> > > after the patch:
> > >
> > > /sys/kernel/debug # cat kprobes/list
> > > ffffff801009ff54  k  _do_fork+0x0    [DISABLED][FTRACE] =20
> >
> > What happens if user puts a probe on _do_fork+4?
> > Is that return -EILSEQ correctly? =20
>=20
> _do_fork+4 can be probed successfully.
>=20
> > =20
> > >
> > > Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
> > > ---
> > >  .../debug/kprobes-on-ftrace/arch-support.txt  |  2 +-
> > >  arch/arm64/Kconfig                            |  1 +
> > >  arch/arm64/include/asm/ftrace.h               |  1 +
> > >  arch/arm64/kernel/probes/Makefile             |  1 +
> > >  arch/arm64/kernel/probes/ftrace.c             | 78 +++++++++++++++++=
++
> > >  5 files changed, 82 insertions(+), 1 deletion(-)
> > >  create mode 100644 arch/arm64/kernel/probes/ftrace.c
> > >
> > > diff --git a/Documentation/features/debug/kprobes-on-ftrace/arch-supp=
ort.txt b/Documentation/features/debug/kprobes-on-ftrace/arch-support.txt
> > > index 4fae0464ddff..f9dd9dd91e0c 100644
> > > --- a/Documentation/features/debug/kprobes-on-ftrace/arch-support.txt
> > > +++ b/Documentation/features/debug/kprobes-on-ftrace/arch-support.txt
> > > @@ -9,7 +9,7 @@
> > >      |       alpha: | TODO |
> > >      |         arc: | TODO |
> > >      |         arm: | TODO |
> > > -    |       arm64: | TODO |
> > > +    |       arm64: |  ok  |
> > >      |         c6x: | TODO |
> > >      |        csky: | TODO |
> > >      |       h8300: | TODO |
> > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > > index b1b4476ddb83..92b9882889ac 100644
> > > --- a/arch/arm64/Kconfig
> > > +++ b/arch/arm64/Kconfig
> > > @@ -166,6 +166,7 @@ config ARM64
> > >       select HAVE_STACKPROTECTOR
> > >       select HAVE_SYSCALL_TRACEPOINTS
> > >       select HAVE_KPROBES
> > > +     select HAVE_KPROBES_ON_FTRACE
> > >       select HAVE_KRETPROBES
> > >       select HAVE_GENERIC_VDSO
> > >       select IOMMU_DMA if IOMMU_SUPPORT
> > > diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm=
/ftrace.h
> > > index 91fa4baa1a93..875aeb839654 100644
> > > --- a/arch/arm64/include/asm/ftrace.h
> > > +++ b/arch/arm64/include/asm/ftrace.h
> > > @@ -20,6 +20,7 @@
> > >
> > >  /* The BL at the callsite's adjusted rec->ip */
> > >  #define MCOUNT_INSN_SIZE     AARCH64_INSN_SIZE
> > > +#define FTRACE_IP_EXTENSION  MCOUNT_INSN_SIZE
> > >
> > >  #define FTRACE_PLT_IDX               0
> > >  #define FTRACE_REGS_PLT_IDX  1
> > > diff --git a/arch/arm64/kernel/probes/Makefile b/arch/arm64/kernel/pr=
obes/Makefile
> > > index 8e4be92e25b1..4020cfc66564 100644
> > > --- a/arch/arm64/kernel/probes/Makefile
> > > +++ b/arch/arm64/kernel/probes/Makefile
> > > @@ -4,3 +4,4 @@ obj-$(CONFIG_KPROBES)         +=3D kprobes.o decode-i=
nsn.o      \
> > >                                  simulate-insn.o
> > >  obj-$(CONFIG_UPROBES)                +=3D uprobes.o decode-insn.o   =
   \
> > >                                  simulate-insn.o
> > > +obj-$(CONFIG_KPROBES_ON_FTRACE)      +=3D ftrace.o
> > > diff --git a/arch/arm64/kernel/probes/ftrace.c b/arch/arm64/kernel/pr=
obes/ftrace.c
> > > new file mode 100644
> > > index 000000000000..0643aa2dacdb
> > > --- /dev/null
> > > +++ b/arch/arm64/kernel/probes/ftrace.c
> > > @@ -0,0 +1,78 @@
> > > +// SPDX-License-Identifier: GPL-2.0-or-later
> > > +/*
> > > + * Dynamic Ftrace based Kprobes Optimization
> > > + *
> > > + * Copyright (C) Hitachi Ltd., 2012
> > > + * Copyright (C) 2019 Jisheng Zhang <jszhang@kernel.org>
> > > + *                 Synaptics Incorporated
> > > + */
> > > +
> > > +#include <linux/kprobes.h>
> > > +
> > > +/*
> > > + * In arm64 FTRACE_WITH_REGS implementation, we patch two nop instru=
ctions:
> > > + * the lr saver and bl ftrace-entry. Both these instructions are cla=
imed
> > > + * by ftrace and we should allow probing on either instruction. =20
> >
> > No, the 2nd bl ftrace-entry must not be probed.
> > The pair of lr-saver and bl ftrace-entry is tightly coupled. You can no=
t
> > decouple it. =20
>=20
> This is the key. different viewing of this results in different implement=
ation.
> I'm just wondering why are the two instructions considered as coupled. I =
think
> here we met similar situation as powerpc: https://lkml.org/lkml/2019/6/18=
/646
> the "mflr r0" equals to lr-saver here, branch to _mcount equals to bl ftr=
ace-entry
> could you please kindly comment more?
>=20
> Thanks in advance
>=20

hmm, I think I may get some part of your opinion. In v7 implementation:

if probe on func+4, that's bl ftrace-entry, similar as mcount call on
other architectures, we allow this probe as normal.

if probe on func+0, the first param ip in kprobe_ftrace_handler() points
to func+4(this is adjusted by ftrace), regs->ip points to func+8, so in
kprobe_ftrace_handler() we modify regs->ip to func+0 to call kprobe
pre handler, then modify regs->ip to func+8 to call kprobe post handler.
As can be seen, the first two instructions are considered as a virtual
mcount call. From this point of view, lr saver and the bl <ftrace-entry>
is coupled.

If we split patch3 into two:
one to support kprobes func+4
the second to support kprobe on func+0
it would be much clearer.

Then the key here is whether we could allow both kprobes on func+0 and func=
+4

Thanks