Re: [BUGFIX PATCH 3/3] kprobes/arm: Fix the return address of multiple kretprobes

["Jon Medhurst (Tixy)" <tixy@linaro.org>]

On Tue, 2017-02-14 at 10:32 +0000, Jon Medhurst (Tixy) wrote:
> On Tue, 2017-02-14 at 00:05 +0900, Masami Hiramatsu wrote:
> > This is arm port of commit 737480a0d525 ("kprobes/x86:
> > Fix the return address of multiple kretprobes").
> > 
> > Fix the return address of subsequent kretprobes when multiple
> > kretprobes are set on the same function.
> > 
> > For example:
> > 
> >   # cd /sys/kernel/debug/tracing
> >   # echo "r:event1 sys_symlink" > kprobe_events
> >   # echo "r:event2 sys_symlink" >> kprobe_events
> >   # echo 1 > events/kprobes/enable
> >   # ln -s /tmp/foo /tmp/bar
> > 
> >  (without this patch)
> > 
> >   # cat trace | grep -v ^#
> >               ln-82    [000] dn.2    68.446525: event1: (kretprobe_trampoline+0x0/0x18 <- SyS_symlink)
> >               ln-82    [000] dn.2    68.447831: event2: (ret_fast_syscall+0x0/0x1c <- SyS_symlink)
> > 
> >  (with this patch)
> > 
> >   # cat trace | grep -v ^#
> >               ln-81    [000] dn.1    39.463469: event1: (ret_fast_syscall+0x0/0x1c <- SyS_symlink)
> >               ln-81    [000] dn.1    39.464701: event2: (ret_fast_syscall+0x0/0x1c <- SyS_symlink)
> > 
> > Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> > Cc: KUMANO Syuhei <kumano.prog@gmail.com>
> > ---
> 
> I don't fully understand this function, but I've checked that the ARM
> version now matches the x86 version (apart from the x86 specific
> register fixup and some comments). So, FWIW
> 
> Acked-by: Jon Medhurst <tixy@linaro.org>
> 
> I ran the before and after test case in the commit log on ARM and
> verified the result is correct. I also tried running the ARM kprobe
> tests with these 3 fixes but the tests fail. However, they also fail
> without any of these changes, so I'll investigate that further...

Bisecting the issue led me back to Linux 4.5 and commit 25362dc496ed
("ARM: 8501/1: mm: flip priority of CONFIG_DEBUG_RODATA")

This sets CONFIG_DEBUG_RODATA to be enabled by default. If I disable
that on 4.10-rc4, with the three patches in this series, then the ARM
kprobes tests pass OK.

I'll stick the DEBUG_RODATA issue on my todo list (it's been around for
a year, so can probably wait a little longer).

-- 
Tixy