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

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

On Wed, 2017-02-15 at 01:01 +0900, Masami Hiramatsu wrote:
> On Tue, 14 Feb 2017 13:47:07 +0000
> "Jon Medhurst (Tixy)" <tixy@linaro.org> wrote:
> 
> > 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).
> 
> Hmm, I'm running arm kernel on qemu, which maybe the reason why
> the test case passed in my environment, since my kconfig also sets
> CONFIG_DEBUG_RODATA=y.
> 
> BTW, would you see that any kprobe_events didn't work with
> CONFIG_DEBUG_RODATA=y? (what the failure messages were?)

The tests I'm running are the ARM specific tests that are enabled by
CONFIG_ARM_KPROBES_TEST=y. I'm running the tests on real multicore ARM
hardware (Versatile Express with a TC2 CoreTile)

For me, sometimes the first test gave:

    Beginning kprobe tests...
    Probe ARM code
        kprobe
    FAIL: test regs not OK

Other times, for the specific instruction emulation tests they return

   FAIL: test_before_handler not run

Not sure how much of the diagnostic appear without setting the tests to
be verbose, which I do with:

  sed -e 's/VERBOSE 0/VERBOSE 1/' -i arch/arm/probes/kprobes/test-core.h

Whilst writing a reply, I looked at the test code in
arch/arm/probes/kprobes/test-core.c (which I wrote some years ago) and
there is possibly a clue staring at us in the comments at the top of the
file...

 *
 * The above would expand to assembler looking something like:
 *
 *	@ TESTCASE_START
 *	bl	__kprobes_test_case_start
 *	.pushsection .rodata
 *	"10:
 *	.ascii "mov r0, r7"	@ text title for test case
 *	.byte	0
 *	.popsection
 *	@ start of inline data...
 *	.word	10b		@ pointer to title in .rodata
section

Note the ".pushsection .rodata" (though I don't see an immediate obvious
reason why that would cause a problem. It certainly seems likely that
the problem is with the ARM test code rather than actual kprobe
implementation itself.

Like I said, this issue has been there for a year or more, so I wasn't
planning on spending time on it for a few more days yet whilst I get on
with other urgent matters.

-- 
Tixy


Basically, m