Re: [PATCH v3 8/9] ARM: kprobes: Make a frame pointer on __kretprobe_trampoline

On Fri, 3 Dec 2021 at 21:38, Arnd Bergmann [off-list ref] wrote:

On Thu, Oct 21, 2021 at 2:55 AM Masami Hiramatsu [off-list ref] wrote:

quoted

Currently kretprobe on ARM just fills r0-r11 of pt_regs, but
that is not enough for the stacktrace. Moreover, from the user
kretprobe handler, stacktrace needs a frame pointer on the
__kretprobe_trampoline.

This adds a frame pointer on __kretprobe_trampoline for both gcc
and clang case. Those have different frame pointer so we need
different but similar stack on pt_regs.

Gcc makes the frame pointer (fp) to point the 'pc' address of
the {fp, ip (=sp), lr, pc}, this means {r11, r13, r14, r15}.
Thus if we save the r11 (fp) on pt_regs->r12, we can make this
set on the end of pt_regs.

On the other hand, Clang makes the frame pointer to point the
'fp' address of {fp, lr} on stack. Since the next to the
pt_regs->lr is pt_regs->sp, I reused the pair of pt_regs->fp
and pt_regs->ip.
So this stores the 'lr' on pt_regs->ip and make the fp to point
pt_regs->fp.

For both cases, saves __kretprobe_trampoline address to
pt_regs->lr, so that the stack tracer can identify this frame
pointer has been made by the __kretprobe_trampoline.

Note that if the CONFIG_FRAME_POINTER is not set, this keeps
fp as is.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

This causes a regression that I see in randconfig builds with gcc-11:

/tmp/ccovvQNw.s: Assembler messages:
/tmp/ccovvQNw.s:32: Error: invalid literal constant: pool needs to be closer
make[5]: *** [/git/arm-soc/scripts/Makefile.build:288:
arch/arm/probes/kprobes/core.o] Error 1

I have two randconfigs that reproduce it locally, here is a .config
[1] and assembly
output[2] for reference. I have not done any further analysis, but
maybe you have
an idea.

Does this help?

diff --git a/arch/arm/probes/kprobes/core.c b/arch/arm/probes/kprobes/core.c
index 9090c3a74dcc..88999ed2cfc4 100644
--- a/arch/arm/probes/kprobes/core.c
+++ b/arch/arm/probes/kprobes/core.c

@@ -408,6 +408,7 @@ void __naked __kprobes __kretprobe_trampoline(void)
 #else
                "mov    pc, lr                  \n\t"
 #endif
+               ".ltorg                         \n\t"
                : : : "memory");
 }

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