Re: [PATCH 7/8] ARM: kprobes: Make a frame pointer on __kretprobe_trampoline

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>
---
 arch/arm/probes/kprobes/core.c |   29 ++++++++++++++++++++++++-----
 1 file changed, 24 insertions(+), 5 deletions(-)

diff --git a/arch/arm/probes/kprobes/core.c b/arch/arm/probes/kprobes/core.c
index 95f23b47ba27..7cbd65a22769 100644
--- a/arch/arm/probes/kprobes/core.c
+++ b/arch/arm/probes/kprobes/core.c

@@ -368,16 +368,35 @@ int __kprobes kprobe_exceptions_notify(struct notifier_block *self,
 /*
  * When a retprobed function returns, trampoline_handler() is called,
  * calling the kretprobe's handler. We construct a struct pt_regs to
- * give a view of registers r0-r11 to the user return-handler.  This is
- * not a complete pt_regs structure, but that should be plenty sufficient
- * for kretprobe handlers which should normally be interested in r0 only
- * anyway.
+ * give a view of registers r0-r11, sp, lr, and pc to the user
+ * return-handler. This is not a complete pt_regs structure, but that
+ * should be enough for stacktrace from the return handler with or
+ * without pt_regs.
  */
 void __naked __kprobes __kretprobe_trampoline(void)
 {
        __asm__ __volatile__ (
-               "sub    sp, sp, #16             \n\t"
+               "ldr    lr, =__kretprobe_trampoline     \n\t"
+               "stmdb  sp!, {sp, lr, pc}       \n\t"
+#ifdef CONFIG_FRAME_POINTER
+       /* __kretprobe_trampoline makes a framepointer on pt_regs. */
+#ifdef CONFIG_CC_IS_CLANG
+               /* In clang case, pt_regs->ip = lr. */
+               "stmdb  sp!, {lr}               \n\t"
                "stmdb  sp!, {r0 - r11}         \n\t"
+               /* fp points regs->r11 (fp) */
+               "add    fp, sp, #44             \n\t"
+#else /* !CONFIG_CC_IS_CLANG */
+               /* In gcc case, pt_regs->ip = fp. */
+               "stmdb  sp!, {fp}               \n\t"
+               "stmdb  sp!, {r0 - r11}         \n\t"
+               /* fp points regs->r15 (pc) */
+               "add    fp, sp, #60             \n\t"