Re: [RFC v2 4/4] arm64: ftrace: add a stack frame for exception handler

[AKASHI Takahiro]

On 08/11/2015 11:57 PM, Jungseok Lee wrote:
> On Aug 4, 2015, at 4:44 PM, AKASHI Takahiro wrote:
>
> Hi Akashi,
>
> > On arm64, an exception handler use the same stack as in non-exception
> > contexts, but doesn't create a stack frame for elx_xx entry, only updating
> > sp register. This behavior results in save_stace_trace() missing a function
> > that is the one when an exception happens.
> >
> > This patch creates a stack frame for this case, and puts an additional
> > entry for the function  in a stack trace list.
> >
> > Signed-off-by: AKASHI Takahiro <takahiro.akashi@xxxxxxxxxx>
> > ---
> > arch/arm64/kernel/entry.S      |    4 ++++
> > arch/arm64/kernel/stacktrace.c |   17 +++++++++++++++++
> > 2 files changed, 21 insertions(+)
> >
> > diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> > index f860bfd..aacb6c6 100644
> > --- a/arch/arm64/kernel/entry.S
> > +++ b/arch/arm64/kernel/entry.S
> > @@ -107,6 +107,8 @@
> > 	str	x21, [sp, #S_SYSCALLNO]
> > 	.endif
> >
> > +	/* create a stack frame for stack tracer */
> > +	mov	x29, sp
> > 	/*
> > 	 * Registers that may be useful after this macro is invoked:
> > 	 *
> > @@ -737,3 +739,5 @@ ENTRY(sys_rt_sigreturn_wrapper)
> > 	mov	x0, sp
> > 	b	sys_rt_sigreturn
> > ENDPROC(sys_rt_sigreturn_wrapper)
> > +
> > +ENTRY(end_of_vectors)
> > diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
> > index d1790eb..22ce7c9 100644
> > --- a/arch/arm64/kernel/stacktrace.c
> > +++ b/arch/arm64/kernel/stacktrace.c
> > @@ -25,6 +25,10 @@
> > #include <asm/stacktrace.h>
> >
> > #define S_FRAME_SIZE sizeof(struct pt_regs) /* asm-offsets.h */
> > +#define S_FP offsetof(struct pt_regs, regs[29])
> > +#define S_LR offsetof(struct pt_regs, regs[30])
> > +
> > +extern unsigned int *vectors, *end_of_vectors;
> >
> > /*
> >   * AArch64 PCS assigns the frame pointer to x29.
> > @@ -50,6 +54,19 @@ int notrace unwind_frame(struct stackframe *frame)
> > 	if (fp < low || fp > high - 0x18 || fp & 0xf)
> > 		return -EINVAL;
> >
> > +	if ((frame->pc >= (unsigned long)&vectors) &&
> > +			(frame->pc < (unsigned long)&end_of_vectors)) {
> > +		/*
> > +		 * Expection handler does not use a normal format of
> > +		 * stack frame, but allocates struct pt_regs.
> > +		 */
> > +		frame->sp = frame->sp + S_FRAME_SIZE;
> > +		frame->fp = *(unsigned long *)(fp + S_FP);
> > +		frame->pc = *(unsigned long *)(fp + S_LR);
>
> Not frame->pc = *(unsigned long *)(fp + S_PC)? Don't we need to look up elr_el1
> since this is an exception?

You are right. Will fix it if I submit the next version.

> > +
> > +		return 0;
> > +	}
> > +
> > 	frame->sp = fp + 0x10;
>
> I'm just curious about this constant, 0x10. Do you have an idea on this value?
> As reviewing objdump of vmlinux, it looks needed to analyze the first store-pair
> instruction of each function.
>
> Please correct me if I'm wrong.

I don't know Catalin's intention here, but fp always points to saved pair of
<fp, lr> and so, in general, "fp + 0x10" is the address of succeeding local variables
in callee function. (Remember my acsii art :)
This can be the easily-approximated (but not accurate) stack pointer of caller unless
we decode function prologues.

Thanks,
-Takahiro AKASHI

> Best Regards
> Jungseok Lee
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