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SubjectRe: Re: [RFC][PATCH 2/4 v4] ftrace/x86: Add save_regs for i386 function calls
(2012/07/14 3:47), Steven Rostedt wrote:
> On Thu, 2012-07-12 at 21:39 +0900, Masami Hiramatsu wrote:
>
>> /*
>> * X86_32 CPUs don't save ss and esp if the CPU is already in kernel mode
>> * when it traps. The previous stack will be directly underneath the saved
>> * registers, and 'sp/ss' won't even have been saved. Thus the '&regs->sp'.
>> *
>> * This is valid only for kernel mode traps.
>> */
>> static inline unsigned long kernel_stack_pointer(struct pt_regs *regs)
>> {
>> #ifdef CONFIG_X86_32
>> return (unsigned long)(&regs->sp);
>> #else
>> return regs->sp;
>> #endif
>> }
>
> I found that regs_get_register() doesn't honor this either. Thus,
> kprobes in tracing gets this:
>
> # echo 'p:ftrace sys_read+4 s=%sp' > /debug/tracing/kprobe_events
> # echo 1 > /debug/tracing/events/kprobes/enable
> # cat trace
> sshd-1345 [000] d... 489.117168: ftrace: (sys_read+0x4/0x70) s=b7e96768
> sshd-1345 [000] d... 489.117191: ftrace: (sys_read+0x4/0x70) s=b7e96768
> cat-1447 [000] d... 489.117392: ftrace: (sys_read+0x4/0x70) s=5a7
> cat-1447 [001] d... 489.118023: ftrace: (sys_read+0x4/0x70) s=b77ad05f
> less-1448 [000] d... 489.118079: ftrace: (sys_read+0x4/0x70) s=b7762e06
> less-1448 [000] d... 489.118117: ftrace: (sys_read+0x4/0x70) s=b7764970
>

Yes, that is by design, since I made it so. :)
Instead of %sp, kprobe tracer provides $stack special argument
for stack address, because "sp" is not always means the stack
address on every arch.

Thanks,

--
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@hitachi.com




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