Re: [RFC][PATCH] x86_32: Return actual stack when requesting sp from regs

[Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>]

(2012/07/14 4:44), Steven Rostedt wrote:
> 
> As x86_32 traps do not save sp when taken in kernel mode, we need to
> accommodate the sp when requesting to get the register.
> 
> This affects kprobes.
> 
> Before:
> 
>  # 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
> 
> After:
>             sshd-1352  [000] d...   362.348016: ftrace: (sys_read+0x4/0x70) s=f3febfa8
>             sshd-1352  [000] d...   362.348048: ftrace: (sys_read+0x4/0x70) s=f3febfa8
>             bash-1355  [001] d...   362.348081: ftrace: (sys_read+0x4/0x70) s=f5075fa8
>             sshd-1352  [000] d...   362.348082: ftrace: (sys_read+0x4/0x70) s=f3febfa8
>             sshd-1352  [000] d...   362.690950: ftrace: (sys_read+0x4/0x70) s=f3febfa8
>             bash-1355  [001] d...   362.691033: ftrace: (sys_read+0x4/0x70) s=f5075fa8
> 
> [ I wonder if this should also go to stable? ]
> 

This obviously makes tracing output better on i386. Original %sp
gives nothing (or just misleading)...

Reviewed-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>

NOTE: if someone needs to get kernel stack address from regs,
I recommend him to use kernel_stack_pointer() directly.

Thank you,

> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
> 
> Index: linux-trace.git/arch/x86/include/asm/ptrace.h
> ===================================================================
> --- linux-trace.git.orig/arch/x86/include/asm/ptrace.h
> +++ linux-trace.git/arch/x86/include/asm/ptrace.h
> @@ -246,6 +246,15 @@ static inline unsigned long regs_get_reg
>  {
>  	if (unlikely(offset > MAX_REG_OFFSET))
>  		return 0;
> +#ifdef CONFIG_X86_32
> +	/*
> +	 * Traps from the kernel do not save sp and ss.
> +	 * Use the helper function to retrieve sp.
> +	 */
> +	if (offset == offsetof(struct pt_regs, sp) &&
> +	    regs->cs == __KERNEL_CS)
> +		return kernel_stack_pointer(regs);
> +#endif
>  	return *(unsigned long *)((unsigned long)regs + offset);
>  }

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