linux-kernel - [RFC][PATCH] x86_32: Return actual stack when requesting sp from regs

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Date:	Fri, 13 Jul 2012 15:44:14 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>, stable <stable@...r.kernel.org>
Subject: [RFC][PATCH] x86_32: Return actual stack when requesting sp from
 regs


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? ]

Signed-off-by: Steven Rostedt <rostedt@...dmis.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);
 }
 


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