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Date: Fri, 13 Jul 2012 14:47:53 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc: linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
Andrew Morton <akpm@...ux-foundation.org>,
Thomas Gleixner <tglx@...utronix.de>,
Frederic Weisbecker <fweisbec@...il.com>,
"H. Peter Anvin" <hpa@...or.com>, yrl.pp-manager.tt@...achi.com
Subject: Re: [RFC][PATCH 2/4 v4] ftrace/x86: Add save_regs for i386 function
calls
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 '®s->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)(®s->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
Note, I added +4 (which is still a normal kprobe, and not a ftrace one)
because it updates the stack where regs->sp would definitely not be a
kernel stack.
This patch fixes it:
diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
index dcfde52..b1e0f53 100644
--- a/arch/x86/include/asm/ptrace.h
+++ b/arch/x86/include/asm/ptrace.h
@@ -246,6 +246,11 @@ static inline unsigned long regs_get_register(struct pt_regs *regs,
{
if (unlikely(offset > MAX_REG_OFFSET))
return 0;
+#ifdef CONFIG_X86_32
+ if (offset == offsetof(struct pt_regs, sp) &&
+ regs->cs == __KERNEL_CS)
+ return kernel_stack_pointer(regs);
+#endif
return *(unsigned long *)((unsigned long)regs + offset);
}
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'll post that patch separately.
> ---
>
> This means that you need a trick here.
>
> sp-> [retaddr]
> (*)-> [orig_stack]
>
> Here is the stack layout when the ftrace_regs_caller is called.
> (*) points the original stack pointer. this means that regs->sp has
> placed at (*). After doing pushf, it changed as below.
>
> (what user expects)
> sp-> [flags] <- regs.cs
> [retaddr] <- regs.flags
> (*)-> [orig_stack] <- regs.sp
>
> So we have to change this stack layout as the user expected. That is
> what I did it in my previous series;
>
> https://lkml.org/lkml/2012/6/5/119
>
> In this patch, I clobbered the return address on the stack and
> stores it in the local stack because of that reason.
>
> + movl 14*4(%esp), %eax /* Load return address */
> + pushl %eax /* Save return address (+4) */
> + subl $MCOUNT_INSN_SIZE, %eax
> + movl %eax, 12*4+4(%esp) /* Store IP */
> + movl 13*4+4(%esp), %edx /* Load flags */
> + movl %edx, 14*4+4(%esp) /* Store flags */
> + movl $__KERNEL_CS, %edx
> + movl %edx, 13*4+4(%esp)
I did something slightly different but basically the same. Here's the
new version:
-- Steve
From: Steven Rostedt <srostedt@...hat.com>
Date: Tue, 5 Jun 2012 20:00:11 -0400
Subject: [PATCH] ftrace/x86: Add save_regs for i386 function calls
Add saving full regs for function tracing on i386.
The saving of regs was influenced by patches sent out by
Masami Hiramatsu.
Cc: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
---
arch/x86/include/asm/ftrace.h | 2 -
arch/x86/kernel/entry_32.S | 68 +++++++++++++++++++++++++++++++++++++++++
arch/x86/kernel/ftrace.c | 4 --
3 files changed, 68 insertions(+), 6 deletions(-)
diff --git a/arch/x86/include/asm/ftrace.h b/arch/x86/include/asm/ftrace.h
index a847501..a6cae0c 100644
--- a/arch/x86/include/asm/ftrace.h
+++ b/arch/x86/include/asm/ftrace.h
@@ -40,10 +40,8 @@
#ifdef CONFIG_DYNAMIC_FTRACE
#define ARCH_SUPPORTS_FTRACE_OPS 1
-#ifdef CONFIG_X86_64
#define ARCH_SUPPORTS_FTRACE_SAVE_REGS
#endif
-#endif
#ifndef __ASSEMBLY__
extern void mcount(void);
diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
index 5da11d1..46caa56 100644
--- a/arch/x86/kernel/entry_32.S
+++ b/arch/x86/kernel/entry_32.S
@@ -1123,6 +1123,7 @@ ftrace_call:
popl %edx
popl %ecx
popl %eax
+ftrace_ret:
#ifdef CONFIG_FUNCTION_GRAPH_TRACER
.globl ftrace_graph_call
ftrace_graph_call:
@@ -1134,6 +1135,73 @@ ftrace_stub:
ret
END(ftrace_caller)
+ENTRY(ftrace_regs_caller)
+ pushf /* push flags before compare (in cs location) */
+ cmpl $0, function_trace_stop
+ jne ftrace_restore_flags
+
+ /*
+ * i386 does not save SS and ESP when coming from kernel.
+ * Instead, to get sp, ®s->sp is used (see ptrace.h).
+ * Unfortunately, that means eflags must be at the same location
+ * as the current return ip is. We move the return ip into the
+ * ip location, and move flags into the return ip location.
+ */
+ pushl 4(%esp) /* save return ip into ip slot */
+ subl $MCOUNT_INSN_SIZE, (%esp) /* Adjust ip */
+
+ pushl $0 /* Load 0 into orig_ax */
+ pushl %gs
+ pushl %fs
+ pushl %es
+ pushl %ds
+ pushl %eax
+ pushl %ebp
+ pushl %edi
+ pushl %esi
+ pushl %edx
+ pushl %ecx
+ pushl %ebx
+
+ movl 13*4(%esp), %eax /* Get the saved flags */
+ movl %eax, 14*4(%esp) /* Move saved flags into regs->flags location */
+ /* clobbering return ip */
+ movl $__KERNEL_CS,13*4(%esp)
+
+ movl 12*4(%esp), %eax /* Load ip (1st parameter) */
+ movl 0x4(%ebp), %edx /* Load parent ip (2cd parameter) */
+ lea (%esp), %ecx
+ pushl %ecx /* Save pt_regs as 4th parameter */
+ leal function_trace_op, %ecx /* Save ftrace_pos in 3rd parameter */
+
+GLOBAL(ftrace_regs_call)
+ call ftrace_stub
+
+ addl $4, %esp /* Skip pt_regs */
+ movl 14*4(%esp), %eax /* Move flags back into cs */
+ movl %eax, 13*4(%esp) /* Needed to keep addl from modifying flags */
+ movl 12*4(%esp), %eax /* Get return ip from regs->ip */
+ addl $MCOUNT_INSN_SIZE, %eax
+ movl %eax, 14*4(%esp) /* Put return ip back for ret */
+
+ popl %ebx
+ popl %ecx
+ popl %edx
+ popl %esi
+ popl %edi
+ popl %ebp
+ popl %eax
+ popl %ds
+ popl %es
+ popl %fs
+ popl %gs
+ addl $8, %esp /* Skip orig_ax and ip */
+ popf /* Pop flags at end (no addl to corrupt flags) */
+ jmp ftrace_ret
+
+ftrace_restore_flags:
+ popf
+ jmp ftrace_stub
#else /* ! CONFIG_DYNAMIC_FTRACE */
ENTRY(mcount)
diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index b90eb1a..1d41402 100644
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -206,7 +206,6 @@ static int
ftrace_modify_code(unsigned long ip, unsigned const char *old_code,
unsigned const char *new_code);
-#ifdef ARCH_SUPPORTS_FTRACE_SAVE_REGS
/*
* Should never be called:
* As it is only called by __ftrace_replace_code() which is called by
@@ -221,7 +220,6 @@ int ftrace_modify_call(struct dyn_ftrace *rec, unsigned long old_addr,
WARN_ON(1);
return -EINVAL;
}
-#endif
int ftrace_update_ftrace_func(ftrace_func_t func)
{
@@ -237,7 +235,6 @@ int ftrace_update_ftrace_func(ftrace_func_t func)
ret = ftrace_modify_code(ip, old, new);
-#ifdef ARCH_SUPPORTS_FTRACE_SAVE_REGS
/* Also update the regs callback function */
if (!ret) {
ip = (unsigned long)(&ftrace_regs_call);
@@ -245,7 +242,6 @@ int ftrace_update_ftrace_func(ftrace_func_t func)
new = ftrace_call_replace(ip, (unsigned long)func);
ret = ftrace_modify_code(ip, old, new);
}
-#endif
atomic_dec(&modifying_ftrace_code);
--
1.7.3.4
--
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