Re: [PATCH] tracing/function-return-tracer: don't trace kfreewhile it frees the return stack

[Ingo Molnar]

* Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote:

> Impact: fix a crash
> 
> While I killed the cat process, I got sometimes the following (but 
> rare) crash:
> 
> [   65.689027] Pid: 2969, comm: cat Not tainted (2.6.28-rc6-tip #83) AMILO Li 2727
> [   65.689027] EIP: 0060:[<00000000>] EFLAGS: 00010082 CPU: 1
> [   65.689027] EIP is at 0x0
> [   65.689027] EAX: 00000000 EBX: f66cd780 ECX: c019a64a EDX: f66cd780
> [   65.689027] ESI: 00000286 EDI: f66cd780 EBP: f630be2c ESP: f630be24
> [   65.689027]  DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0068
> [   65.689027] Process cat (pid: 2969, ti=f630a000 task=f66cd780 task.ti=f630a000)
> [   65.689027] Stack:
> [   65.689027]  00000012 f630bd54 f630be7c c012c853 00000000 c0133cc9 f66cda54 f630be5c
> [   65.689027]  f630be68 f66cda54 f66cd88c f66cd878 f7070000 00000001 f630be90 c0135dbc
> [   65.689027]  f614a614 f630be68 f630be68 f65ba200 00000002 f630bf10 f630be90 c012cad6
> [   65.689027] Call Trace:
> [   65.689027]  [<c012c853>] ? do_exit+0x603/0x850
> [   65.689027]  [<c0133cc9>] ? next_signal+0x9/0x40
> [   65.689027]  [<c0135dbc>] ? dequeue_signal+0x8c/0x180
> [   65.689027]  [<c012cad6>] ? do_group_exit+0x36/0x90
> [   65.689027]  [<c013709c>] ? get_signal_to_deliver+0x20c/0x390
> [   65.689027]  [<c0102b69>] ? do_notify_resume+0x99/0x8b0
> [   65.689027]  [<c02e6d1a>] ? tty_ldisc_deref+0x5a/0x80
> [   65.689027]  [<c014db9b>] ? trace_hardirqs_on+0xb/0x10
> [   65.689027]  [<c02e6d1a>] ? tty_ldisc_deref+0x5a/0x80
> [   65.689027]  [<c02e39b0>] ? n_tty_write+0x0/0x340
> [   65.689027]  [<c02e1812>] ? redirected_tty_write+0x82/0x90
> [   65.689027]  [<c019ee99>] ? vfs_write+0x99/0xd0
> [   65.689027]  [<c02e1790>] ? redirected_tty_write+0x0/0x90
> [   65.689027]  [<c019f342>] ? sys_write+0x42/0x70
> [   65.689027]  [<c01035ca>] ? work_notifysig+0x13/0x19
> [   65.689027] Code:  Bad EIP value.
> [   65.689027] EIP: [<00000000>] 0x0 SS:ESP 0068:f630be24
> 
> This is because on do_exit(), kfree is called to free the return addresses stack
> but kfree is traced and stored its return address in this stack.
> This patch fixes it.
> 
> Signed-off-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 90d99fb..ffff7ec 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -1628,8 +1628,9 @@ void ftrace_retfunc_init_task(struct task_struct *t)
>  
>  void ftrace_retfunc_exit_task(struct task_struct *t)
>  {
> -	kfree(t->ret_stack);
> +	struct ftrace_ret_stack	*ret_stack = t->ret_stack;
>  	t->ret_stack = NULL;
> +	kfree(ret_stack);

heh, nice one :)

note that we also need to keep gcc from reordering things here (no 
matter how unlikely in this particular case).

(also, small detail: we prefer a newline after variable definitions.)

Full commit attached below.

	Ingo

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