Subject: [PATCH] arm64: ftrace: fix function_graph tracer panic

From: [email-masked] (AKASHI Takahiro)
Date: Thu, 1 Oct 2015 15:11:29 +0900

On 09/30/2015 11:49 AM, Li Bin wrote:
> When function graph tracer is enabled, the following operation
> will trigger panic:
>
> mount -t debugfs nodev /sys/kernel
> echo next_tgid > /sys/kernel/tracing/set_ftrace_filter
> echo function_graph > /sys/kernel/tracing/current_tracer
> ls /proc/
>
> ------------[ cut here ]------------
> [  198.501417] Unable to handle kernel paging request at virtual address cb88537fdc8ba316
> [  198.506126] pgd = ffffffc008f79000
> [  198.509363] [cb88537fdc8ba316] *pgd=00000000488c6003, *pud=00000000488c6003, *pmd=0000000000000000
> [  198.517726] Internal error: Oops: 94000005 [#1] SMP
> [  198.518798] Modules linked in:
> [  198.520582] CPU: 1 PID: 1388 Comm: ls Tainted: G
> [  198.521800] Hardware name: linux,dummy-virt (DT)
> [  198.522852] task: ffffffc0fa9e8000 ti: ffffffc0f9ab0000 task.ti: ffffffc0f9ab0000
> [  198.524306] PC is at next_tgid+0x30/0x100
> [  198.525205] LR is at return_to_handler+0x0/0x20
> [  198.526090] pc : [<ffffffc0002a1070>] lr : [<ffffffc0000907c0>] pstate: 60000145
> [  198.527392] sp : ffffffc0f9ab3d40
> [  198.528084] x29: ffffffc0f9ab3d40 x28: ffffffc0f9ab0000
> [  198.529406] x27: ffffffc000d6a000 x26: ffffffc000b786e8
> [  198.530659] x25: ffffffc0002a1900 x24: ffffffc0faf16c00
> [  198.531942] x23: ffffffc0f9ab3ea0 x22: 0000000000000002
> [  198.533202] x21: ffffffc000d85050 x20: 0000000000000002
> [  198.534446] x19: 0000000000000002 x18: 0000000000000000
> [  198.535719] x17: 000000000049fa08 x16: ffffffc000242efc
> [  198.537030] x15: 0000007fa472b54c x14: ffffffffff000000
> [  198.538347] x13: ffffffc0fada84a0 x12: 0000000000000001
> [  198.539634] x11: ffffffc0f9ab3d70 x10: ffffffc0f9ab3d70
> [  198.540915] x9 : ffffffc0000907c0 x8 : ffffffc0f9ab3d40
> [  198.542215] x7 : 0000002e330f08f0 x6 : 0000000000000015
> [  198.543508] x5 : 0000000000000f08 x4 : ffffffc0f9835ec0
> [  198.544792] x3 : cb88537fdc8ba316 x2 : cb88537fdc8ba306
> [  198.546108] x1 : 0000000000000002 x0 : ffffffc000d85050
> [  198.547432]
> [  198.547920] Process ls (pid: 1388, stack limit = 0xffffffc0f9ab0020)
> [  198.549170] Stack: (0xffffffc0f9ab3d40 to 0xffffffc0f9ab4000)
> [  198.582568] Call trace:
> [  198.583313] [<ffffffc0002a1070>] next_tgid+0x30/0x100
> [  198.584359] [<ffffffc0000907bc>] ftrace_graph_caller+0x6c/0x70
> [  198.585503] [<ffffffc0000907bc>] ftrace_graph_caller+0x6c/0x70
> [  198.586574] [<ffffffc0000907bc>] ftrace_graph_caller+0x6c/0x70
> [  198.587660] [<ffffffc0000907bc>] ftrace_graph_caller+0x6c/0x70
> [  198.588896] Code: aa0003f5 2a0103f4 b4000102 91004043 (885f7c60)
> [  198.591092] ---[ end trace 6a346f8f20949ac8 ]---
>
> This is because when using function graph tracer, if the traced
> function return value is in multi regs ([0x-07]), return_to_handler
> may corrupt them. So in return_to_handler, the parameter regs should
> be protected properly.

You're right. we should preserve x0-x7 around a call to ftrace_return_to_handler()
just in case they might be used as a "composite type" (ie. struct) of return value.

Thanks,
-Takahiro AKASHI

> Cc: <[email-masked]> # 3.18+
> Signed-off-by: Li Bin <[email-masked]>
> ---
>   arch/arm64/kernel/entry-ftrace.S |   22 ++++++++++++++++++++--
>   1 files changed, 20 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/kernel/entry-ftrace.S b/arch/arm64/kernel/entry-ftrace.S
> index 08cafc5..0f03a8f 100644
> --- a/arch/arm64/kernel/entry-ftrace.S
> +++ b/arch/arm64/kernel/entry-ftrace.S
> @@ -178,6 +178,24 @@ ENTRY(ftrace_stub)
>   ENDPROC(ftrace_stub)
>
>   #ifdef CONFIG_FUNCTION_GRAPH_TRACER
> +	/* save return value regs*/
> +	.macro save_return_regs
> +	sub sp, sp, #64
> +	stp x0, x1, [sp]
> +	stp x2, x3, [sp, #16]
> +	stp x4, x5, [sp, #32]
> +	stp x6, x7, [sp, #48]
> +	.endm
> +
> +	/* restore return value regs*/
> +	.macro restore_return_regs
> +	ldp x0, x1, [sp]
> +	ldp x2, x3, [sp, #16]
> +	ldp x4, x5, [sp, #32]
> +	ldp x6, x7, [sp, #48]
> +	add sp, sp, #64
> +	.endm
> +
>   /*
>    * void ftrace_graph_caller(void)
>    *
> @@ -204,11 +222,11 @@ ENDPROC(ftrace_graph_caller)
>    * only when CONFIG_HAVE_FUNCTION_GRAPH_FP_TEST is enabled.
>    */
>   ENTRY(return_to_handler)
> -	str	x0, [sp, #-16]!
> +	save_return_regs
>   	mov	x0, x29			//     parent's fp
>   	bl	ftrace_return_to_handler// addr = ftrace_return_to_hander(fp);
>   	mov	x30, x0			// restore the original return address
> -	ldr	x0, [sp], #16
> +	restore_return_regs
>   	ret
>   END(return_to_handler)
>   #endif /* CONFIG_FUNCTION_GRAPH_TRACER */
>