Re: [RFC][PATCH 0/3] ftrace: Add dynamically allocated trampolines

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

(2014/07/11 23:29), Josh Poimboeuf wrote:
[...]
> 
>>From 951d2aec17885a62905df6b910dc705d99c63993 Mon Sep 17 00:00:00 2001
> From: Josh Poimboeuf <jpoimboe@redhat.com>
> Date: Fri, 11 Jul 2014 08:58:33 -0500
> Subject: [PATCH] x86/dumpstack: fix stack traces for generated code
> 
> If a function in the stack trace is dynamically generated, for example an
> ftrace dynamically generated trampoline, print_context_stack() gets confused
> and ends up showing all the following addresses as unreliable:
> 
>   [  934.546013]  [<ffffffff81700312>] dump_stack+0x45/0x56
>   [  934.546020]  [<ffffffff8125f5b0>] ? meminfo_proc_open+0x30/0x30
>   [  934.546027]  [<ffffffffa080a494>] kpatch_ftrace_handler+0x14/0xf0 [kpatch]
>   [  934.546058]  [<ffffffff812143ae>] ? seq_read+0x2de/0x3b0
>   [  934.546062]  [<ffffffff812143ae>] ? seq_read+0x2de/0x3b0
>   [  934.546067]  [<ffffffff8125f5b5>] ? meminfo_proc_show+0x5/0x5e0
>   [  934.546071]  [<ffffffff8125f5b5>] ? meminfo_proc_show+0x5/0x5e0
>   [  934.546075]  [<ffffffff8121423a>] ? seq_read+0x16a/0x3b0
>   [  934.546081]  [<ffffffff8125768d>] ? proc_reg_read+0x3d/0x80
>   [  934.546088]  [<ffffffff811f0668>] ? vfs_read+0x98/0x170
>   [  934.546093]  [<ffffffff811f1345>] ? SyS_read+0x55/0xd0
>   [  934.546099]  [<ffffffff81707969>] ? system_call_fastpath+0x16/0x1b
> 
> Once it encounters an address which is not in the kernel's text area, it gets
> confused and stops updating the frame pointer.

Right, this uses a module_alloc to get a memory for trampline, but
it just allocates a page in executable vmalloc area. We need a hack
in __kernel_text_address if we really want to use that.

> The __kernel_text_address() check isn't needed when determining whether an
> address is reliable.  It's only needed when deciding whether to print an
> unreliable address.

Yeah, I guess that is for the case that the frame pointer is broken.

> 
> Here's the same stack trace with this patch:
> 
>   [ 1314.612287]  [<ffffffff81700312>] dump_stack+0x45/0x56
>   [ 1314.612290]  [<ffffffff8125f5b0>] ? meminfo_proc_open+0x30/0x30
>   [ 1314.612293]  [<ffffffffa080a494>] kpatch_ftrace_handler+0x14/0xf0 [kpatch]
>   [ 1314.612306]  [<ffffffffa00160c4>] 0xffffffffa00160c3

Here, this still has a wrong entry. Maybe the trampline doesn't setup
frame pointer (bp) correctly.

Thank you,

>   [ 1314.612309]  [<ffffffff812143ae>] ? seq_read+0x2de/0x3b0
>   [ 1314.612311]  [<ffffffff812143ae>] ? seq_read+0x2de/0x3b0
>   [ 1314.612312]  [<ffffffff8125f5b5>] ? meminfo_proc_show+0x5/0x5e0
>   [ 1314.612314]  [<ffffffff8125f5b5>] ? meminfo_proc_show+0x5/0x5e0
>   [ 1314.612315]  [<ffffffff8121423a>] ? seq_read+0x16a/0x3b0
>   [ 1314.612318]  [<ffffffff8125768d>] proc_reg_read+0x3d/0x80
>   [ 1314.612320]  [<ffffffff811f0668>] vfs_read+0x98/0x170
>   [ 1314.612322]  [<ffffffff811f1345>] SyS_read+0x55/0xd0
>   [ 1314.612324]  [<ffffffff81707969>] system_call_fastpath+0x16/0x1b
> ---
>  arch/x86/kernel/dumpstack.c | 15 +++++++--------
>  1 file changed, 7 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
> index b74ebc7..db0a33c 100644
> --- a/arch/x86/kernel/dumpstack.c
> +++ b/arch/x86/kernel/dumpstack.c
> @@ -102,14 +102,13 @@ print_context_stack(struct thread_info *tinfo,
>  		unsigned long addr;
>  
>  		addr = *stack;
> -		if (__kernel_text_address(addr)) {
> -			if ((unsigned long) stack == bp + sizeof(long)) {
> -				ops->address(data, addr, 1);
> -				frame = frame->next_frame;
> -				bp = (unsigned long) frame;
> -			} else {
> -				ops->address(data, addr, 0);
> -			}
> +		if ((unsigned long) stack == bp + sizeof(long)) {
> +			ops->address(data, addr, 1);
> +			frame = frame->next_frame;
> +			bp = (unsigned long) frame;
> +			print_ftrace_graph_addr(addr, data, ops, tinfo, graph);
> +		} else if (__kernel_text_address(addr)) {
> +			ops->address(data, addr, 0);
>  			print_ftrace_graph_addr(addr, data, ops, tinfo, graph);
>  		}
>  		stack++;
> 


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