linux-kernel - Re: [PATCH] x86/unwind/orc: Fix unwind for newly forked tasks

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Message-ID: <20240913231108.a3ukhoyhp2xactdq@treble>
Date: Fri, 13 Sep 2024 16:11:08 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Zheng Yejian <zhengyejian@...weicloud.com>
Cc: peterz@...radead.org, tglx@...utronix.de, mingo@...hat.com,
	bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com,
	brgerst@...il.com, samitolvanen@...gle.com, kees@...nel.org,
	x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/unwind/orc: Fix unwind for newly forked tasks

On Fri, Sep 13, 2024 at 10:45:01AM +0800, Zheng Yejian wrote:
> When arch_stack_walk_reliable() is called to unwind for newly forked
> tasks, the return value is negative which means the call stack is
> unreliable. This obviously does not meet expectations.
> 
> The root cause is that after commit 3aec4ecb3d1f ("x86: Rewrite
>  ret_from_fork() in C"), the 'ret_addr' of newly forked task is changed
> to 'ret_from_fork_asm' (see copy_thread()), then at the start of the
> unwind, it is incorrectly interprets not as a "signal" one because
> 'ret_from_fork' is still used to determine the initial "signal" (see
> __unwind_start()). Then the address gets incorrectly decremented in the
> call to orc_find() (see unwind_next_frame()) and resulting in the
> incorrect ORC data.
> 
> To fix it, check 'ret_from_fork_asm' rather than 'ret_from_fork' in
> __unwind_start().
> 
> Fixes: 3aec4ecb3d1f ("x86: Rewrite ret_from_fork() in C")
> Signed-off-by: Zheng Yejian <zhengyejian@...weicloud.com>

Thanks!

Acked-by: Josh Poimboeuf <jpoimboe@...nel.org>

-- 
Josh