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Date: Wed, 6 Mar 2024 08:02:48 -0800
From: Josh Poimboeuf <jpoimboe@kernel.org>
To: "Russell King (Oracle)" <linux@armlinux.org.uk>
Cc: Jiangfeng Xiao <xiaojiangfeng@huawei.com>,
	Kees Cook <keescook@chromium.org>, Jann Horn <jannh@google.com>,
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	douzhaolei@huawei.com, linux-arm-kernel@lists.infradead.org,
	Ard Biesheuvel <ardb@kernel.org>
Subject: Re: [PATCH] usercopy: delete __noreturn from usercopy_abort
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On Wed, Mar 06, 2024 at 09:52:01AM +0000, Russell King (Oracle) wrote:
> On Tue, Mar 05, 2024 at 09:58:46AM -0800, Josh Poimboeuf wrote:
> > This is an off-by-one bug which is common in unwinders, due to the fact
> > that the address on the stack points to the return address rather than
> > the call address.
> > 
> > So, for example, when the last instruction of a function is a function
> > call (e.g., to a noreturn function), it can cause the unwinder to
> > incorrectly try to unwind from the function *after* the callee.
> 
> I suppose this can only happen in __noreturn functions because that
> can be:
> 
> foo:
> ...
> 	bl	bar
> ... end of function and thus next function ...
> 
> which results in LR pointing into the next function.
> 
> Would it make better sense to lookup the LR value winding it back by
> one instruction like ORC on x86 does (as you mention) rather than
> the patch you proposed which looks rather large and complicated?

That patch *is* an attempt to make it match ORC's behavior.  What
specifically looks complicated about it?

-- 
Josh