linux-hardening - Re: [PATCH v2] ARM: unwind: improve unwinders for noreturn case

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Date: Thu, 21 Mar 2024 11:23:51 +0000
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: David Laight <David.Laight@...lab.com>,
	Ard Biesheuvel <ardb@...nel.org>
Cc: 'Jiangfeng Xiao' <xiaojiangfeng@...wei.com>,
	"arnd@...db.de" <arnd@...db.de>,
	"keescook@...omium.org" <keescook@...omium.org>,
	"haibo.li@...iatek.com" <haibo.li@...iatek.com>,
	"angelogioacchino.delregno@...labora.com" <angelogioacchino.delregno@...labora.com>,
	"amergnat@...libre.com" <amergnat@...libre.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
	"douzhaolei@...wei.com" <douzhaolei@...wei.com>,
	"gustavoars@...nel.org" <gustavoars@...nel.org>,
	"jpoimboe@...nel.org" <jpoimboe@...nel.org>,
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	"kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
	"linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
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	"peterz@...radead.org" <peterz@...radead.org>,
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	"jannh@...gle.com" <jannh@...gle.com>,
	"willy@...radead.org" <willy@...radead.org>
Subject: Re: [PATCH v2] ARM: unwind: improve unwinders for noreturn case

On Thu, Mar 21, 2024 at 10:22:30AM +0000, David Laight wrote:
> How aggressively does the compiler optimise 'noreturn' functions?

I've seen cases where the compiler emits a BL instruction as the very
last thing in the function, and nothing after it.

This is where the problem lies - because the link register value
created by the BL instruction will point to the instruction after the
BL which will _not_ part of the function that invoked the BL. That
will probably cause issues for the ELF unwinder, which means this
issue probably goes beyond _just_ printing the function name.

I have vague memories that Ard has been involved in the unwinder,
maybe he could comment on this problem? Maybe we need the unwinder
itself to do the correction? I also wonder whether we should only
do the correction if we detect that we're pointing at the first
instruction of a function, and the previous instruction in the
text segment was a BL.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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