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

From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: David Laight <David.Laight@aculab.com>, Ard Biesheuvel <ardb@kernel.org>
Cc: 'Jiangfeng Xiao' <xiaojiangfeng@huawei.com>,
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Subject: Re: [PATCH v2] ARM: unwind: improve unwinders for noreturn case
Date: Thu, 21 Mar 2024 11:23:51 +0000	[thread overview]
Message-ID: <ZfwYx/8k8FVONg6+@shell.armlinux.org.uk> (raw)
In-Reply-To: <bbcca1e205384cf0b42236e17f3969f7@AcuMS.aculab.com>

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.

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