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Date: Thu, 21 Mar 2024 12:22:58 +0000
From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: David Laight <David.Laight@aculab.com>
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Subject: Re: [PATCH v2] ARM: unwind: improve unwinders for noreturn case
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Sender: Russell King (Oracle) <linux@armlinux.org.uk>

On Thu, Mar 21, 2024 at 12:07:51PM +0000, David Laight wrote:
> From: Russell King
> > Sent: 21 March 2024 11:24
> > 
> > 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.
> 
> I've also seen the compiler defer generating a stack frame until
> after an initial conditional.

. which is why we pass -mno-sched-prolog to GCC.

> That might mean you can get the BL in the middle of a function
> but where the following instruction is for the 'no stack frame'
> side of the branch.
> That is very likely to break any stack offset calculations. 

No it can't. At any one point in the function, the stack has to be in
a well defined state, so that access to local variables can work, and
also the stack can be correctly unwound. If there exists a point in
the function body which can be reached where the stack could be in two
different states, then the stack can't be restored to the parent
context.

> > 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.
> 
> Isn't this already in the unwinder?
> A BL itself isn't going to fault with PC = next-instruction.

You are missing the fact that the PC can be the saved LR, and thus
can very well be the next instruction.

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