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Date: Thu, 21 Mar 2024 12:07:51 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Russell King' <linux@...linux.org.uk>, Ard Biesheuvel <ardb@...nel.org>
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Subject: RE: [PATCH v2] ARM: unwind: improve unwinders for noreturn case

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.
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. 

> 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.

For pretty much all code isn't *(LR-4) going to be BL?
On arm that is probably testable.
(It is pretty much impossible to detect a ACLL on x86.)
If it is a direct BL then you'd normally expect to the be
a call the function containing the current 'PC'.
The obvious exception is if there was a tail call, and printing
the called address would then be useful.
(It might help with leaf functions that don't generate a stack frame.)

I remember issues with the solaris sparc backtrace that used to
get confused by leaf functions...

> 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.

It might be enough to depend on whether the address is that of a
fault (where the instruction could be retried) or from a call/trap
instruction where it will be the following instruction.

	David

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