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Date: Thu, 21 Mar 2024 13:08:02 +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 12:57:07PM +0000, David Laight wrote:
> From: Russell King
> > Sent: 21 March 2024 12:23
> ...
> > > 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.
> 
> Actually you can get there with a function that has a lot of args.
> So you can have:
> 	if (...) {
> 		push x
> 		bl func
> 		add %sp, #8
> 	}
> 	code;
> which is fine.

No you can't.... and that isn't even Arm code. Arm doesn't use %sp.
Moreover, that "bl" will stomp over the link register, meaning this
function can not return.

> But if 'func' is 'noreturn' then the 'add %sp, #8' can be discarded
> and then the saved LR is that of 'code' - but the stack offset is wrong.

If func is noreturn, then the remainder of that path isn't expected
to be executed, so anything that happens after the "bl" is irrelevant.

> A PC from LR will always be the next instruction.
> It is only the PC from a fault frame that is the current one.

That sentence makes no sense to me, as I don't think it's even proper
English, so I can't parse it.

> The unwinder probably need to be told which one it has.
> (Or add 4 the fault frame PC so that the unwinder can subtract
> 4 from it.)

That's basically what I said.

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