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Date: Thu, 21 Mar 2024 14:56:09 +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
Message-ID: <ZfxKiQuNM/zruxSd@shell.armlinux.org.uk>
References: <84a57ca8-8963-ca24-8bd1-ddc5c33bf4da@huawei.com>
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Sender: Russell King (Oracle) <linux@armlinux.org.uk>

On Thu, Mar 21, 2024 at 02:37:28PM +0000, David Laight wrote:
> From: Russell King
> > Sent: 21 March 2024 13:08
> > 
> > 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.
> 
> With 9+ arguments they spill to see https://godbolt.org/z/Yj3ovd8bY
> 
> Where the compiler generates:
> f9:
>         cmp     w0, 0
>         ble     .L2
>         sub     sp, sp, #32
>         mov     w7, w0
>         mov     w6, w0
>         mov     w5, w0
>         mov     w4, w0
>         mov     w3, w0
>         stp     x29, x30, [sp, 16]
>         add     x29, sp, 16
>         mov     w2, w0
>         mov     w1, w0
>         str     w0, [sp]
>         bl      f
> .L2:
>         ret

Don't show me Arm64 assembly when we're discussing Arm32.

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