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Date: Mon, 19 Feb 2024 07:12:16 -0600
From: Rob Landley <rob@...dley.net>
To: Rich Felker <dalias@...c.org>, Valery Ushakov <uwe@...err.spb.ru>
Cc: musl@...ts.openwall.com, toybox <toybox@...ts.landley.net>
Subject: Re: Re: Not sure how to debug this one.
On 2/17/24 19:34, Rich Felker wrote:
> On Sun, Feb 18, 2024 at 12:45:35AM +0300, Valery Ushakov wrote:
>> On Fri, Feb 16, 2024 at 19:48:27 -0600, Rob Landley wrote:
>>
>> > https://git.musl-libc.org/cgit/musl/tree/src/signal/sh/sigsetjmp.s
FYI the next round of debugging I had queued up was cutting and pasting the
sigsetjmp+setjmp assembly blob inline in the code (either as a C asm { }
directive or my own .s->.o nailed to the side of the link) and removing one
instruction from the end of the testfunc() at a time until the crash went away.
(Doesn't matter if it _works_, the point is does it _return_. Find the
instruction that poked the bear.)
I was just AFK most of the weekend...
> The only problem with the sh implementation is that, due to limited
> displacement range for load/store instructions, we have to add 60 to
> the sigjmp_buf base address to get the base register for accessing the
> saved return address and r8. This takes place in a temp register r6.
> However, the last line of this code path, the delay slot after the
> tail call to __sigsetjmp_tail, erroneously uses r4 (the base
> sigjmp_buf pointer) as the base for restoring r8:
>
> mov.l @(4+8,r4), r8
>
> This corrupts the caller's saved register file. If I'm not mistaken,
> it overwrites the caller's r8 with the caller's r11.
>
> Presumably Rob didn't actually hit a crash in sigsetjmp, but just
> inferred the crash was there via printf debugging. The actual crash
> must be in the caller once it gets back control with the wrong value
> in r8.
I saw that the printf after the sigsetjmp did not trigger, yes. Why was the
pending question...
> This has been an area I've wanted to have testing for for a long time,
> but I don't have a really good idea for how to implement the test. We
> would want the compiler to generate code that puts lots of
> intermediates in as many call-saved registers as possible, calls
> setjmp, then calls another function that *also* puts pressure on the
> register allocation and then calls longjmp. If it's okay for the test
> to be arch-specific, this could just work via __asm__ register
> constraints and a call to setjmp from asm, but I'm not sure if that
> would be a good way to do things in libc-test...
>
> In any case, thanks to everyone who participated in this. This is a
> rather bad bug and a nice one to get fixed up in the release window!
Very much so. Thank you.
I pulled musl-git but didn't see a commit yet, lemme know when I can build a new
test toolchain to confirm the fix.
> Rich
Thanks,
Rob
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