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From: Michael Schmitz <schmitzmic@gmail.com>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-m68k@lists.linux-m68k.org,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	Greg Ungerer <gerg@linux-m68k.org>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/3] m68k: leave stack mangling to asm wrapper of sigreturn()
Date: Thu, 16 Sep 2021 12:53:53 +1200	[thread overview]
Message-ID: <48dafad1-4f0c-4ab7-792c-b34a81d26799@gmail.com> (raw)
In-Reply-To: <YUKNn3erTbH+ytpM@zeniv-ca.linux.org.uk>

Hi Al,

On 16/09/21 12:19, Al Viro wrote:
> On Thu, Sep 16, 2021 at 11:35:05AM +1200, Michael Schmitz wrote:
>
>> This one's a little harder - you use a 84 byte gap on each sigreturn, no
>> matter what the frame size we need to restore. The original
>> mangle_kernel_stack() only makes room on the stack when it has no other
>> option (using twice as much size - correct me if I'm wrong).
>>
>> Ideally, we'd only leave a gap for mangle_kernel_stack() to use if the frame
>> size requires us to do so. Working that out in asm glue would be
>> sufficiently convoluted as to cancel out the benefits of cleaning up the C
>> sigreturn part. Probably not worth it.
>
> You'd need to
> 	* load the frame type from sigcontext (and deal with EFAULT, etc.)
> 	* make decision based on that
> 	* pass the type down into sigreturn(), so we wouldn't run into
> mismatches.
>
> And all that just to avoid a single "subtract a constant from stack pointer"
> insn.  We are on a very shallow kernel stack here - it's a syscall entry,
> after all.  And the stack footprint of do_sigreturn() is fairly small - e.g.
> stat(2) eats a lot more.

Thanks, that's what I was wondering. Not worth the extra complexity then.

>
> We are not initializing the gap either - it's just reserved on stack; we only
> access it if we need to enlarge the stack frame.
>
> IOW, what would be the benefit of trying to avoid unconditional gap there?

Avoiding a kernel stack overflow - there are comments in the code that 
warn against that, but those may be largely historic...

Cheers,

	Michael

  reply	other threads:[~2021-09-16  0:55 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-25 17:18 [RFC][CFT] signal handling fixes Al Viro
2021-07-25 17:19 ` [PATCH 1/3] m68k: handle arrivals of multiple signals correctly Al Viro
2021-09-15 22:08   ` Michael Schmitz
2021-07-25 17:19 ` [PATCH 2/3] m68k: update ->thread.esp0 before calling syscall_trace() in ret_from_signal Al Viro
2021-09-15 22:19   ` Michael Schmitz
2021-07-25 17:20 ` [PATCH 3/3] m68k: leave stack mangling to asm wrapper of sigreturn() Al Viro
2021-09-15 23:35   ` Michael Schmitz
2021-09-16  0:19     ` Al Viro
2021-09-16  0:53       ` Michael Schmitz [this message]
2021-09-16  3:21         ` Al Viro
2021-09-16  5:02           ` Michael Schmitz
2021-09-16 16:14             ` Al Viro
2021-07-27 10:21 ` [RFC][CFT] signal handling fixes Finn Thain
2021-07-27 14:42   ` Al Viro
2021-07-28  1:23     ` Finn Thain
2021-08-11  1:42     ` Finn Thain
2021-09-16  9:03 ` Finn Thain
2021-09-23 14:43   ` Geert Uytterhoeven
2021-09-23 14:45 ` Geert Uytterhoeven

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