linux-kernel - Re: [PATCH 3/3] m68k: leave stack mangling to asm wrapper of sigreturn()

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Date:   Thu, 16 Sep 2021 00:19:43 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Michael Schmitz <schmitzmic@...il.com>
Cc:     linux-m68k@...ts.linux-m68k.org,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Greg Ungerer <gerg@...ux-m68k.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] m68k: leave stack mangling to asm wrapper of
 sigreturn()

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.

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?