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SubjectRe: [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?

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