Messages in this thread | | | Date | Thu, 16 Sep 2021 00:19:43 +0000 | From | Al Viro <> | Subject | Re: [PATCH 3/3] m68k: leave stack mangling to asm wrapper of sigreturn() |
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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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