Messages in this thread | | | Subject | Re: [PATCH 3/3] m68k: leave stack mangling to asm wrapper of sigreturn() | From | Michael Schmitz <> | Date | Thu, 16 Sep 2021 12:53:53 +1200 |
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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
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