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
