From: Michael Schmitz <firstname.lastname@example.org> To: Al Viro <email@example.com> Cc: firstname.lastname@example.org, Geert Uytterhoeven <email@example.com>, Greg Ungerer <firstname.lastname@example.org>, email@example.com Subject: Re: [PATCH 3/3] m68k: leave stack mangling to asm wrapper of sigreturn() Date: Thu, 16 Sep 2021 12:53:53 +1200 [thread overview] Message-ID: <firstname.lastname@example.org> (raw) In-Reply-To: <YUKNn3erTbH+ytpM@zeniv-ca.linux.org.uk> 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
next prev parent reply other threads:[~2021-09-16 0:55 UTC|newest] Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-07-25 17:18 [RFC][CFT] signal handling fixes Al Viro 2021-07-25 17:19 ` [PATCH 1/3] m68k: handle arrivals of multiple signals correctly Al Viro 2021-09-15 22:08 ` Michael Schmitz 2021-07-25 17:19 ` [PATCH 2/3] m68k: update ->thread.esp0 before calling syscall_trace() in ret_from_signal Al Viro 2021-09-15 22:19 ` Michael Schmitz 2021-07-25 17:20 ` [PATCH 3/3] m68k: leave stack mangling to asm wrapper of sigreturn() Al Viro 2021-09-15 23:35 ` Michael Schmitz 2021-09-16 0:19 ` Al Viro 2021-09-16 0:53 ` Michael Schmitz [this message] 2021-09-16 3:21 ` Al Viro 2021-09-16 5:02 ` Michael Schmitz 2021-09-16 16:14 ` Al Viro 2021-07-27 10:21 ` [RFC][CFT] signal handling fixes Finn Thain 2021-07-27 14:42 ` Al Viro 2021-07-28 1:23 ` Finn Thain 2021-08-11 1:42 ` Finn Thain 2021-09-16 9:03 ` Finn Thain 2021-09-23 14:43 ` Geert Uytterhoeven 2021-09-23 14:45 ` Geert Uytterhoeven
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