linux-kernel - Re: [RFC] de-asmify the x86-64 system call slowpath

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Message-ID: <CA+55aFzWRjNC4kPeVad3isOOW+vZhnzX0Gk5_8QzD16GZEw5Kg@mail.gmail.com>
Date:	Mon, 27 Jan 2014 14:17:23 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Al Viro <viro@...iv.linux.org.uk>, Peter Anvin <hpa@...or.com>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] de-asmify the x86-64 system call slowpath

On Mon, Jan 27, 2014 at 2:06 PM, Andy Lutomirski <luto@...capital.net> wrote:
>
> It's not just ip and sp -- cs matters here, too, I think.

For signal *delivery*, CS will always be __USER_CS, and %rcx can be
crap, so sysret should be fine. We could easily check that %rip is
valid in the whole slow-path instead of saying "return 1 if we did
do_signal()".

Now, it's a different thing wrt signal handler *return*, because at
that point we really cannot return with some random value in %rcx. We
absolutely do need to use 'iretq' in that whole [rt_]sigreturn() path,
but on x86-64 that is all handled by the system call itself (see the
stub_*_sigreturn stuff in entry_64.S) and it very much uses iret
explicitly (the 32-bit case also does that, by forcing the sigreturn
to be done with an "int 0x80" instruction - we could change that to
use syscall+iret, but frankly, I'm not all that inclined to care,
although it might be worth trying to do just to unify the models a
bit).

                   Linus
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