From: Andy Lutomirski <luto@amacapital.net>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Herbert Xu <herbert@gondor.apana.org.au>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
Ingo Molnar <mingo@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Andy Lutomirski <luto@kernel.org>,
Ard Biesheuvel <ard.biesheuvel@linaro.org>
Subject: Re: x86-64: Maintain 16-byte stack alignment
Date: Thu, 12 Jan 2017 19:23:18 -0800 [thread overview]
Message-ID: <CALCETrUFCFn-rKnr+NG3SU7J78ree9siJC=Kz8f_Bk6eG2HyPA@mail.gmail.com> (raw)
In-Reply-To: <20170113031107.mgitq54fmjnrvi6f@treble>
On Thu, Jan 12, 2017 at 7:11 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> On Thu, Jan 12, 2017 at 05:46:55PM -0800, Andy Lutomirski wrote:
>> On Thu, Jan 12, 2017 at 12:15 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>> > On Thu, Jan 12, 2017 at 12:08:07PM -0800, Andy Lutomirski wrote:
>> >> On Thu, Jan 12, 2017 at 11:51 AM, Linus Torvalds
>> >> <torvalds@linux-foundation.org> wrote:
>> >> > On Thu, Jan 12, 2017 at 6:02 AM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>> >> >>
>> >> >> Just to clarify, I think you're asking if, for versions of gcc which
>> >> >> don't support -mpreferred-stack-boundary=3, objtool can analyze all C
>> >> >> functions to ensure their stacks are 16-byte aligned.
>> >> >>
>> >> >> It's certainly possible, but I don't see how that solves the problem.
>> >> >> The stack will still be misaligned by entry code. Or am I missing
>> >> >> something?
>> >> >
>> >> > I think the argument is that we *could* try to align things, if we
>> >> > just had some tool that actually then verified that we aren't missing
>> >> > anything.
>> >> >
>> >> > I'm not entirely happy with checking the generated code, though,
>> >> > because as Ingo says, you have a 50:50 chance of just getting it right
>> >> > by mistake. So I'd much rather have some static tool that checks
>> >> > things at a code level (ie coccinelle or sparse).
>> >>
>> >> What I meant was checking the entry code to see if it aligns stack
>> >> frames, and good luck getting sparse to do that. Hmm, getting 16-byte
>> >> alignment for real may actually be entirely a lost cause. After all,
>> >> I think we have some inline functions that do asm volatile ("call
>> >> ..."), and I don't see any credible way of forcing alignment short of
>> >> generating an entirely new stack frame and aligning that.
>> >
>> > Actually we already found all such cases and fixed them by forcing a new
>> > stack frame, thanks to objtool. For example, see 55a76b59b5fe.
>>
>> What I mean is: what guarantees that the stack is properly aligned for
>> the subroutine call? gcc promises to set up a stack frame, but does
>> it promise that rsp will be properly aligned to call a C function?
>
> Yes, I did an experiment and you're right. I had naively assumed that
> all stack frames would be aligned.
Just to check: did you do your experiment with -mpreferred-stack-boundary=4?
--Andy
next prev parent reply other threads:[~2017-01-13 3:23 UTC|newest]
Thread overview: 54+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-01-10 14:33 x86-64: Maintain 16-byte stack alignment Herbert Xu
2017-01-10 14:39 ` Herbert Xu
2017-01-10 17:05 ` Linus Torvalds
2017-01-10 17:09 ` Andy Lutomirski
2017-01-11 3:11 ` Herbert Xu
2017-01-11 3:30 ` Linus Torvalds
2017-01-11 4:17 ` Linus Torvalds
2017-01-11 4:35 ` Herbert Xu
2017-01-11 6:01 ` Andy Lutomirski
2017-01-12 6:21 ` Andy Lutomirski
2017-01-12 7:40 ` Ingo Molnar
2017-01-12 14:02 ` Josh Poimboeuf
2017-01-12 19:51 ` Linus Torvalds
2017-01-12 20:08 ` Andy Lutomirski
2017-01-12 20:15 ` Josh Poimboeuf
2017-01-12 20:55 ` Josh Poimboeuf
2017-01-12 21:40 ` Linus Torvalds
2017-01-13 8:38 ` Herbert Xu
2017-01-13 1:46 ` Andy Lutomirski
2017-01-13 3:11 ` Josh Poimboeuf
2017-01-13 3:23 ` Andy Lutomirski [this message]
2017-01-13 4:27 ` Josh Poimboeuf
[not found] ` <CA+55aFzRrSwGxxfZk-RUEnsz=xhcSmOwE1CenfCPBWtsS9MwDw@mail.gmail.com>
2017-01-13 5:07 ` Josh Poimboeuf
2017-01-13 8:43 ` Herbert Xu
2017-01-13 8:42 ` Herbert Xu
2017-01-13 8:39 ` Herbert Xu
2017-01-13 8:36 ` Herbert Xu
2017-01-13 13:07 ` Josh Poimboeuf
[not found] ` <CA+55aFw+Z_ieo6DzTVB6_-TvQ0jj60s=T0mvXfqkBVFdKFPw_Q@mail.gmail.com>
2017-01-11 8:06 ` Ard Biesheuvel
2017-01-11 8:09 ` Herbert Xu
2017-01-11 18:20 ` Andy Lutomirski
2017-01-12 7:05 ` Herbert Xu
2017-01-12 7:46 ` Ingo Molnar
2017-01-12 14:49 ` Josh Poimboeuf
2017-01-12 7:51 ` Andy Lutomirski
2017-01-12 8:04 ` Herbert Xu
2017-01-12 8:18 ` Ingo Molnar
2017-01-12 15:03 ` Josh Poimboeuf
2017-01-12 15:06 ` Herbert Xu
2017-01-12 15:18 ` Josh Poimboeuf
2017-01-12 15:10 ` Josh Poimboeuf
2017-01-10 17:30 ` Ard Biesheuvel
2017-01-10 19:00 ` Andy Lutomirski
2017-01-10 19:16 ` Ard Biesheuvel
2017-01-10 19:22 ` Andy Lutomirski
2017-01-10 20:00 ` Ard Biesheuvel
2017-01-10 23:25 ` Andy Lutomirski
2017-01-11 3:26 ` Herbert Xu
2017-01-11 3:26 ` Herbert Xu
2017-01-11 3:16 ` Herbert Xu
2017-01-11 3:15 ` Herbert Xu
2017-01-12 6:12 ` Herbert Xu
2017-01-12 8:01 ` Ard Biesheuvel
2017-01-12 8:06 ` Herbert Xu
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