From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Matthias Kaehlcke <mka@chromium.org>
Cc: Chris J Arges <chris.j.arges@canonical.com>,
Borislav Petkov <bp@suse.de>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, "H . Peter Anvin" <hpa@zytor.com>,
x86@kernel.org, linux-kernel@vger.kernel.org,
Douglas Anderson <dianders@chromium.org>,
Michael Davidson <md@google.com>,
Greg Hackmann <ghackmann@google.com>,
Nick Desaulniers <ndesaulniers@google.com>,
Stephen Hines <srhines@google.com>,
Kees Cook <keescook@chromium.org>, Arnd Bergmann <arnd@arndb.de>,
Bernhard.Rosenkranzer@linaro.org
Subject: Re: [PATCH] Revert "x86/uaccess: Add stack frame output operand in get_user() inline asm"
Date: Thu, 13 Jul 2017 14:25:54 -0500 [thread overview]
Message-ID: <20170713192554.f4xznyxjkdtrmh3f@treble> (raw)
In-Reply-To: <20170713184748.GF95735@google.com>
On Thu, Jul 13, 2017 at 11:47:48AM -0700, Matthias Kaehlcke wrote:
> > What happens if you try the below patch instead of the revert? Any
> > chance the offending instruction goes away?
> >
> > diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> > index 11433f9..beac907 100644
> > --- a/arch/x86/include/asm/uaccess.h
> > +++ b/arch/x86/include/asm/uaccess.h
> > @@ -171,7 +171,7 @@ __typeof__(__builtin_choose_expr(sizeof(x) > sizeof(0UL), 0ULL, 0UL))
> > might_fault(); \
> > asm volatile("call __get_user_%P4" \
> > : "=a" (__ret_gu), "=r" (__val_gu), "+r" (__sp) \
> > - : "0" (ptr), "i" (sizeof(*(ptr)))); \
> > + : "0" (ptr), "i" (sizeof(*(ptr))), "r" (__sp)); \
> > (x) = (__force __typeof__(*(ptr))) __val_gu; \
> > __builtin_expect(__ret_gu, 0); \
> > })
>
> The generated code is basically the same, only that now the value from
> the stack is stored in a register and written twice to RSP:
>
> ffffffff813676ba: 31 c0 xor %eax,%eax
> ffffffff813676bc: 48 89 45 c8 mov %rax,-0x38(%rbp)
> ffffffff813676c0: 45 31 ff xor %r15d,%r15d
> ffffffff813676c3: 48 89 45 a8 mov %rax,-0x58(%rbp)
> ...
> ffffffff81367918: 48 8b 4d a8 mov -0x58(%rbp),%rcx
> ffffffff8136791c: 48 89 cc mov %rcx,%rsp
> ffffffff8136791f: 48 89 cc mov %rcx,%rsp
> ffffffff81367922: e8 69 26 f1 ff callq ffffffff81279f90 <__get_user_4>
LOL. Why corrupt the stack pointer with a single instruction (reading a
zero from memory, no less) when you can instead do it with three
instructions, including two duplicates?
Anyway this seems like a clang bug to me. If I specify RSP as an input
register then the compiler shouldn't overwrite it first. For that
matter it has no reason to overwrite it if it's an output register
either.
--
Josh
next prev parent reply other threads:[~2017-07-13 19:25 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-07-12 21:27 [PATCH] Revert "x86/uaccess: Add stack frame output operand in get_user() inline asm" Matthias Kaehlcke
2017-07-12 22:12 ` Josh Poimboeuf
2017-07-12 22:20 ` Matthias Kaehlcke
2017-07-12 22:35 ` Josh Poimboeuf
2017-07-12 22:36 ` Josh Poimboeuf
2017-07-12 23:22 ` Matthias Kaehlcke
2017-07-13 18:00 ` Josh Poimboeuf
2017-07-13 18:47 ` Matthias Kaehlcke
2017-07-13 19:25 ` Josh Poimboeuf [this message]
2017-07-13 19:38 ` Michael Davidson
2017-07-13 20:18 ` Josh Poimboeuf
2017-07-13 20:20 ` Andrey Rybainin
2017-07-13 20:34 ` Josh Poimboeuf
2017-07-13 21:12 ` Matthias Kaehlcke
2017-07-13 21:34 ` Josh Poimboeuf
2017-07-13 21:57 ` Matthias Kaehlcke
2017-07-19 17:46 ` Josh Poimboeuf
2017-07-19 21:50 ` Matthias Kaehlcke
2017-07-20 10:01 ` Andrey Ryabinin
2017-07-20 15:18 ` Josh Poimboeuf
2017-07-20 15:30 ` Andrey Ryabinin
2017-07-20 20:56 ` Josh Poimboeuf
2017-07-21 9:13 ` Andrey Ryabinin
2017-07-21 13:24 ` Josh Poimboeuf
2017-07-29 0:38 ` Matthias Kaehlcke
2017-07-29 0:55 ` Josh Poimboeuf
2017-07-29 0:58 ` Josh Poimboeuf
2017-07-29 1:06 ` Matthias Kaehlcke
2017-07-13 21:14 ` Matthias Kaehlcke
2017-07-13 21:25 ` Andrey Rybainin
2017-07-13 21:43 ` Matthias Kaehlcke
2017-07-13 21:52 ` Josh Poimboeuf
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