From: Segher Boessenkool <segher@kernel.crashing.org>
To: David Laight <David.Laight@aculab.com>
Cc: "'Christophe Leroy'" <christophe.leroy@csgroup.eu>,
"'Linus Torvalds'" <torvalds@linux-foundation.org>,
linux-arch <linux-arch@vger.kernel.org>,
Kees Cook <keescook@chromium.org>,
the arch/x86 maintainers <x86@kernel.org>,
Nick Desaulniers <ndesaulniers@google.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Alexey Dobriyan <adobriyan@gmail.com>,
Luis Chamberlain <mcgrof@kernel.org>,
Al Viro <viro@zeniv.linux.org.uk>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
Christoph Hellwig <hch@lst.de>
Subject: Re: remove the last set_fs() in common code, and remove it for x86 and powerpc v3
Date: Thu, 10 Sep 2020 10:20:30 -0500 [thread overview]
Message-ID: <20200910152030.GJ28786@gate.crashing.org> (raw)
In-Reply-To: <5050b43687c84515a49b345174a98822@AcuMS.aculab.com>
On Thu, Sep 10, 2020 at 12:26:53PM +0000, David Laight wrote:
> Actually this is pretty sound:
> __label__ label;
> register int eax asm ("eax");
> // Ensure eax can't be reloaded from anywhere
> // In particular it can't be reloaded after the asm goto line
> asm volatile ("" : "=r" (eax));
This asm is fine. It says it writes the "eax" variable, which lives in
the eax register *in that asm* (so *not* guaranteed after it!).
> // Provided gcc doesn't save eax here...
> asm volatile goto ("xxxxx" ::: "eax" : label);
So this is incorrect.
> // ... and reload the saved value here.
> // The input value here will be that modified by the 'asm goto'.
> // Since this modifies eax it can't be moved before the 'asm goto'.
> asm volatile ("" : "+r" (eax));
> // So here eax must contain the value set by the "xxxxx" instructions.
No, the register eax will contain the value of the eax variable. In the
asm; it might well be there before or after the asm as well, but none of
that is guaranteed.
Segher
WARNING: multiple messages have this Message-ID (diff)
From: Segher Boessenkool <segher@kernel.crashing.org>
To: David Laight <David.Laight@aculab.com>
Cc: linux-arch <linux-arch@vger.kernel.org>,
Kees Cook <keescook@chromium.org>,
linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
the arch/x86 maintainers <x86@kernel.org>,
Nick Desaulniers <ndesaulniers@google.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Christoph Hellwig <hch@lst.de>,
Luis Chamberlain <mcgrof@kernel.org>,
Al Viro <viro@zeniv.linux.org.uk>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
'Linus Torvalds' <torvalds@linux-foundation.org>,
Alexey Dobriyan <adobriyan@gmail.com>
Subject: Re: remove the last set_fs() in common code, and remove it for x86 and powerpc v3
Date: Thu, 10 Sep 2020 10:20:30 -0500 [thread overview]
Message-ID: <20200910152030.GJ28786@gate.crashing.org> (raw)
In-Reply-To: <5050b43687c84515a49b345174a98822@AcuMS.aculab.com>
On Thu, Sep 10, 2020 at 12:26:53PM +0000, David Laight wrote:
> Actually this is pretty sound:
> __label__ label;
> register int eax asm ("eax");
> // Ensure eax can't be reloaded from anywhere
> // In particular it can't be reloaded after the asm goto line
> asm volatile ("" : "=r" (eax));
This asm is fine. It says it writes the "eax" variable, which lives in
the eax register *in that asm* (so *not* guaranteed after it!).
> // Provided gcc doesn't save eax here...
> asm volatile goto ("xxxxx" ::: "eax" : label);
So this is incorrect.
> // ... and reload the saved value here.
> // The input value here will be that modified by the 'asm goto'.
> // Since this modifies eax it can't be moved before the 'asm goto'.
> asm volatile ("" : "+r" (eax));
> // So here eax must contain the value set by the "xxxxx" instructions.
No, the register eax will contain the value of the eax variable. In the
asm; it might well be there before or after the asm as well, but none of
that is guaranteed.
Segher
next prev parent reply other threads:[~2020-09-10 19:22 UTC|newest]
Thread overview: 115+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-03 14:22 remove the last set_fs() in common code, and remove it for x86 and powerpc v3 Christoph Hellwig
2020-09-03 14:22 ` Christoph Hellwig
2020-09-03 14:22 ` [PATCH 01/14] proc: remove a level of indentation in proc_get_inode Christoph Hellwig
2020-09-03 14:22 ` Christoph Hellwig
2020-09-03 14:22 ` [PATCH 02/14] proc: cleanup the compat vs no compat file ops Christoph Hellwig
2020-09-03 14:22 ` Christoph Hellwig
2020-09-03 14:22 ` [PATCH 03/14] proc: add a read_iter method to proc proc_ops Christoph Hellwig
2020-09-03 14:22 ` Christoph Hellwig
2020-09-03 14:22 ` [PATCH 04/14] sysctl: Convert to iter interfaces Christoph Hellwig
2020-09-03 14:22 ` Christoph Hellwig
2020-09-03 14:22 ` [PATCH 05/14] fs: don't allow kernel reads and writes without iter ops Christoph Hellwig
2020-09-03 14:22 ` Christoph Hellwig
2020-10-01 22:38 ` Eric Biggers
2020-10-01 22:38 ` Eric Biggers
2020-10-01 22:40 ` Al Viro
2020-10-01 22:40 ` Al Viro
2020-10-02 16:27 ` Linus Torvalds
2020-10-02 16:27 ` Linus Torvalds
2020-10-09 22:06 ` Eric Biggers
2020-10-09 22:06 ` Eric Biggers
2020-10-10 1:03 ` Linus Torvalds
2020-10-10 1:03 ` Linus Torvalds
2020-10-10 1:19 ` Eric Biggers
2020-10-10 1:19 ` Eric Biggers
2020-10-10 1:29 ` Linus Torvalds
2020-10-10 1:29 ` Linus Torvalds
2020-10-10 1:55 ` Alexander Viro
2020-10-10 1:55 ` Alexander Viro
2020-10-14 5:51 ` Christoph Hellwig
2020-10-14 5:51 ` Christoph Hellwig
2020-09-03 14:22 ` [PATCH 06/14] fs: don't allow splice read/write without explicit ops Christoph Hellwig
2020-09-03 14:22 ` Christoph Hellwig
2020-09-03 14:22 ` [PATCH 07/14] uaccess: add infrastructure for kernel builds with set_fs() Christoph Hellwig
2020-09-03 14:22 ` Christoph Hellwig
2020-09-03 14:22 ` [PATCH 08/14] test_bitmap: remove user bitmap tests Christoph Hellwig
2020-09-03 14:22 ` Christoph Hellwig
2020-09-03 14:22 ` [PATCH 09/14] lkdtm: remove set_fs-based tests Christoph Hellwig
2020-09-03 14:22 ` Christoph Hellwig
2020-09-03 14:22 ` [PATCH 10/14] x86: move PAGE_OFFSET, TASK_SIZE & friends to page_{32,64}_types.h Christoph Hellwig
2020-09-03 14:22 ` [PATCH 10/14] x86: move PAGE_OFFSET, TASK_SIZE & friends to page_{32, 64}_types.h Christoph Hellwig
2020-09-03 14:22 ` [PATCH 11/14] x86: make TASK_SIZE_MAX usable from assembly code Christoph Hellwig
2020-09-03 14:22 ` Christoph Hellwig
2020-09-03 14:22 ` [PATCH 12/14] x86: remove address space overrides using set_fs() Christoph Hellwig
2020-09-03 14:22 ` Christoph Hellwig
2020-09-03 21:30 ` David Laight
2020-09-03 21:30 ` David Laight
2020-09-03 23:25 ` Linus Torvalds
2020-09-03 23:25 ` Linus Torvalds
2020-09-04 7:59 ` David Laight
2020-09-04 7:59 ` David Laight
2020-09-04 2:55 ` Al Viro
2020-09-04 2:55 ` Al Viro
2020-09-04 4:41 ` Al Viro
2020-09-04 4:41 ` Al Viro
2020-09-04 6:38 ` Christoph Hellwig
2020-09-04 6:38 ` Christoph Hellwig
2020-09-04 7:47 ` Christoph Hellwig
2020-09-04 7:47 ` Christoph Hellwig
2020-09-03 14:22 ` [PATCH 13/14] powerpc: use non-set_fs based maccess routines Christoph Hellwig
2020-09-03 14:22 ` Christoph Hellwig
2020-09-03 14:22 ` [PATCH 14/14] powerpc: remove address space overrides using set_fs() Christoph Hellwig
2020-09-03 14:22 ` Christoph Hellwig
2020-09-03 15:43 ` Christophe Leroy
2020-09-03 15:49 ` Christoph Hellwig
2020-09-03 15:49 ` Christoph Hellwig
2020-09-03 15:56 ` Christoph Hellwig
2020-09-03 15:56 ` Christoph Hellwig
2020-09-03 16:03 ` Christophe Leroy
2020-09-03 16:03 ` Christophe Leroy
2020-09-03 14:28 ` remove the last set_fs() in common code, and remove it for x86 and powerpc v3 Al Viro
2020-09-03 14:28 ` Al Viro
2020-09-03 14:30 ` Christoph Hellwig
2020-09-03 14:30 ` Christoph Hellwig
2020-09-03 14:36 ` Al Viro
2020-09-03 14:36 ` Al Viro
2020-09-03 14:40 ` Christoph Hellwig
2020-09-03 14:40 ` Christoph Hellwig
2020-09-09 17:31 ` Linus Torvalds
2020-09-09 17:31 ` Linus Torvalds
2020-09-09 18:40 ` Segher Boessenkool
2020-09-09 18:40 ` Segher Boessenkool
2020-09-09 21:33 ` Linus Torvalds
2020-09-09 21:33 ` Linus Torvalds
2020-09-10 8:04 ` David Laight
2020-09-10 8:04 ` David Laight
2020-09-10 8:13 ` Christophe Leroy
2020-09-10 8:13 ` Christophe Leroy
2020-09-10 9:26 ` David Laight
2020-09-10 9:26 ` David Laight
2020-09-10 12:26 ` David Laight
2020-09-10 12:26 ` David Laight
2020-09-10 15:20 ` Segher Boessenkool [this message]
2020-09-10 15:20 ` Segher Boessenkool
2020-09-10 15:31 ` David Laight
2020-09-10 15:31 ` David Laight
2020-09-10 17:15 ` Segher Boessenkool
2020-09-10 17:15 ` Segher Boessenkool
2020-09-10 15:16 ` Segher Boessenkool
2020-09-10 15:16 ` Segher Boessenkool
2020-09-10 15:44 ` Segher Boessenkool
2020-09-10 15:44 ` Segher Boessenkool
2020-09-03 15:49 ` Linus Torvalds
2020-09-03 15:49 ` Linus Torvalds
2020-09-04 6:00 ` Ingo Molnar
2020-09-04 6:00 ` Ingo Molnar
2020-09-04 17:58 ` Alexey Dobriyan
2020-09-04 17:58 ` Alexey Dobriyan
2020-09-04 18:42 ` Linus Torvalds
2020-09-04 18:42 ` Linus Torvalds
2020-09-04 21:01 ` David Laight
2020-09-04 21:01 ` David Laight
2020-09-05 7:16 ` Christophe Leroy
2020-09-05 7:16 ` Christophe Leroy
2020-09-05 10:13 ` David Laight
2020-09-05 10:13 ` David Laight
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