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From: Arvind Sankar <nivedita@alum.mit.edu>
To: David Laight <David.Laight@ACULAB.COM>
Cc: 'Arvind Sankar' <nivedita@alum.mit.edu>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
	Sedat Dilek <sedat.dilek@gmail.com>,
	Segher Boessenkool <segher@kernel.crashing.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Nick Desaulniers <ndesaulniers@google.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Ingo Molnar <mingo@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
	Borislav Petkov <bp@alien8.de>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Kees Cook <keescook@chromium.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Juergen Gross <jgross@suse.com>,
	Andy Lutomirski <luto@kernel.org>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	LKML <linux-kernel@vger.kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Will Deacon <will@kernel.org>,
	"nadav.amit@gmail.com" <nadav.amit@gmail.com>,
	Nathan Chancellor <natechancellor@gmail.com>
Subject: Re: [PATCH v2] x86/asm: Replace __force_order with memory clobber
Date: Wed, 2 Sep 2020 12:14:29 -0400
Message-ID: <20200902161429.GB3297881@rani.riverdale.lan> (raw)
In-Reply-To: <55a35a3d8fba417aabe63ad13d519198@AcuMS.aculab.com>

On Wed, Sep 02, 2020 at 03:58:38PM +0000, David Laight wrote:
> From: Arvind Sankar
> > Sent: 02 September 2020 16:34
> > 
> > The CRn accessor functions use __force_order as a dummy operand to
> > prevent the compiler from reordering the inline asm.
> > 
> > The fact that the asm is volatile should be enough to prevent this
> > already, however older versions of GCC had a bug that could sometimes
> > result in reordering. This was fixed in 8.1, 7.3 and 6.5. Versions prior
> > to these, including 5.x and 4.9.x, may reorder volatile asm.
> > 
> > There are some issues with __force_order as implemented:
> > - It is used only as an input operand for the write functions, and hence
> >   doesn't do anything additional to prevent reordering writes.
> > - It allows memory accesses to be cached/reordered across write
> >   functions, but CRn writes affect the semantics of memory accesses, so
> >   this could be dangerous.
> > - __force_order is not actually defined in the kernel proper, but the
> >   LLVM toolchain can in some cases require a definition: LLVM (as well
> >   as GCC 4.9) requires it for PIE code, which is why the compressed
> >   kernel has a definition, but also the clang integrated assembler may
> >   consider the address of __force_order to be significant, resulting in
> >   a reference that requires a definition.
> > 
> > Fix this by:
> > - Using a memory clobber for the write functions to additionally prevent
> >   caching/reordering memory accesses across CRn writes.
> > - Using a dummy input operand with an arbitrary constant address for the
> >   read functions, instead of a global variable. This will prevent reads
> >   from being reordered across writes, while allowing memory loads to be
> >   cached/reordered across CRn reads, which should be safe.
> 
> How much does using a full memory clobber for the reads cost?
> 
> It would remove any chance that the compiler decides it needs to
> get the address of the 'dummy' location into a register so that
> it can be used as a memory reference in a generated instruction
> (which is probably what was happening for PIE compiles).
> 
> 	David
> 

It doesn't cost much. When I tested it, the only differences were in
startup code and sleep/hibernate/reboot code.

The compiler doesn't load 0x1000 into a register even for PIE code, the
reason it was doing it with a real symbol is to go through the GOT.

Thanks.

  reply index

Thread overview: 63+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-27 13:53 [PATCH] x86: work around clang IAS bug referencing __force_order Arnd Bergmann
2020-08-01 11:50 ` Sedat Dilek
2020-08-06 22:13   ` Thomas Gleixner
2020-08-07  7:03     ` Sedat Dilek
2020-08-04  0:09 ` Nick Desaulniers
2020-08-14 17:29   ` Sedat Dilek
2020-08-14 21:19     ` Sedat Dilek
2020-08-14 22:57       ` Nick Desaulniers
2020-08-15  0:26         ` Nick Desaulniers
2020-08-15  3:28           ` Sedat Dilek
2020-08-15  8:23             ` Sedat Dilek
2020-08-15 10:46               ` Sedat Dilek
2020-08-15 14:39                 ` Sedat Dilek
2020-08-16  9:37                   ` Sedat Dilek
2020-08-06 22:11 ` Thomas Gleixner
2020-08-13  0:12   ` Nick Desaulniers
2020-08-13  8:49     ` David Laight
2020-08-13 17:20     ` Arvind Sankar
2020-08-13 17:28     ` Thomas Gleixner
2020-08-13 17:37       ` Paul E. McKenney
2020-08-13 18:09         ` Arvind Sankar
2020-08-13 18:20           ` Paul E. McKenney
2020-08-20 10:44           ` Thomas Gleixner
2020-08-20 13:06             ` Arvind Sankar
2020-08-21  0:37               ` Thomas Gleixner
2020-08-21 23:04                 ` Arvind Sankar
2020-08-21 23:16                   ` Nick Desaulniers
2020-08-21 23:25                     ` Arvind Sankar
2020-08-22  0:43                     ` Thomas Gleixner
2020-08-22  3:55                       ` Arvind Sankar
2020-08-22  8:41                         ` Segher Boessenkool
2020-08-22  9:23                           ` Sedat Dilek
2020-08-22  9:51                             ` Sedat Dilek
2020-08-22 10:26                               ` Segher Boessenkool
2020-08-22 10:35                                 ` Arnd Bergmann
2020-08-22 18:17                               ` Miguel Ojeda
2020-08-22 21:08                                 ` Linus Torvalds
2020-08-22 23:10                                   ` Arvind Sankar
2020-08-23  0:10                                     ` Linus Torvalds
2020-08-23  1:16                                       ` Arvind Sankar
2020-08-23 21:25                                         ` [PATCH] x86/asm: Replace __force_order with memory clobber Arvind Sankar
2020-08-24 17:50                                           ` Nathan Chancellor
2020-08-24 19:13                                           ` Miguel Ojeda
2020-08-25 15:19                                             ` Arvind Sankar
2020-08-25 15:21                                               ` Sedat Dilek
2020-09-02 15:33                                           ` [PATCH v2] " Arvind Sankar
2020-09-02 15:58                                             ` David Laight
2020-09-02 16:14                                               ` Arvind Sankar [this message]
2020-09-02 16:08                                             ` Arvind Sankar
2020-09-02 20:26                                               ` David Laight
2020-09-02 17:16                                             ` Segher Boessenkool
2020-09-02 17:36                                               ` Arvind Sankar
2020-09-02 18:19                                             ` Miguel Ojeda
2020-09-02 18:24                                               ` Arvind Sankar
2020-09-02 23:21                                           ` [PATCH v3] " Arvind Sankar
2020-09-03  2:17                                             ` Kees Cook
2020-09-03  5:34                                             ` Miguel Ojeda
2020-09-30 20:50                                             ` Kees Cook
2020-10-01 10:12                                             ` [tip: x86/asm] x86/asm: Replace __force_order with a " tip-bot2 for Arvind Sankar
2020-10-13  9:30                                             ` tip-bot2 for Arvind Sankar
2020-08-22 21:17                                 ` [PATCH] x86: work around clang IAS bug referencing __force_order Arvind Sankar
2020-08-23 13:31                                   ` David Laight
2020-09-08 22:25                               ` Pavel Machek

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