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From: Will Deacon <will@kernel.org>
To: Marco Elver <elver@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Ingo Molnar <mingo@kernel.org>,
	Dmitry Vyukov <dvyukov@google.com>
Subject: Re: [PATCH v5 00/18] Rework READ_ONCE() to improve codegen
Date: Wed, 13 May 2020 18:47:48 +0100	[thread overview]
Message-ID: <20200513174747.GB24836@willie-the-truck> (raw)
In-Reply-To: <CANpmjNN=n59ue06s0MfmRFvKX=WB2NgLgbP6kG_MYCGy2R6PHg@mail.gmail.com>

On Wed, May 13, 2020 at 07:32:58PM +0200, Marco Elver wrote:
> On Wed, 13 May 2020 at 18:50, Will Deacon <will@kernel.org> wrote:
> >
> > On Wed, May 13, 2020 at 03:15:55PM +0200, Marco Elver wrote:
> > > On Wed, 13 May 2020 at 14:40, Will Deacon <will@kernel.org> wrote:
> > > >
> > > > On Wed, May 13, 2020 at 02:32:43PM +0200, Peter Zijlstra wrote:
> > > > > On Wed, May 13, 2020 at 01:48:41PM +0200, Marco Elver wrote:
> > > > >
> > > > > > Disabling most instrumentation for arch/x86 is reasonable. Also fine
> > > > > > with the __READ_ONCE/__WRITE_ONCE changes (your improved
> > > > > > compiler-friendlier version).
> > > > > >
> > > > > > We likely can't have both: still instrument __READ_ONCE/__WRITE_ONCE
> > > > > > (as Will suggested) *and* avoid double-instrumentation in arch_atomic.
> > > > > > If most use-cases of __READ_ONCE/__WRITE_ONCE are likely to use
> > > > > > data_race() or KCSAN_SANITIZE := n anyway, I'd say it's reasonable for
> > > > > > now.
> > > >
> > > > I agree that Peter's patch is the right thing to do for now. I was hoping we
> > > > could instrument __{READ,WRITE}_ONCE(), but that we before I realised that
> > > > __no_sanitize_or_inline doesn't seem to do anything.
> > > >
> > > > > Right, if/when people want sanitize crud enabled for x86 I need
> > > > > something that:
> > > > >
> > > > >  - can mark a function 'no_sanitize' and all code that gets inlined into
> > > > >    that function must automagically also not get sanitized. ie. make
> > > > >    inline work like macros (again).
> > > > >
> > > > > And optionally:
> > > > >
> > > > >  - can mark a function explicitly 'sanitize', and only when an explicit
> > > > >    sanitize and no_sanitize mix in inlining give the current
> > > > >    incompatible attribute splat.
> > > > >
> > > > > That way we can have the noinstr function attribute imply no_sanitize
> > > > > and frob the DEFINE_IDTENTRY*() macros to use (a new) sanitize_or_inline
> > > > > helper instead of __always_inline for __##func().
> > > >
> > > > Sounds like a good plan to me, assuming the compiler folks are onboard.
> > > > In the meantime, can we kill __no_sanitize_or_inline and put it back to
> > > > the old __no_kasan_or_inline, which I think simplifies compiler.h and
> > > > doesn't mislead people into using the function annotation to avoid KCSAN?
> > > >
> > > > READ_ONCE_NOCHECK should also probably be READ_ONCE_NOKASAN, but I
> > > > appreciate that's a noisier change.
> > >
> > > So far so good, except: both __no_sanitize_or_inline and
> > > __no_kcsan_or_inline *do* avoid KCSAN instrumenting plain accesses, it
> > > just doesn't avoid explicit kcsan_check calls, like those in
> > > READ/WRITE_ONCE if KCSAN is enabled for the compilation unit. That's
> > > just because macros won't be redefined just for __no_sanitize
> > > functions. Similarly, READ_ONCE_NOCHECK does work as expected, and its
> > > access is unchecked.
> > >
> > > This will have the expected result:
> > > __no_sanitize_or_inline void foo(void) { x++; } // no data races reported
> > >
> > > This will not work as expected:
> > > __no_sanitize_or_inline void foo(void) { READ_ONCE(x); }  // data
> > > races are reported
> >
> > But the problem is that *this* does not work as expected:
> >
> > unsigned long __no_sanitize_or_inline foo(unsigned long *ptr)
> > {
> >         return READ_ONCE_NOCHECK(*ptr);
> > }
> >
> > which I think means that the function annotation is practically useless.
> 
> Let me understand the problem better:
> 
> - We do not want __tsan_func_entry/exit (looking at the disassembly,
> these aren't always generated).
> - We do not want kcsan_disable/enable calls (with the new __READ_ONCE version).
> - We do *not* want the call to __read_once_word_nocheck if we have
> __no_sanitize_or_inline. AFAIK that's the main problem -- this applies
> to both KASAN and KCSAN.

Sorry, I should've been more explicit. The code above, with KASAN enabled,
compiles to:

ffffffff810a2d50 <foo>:
ffffffff810a2d50:       48 8b 07                mov    (%rdi),%rax
ffffffff810a2d53:       c3                      retq

but with KCSAN enabled, compiles to:

ffffffff8109ecd0 <foo>:
ffffffff8109ecd0:       53                      push   %rbx
ffffffff8109ecd1:       48 89 fb                mov    %rdi,%rbx
ffffffff8109ecd4:       48 8b 7c 24 08          mov    0x8(%rsp),%rdi
ffffffff8109ecd9:       e8 52 9c 1a 00          callq  ffffffff81248930 <__tsan_func_entry>
ffffffff8109ecde:       48 89 df                mov    %rbx,%rdi
ffffffff8109ece1:       e8 1a 00 00 00          callq  ffffffff8109ed00 <__read_once_word_nocheck>
ffffffff8109ece6:       48 89 c3                mov    %rax,%rbx
ffffffff8109ece9:       e8 52 9c 1a 00          callq  ffffffff81248940 <__tsan_func_exit>
ffffffff8109ecee:       48 89 d8                mov    %rbx,%rax
ffffffff8109ecf1:       5b                      pop    %rbx
ffffffff8109ecf2:       c3                      retq

Is that expected? There don't appear to be any more annotations to throw
at it.

> From what I gather, we want to just compile the function as if the
> sanitizer was never enabled. One reason for why this doesn't quite
> work is because of the preprocessor.
> 
> Note that the sanitizers won't complain about these accesses, which
> unfortunately is all these attributes ever were documented to do. So
> the attributes aren't completely useless. Why doesn't
> K[AC]SAN_SANITIZE := n work?

I just don't get the point in having a function annotation if you then have to
pass flags at the per-object level. That also then necessitates either weird
refactoring and grouping of code into "noinstrument.c" type files, or blanket
disabling of instrumentation for things like arch/x86/

Will

  reply	other threads:[~2020-05-13 17:47 UTC|newest]

Thread overview: 127+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-11 20:41 Will Deacon
2020-05-11 20:41 ` [PATCH v5 01/18] sparc32: mm: Fix argument checking in __srmmu_get_nocache() Will Deacon
2020-05-12 14:37   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-11 20:41 ` [PATCH v5 02/18] sparc32: mm: Restructure sparc32 MMU page-table layout Will Deacon
2020-05-12 14:37   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-11 20:41 ` [PATCH v5 03/18] sparc32: mm: Change pgtable_t type to pte_t * instead of struct page * Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-11 20:41 ` [PATCH v5 04/18] sparc32: mm: Reduce allocation size for PMD and PTE tables Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-17  0:00   ` [PATCH v5 04/18] " Guenter Roeck
2020-05-17  0:07     ` Guenter Roeck
2020-05-18  8:37       ` Will Deacon
2020-05-18  9:18         ` Mike Rapoport
2020-05-18  9:48         ` Guenter Roeck
2020-05-18 14:23           ` Mike Rapoport
2020-05-18 16:08             ` Guenter Roeck
2020-05-18 18:11               ` Ira Weiny
2020-05-18 18:14               ` Ira Weiny
2020-05-18 18:09             ` Guenter Roeck
2020-05-18 18:21               ` Ira Weiny
2020-05-18 19:15               ` Mike Rapoport
2020-05-19 16:40                 ` Guenter Roeck
2020-05-20 17:03         ` Mike Rapoport
2020-05-20 19:03           ` Guenter Roeck
2020-05-20 19:51             ` Mike Rapoport
2020-05-21 23:02               ` Guenter Roeck
2020-05-24 12:32                 ` Mike Rapoport
2020-05-24 14:01                   ` Guenter Roeck
2020-05-26 13:26                   ` Will Deacon
2020-05-26 14:01                     ` Will Deacon
2020-05-26 15:21                       ` Mike Rapoport
2020-05-26 16:18                       ` Guenter Roeck
2020-05-26 16:29                         ` Mike Rapoport
2020-05-26 17:15                           ` Guenter Roeck
2020-05-11 20:41 ` [PATCH v5 05/18] compiler/gcc: Raise minimum GCC version for kernel builds to 4.8 Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-11 20:41 ` [PATCH v5 06/18] netfilter: Avoid assigning 'const' pointer to non-const pointer Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-11 20:41 ` [PATCH v5 07/18] net: tls: " Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-11 20:41 ` [PATCH v5 08/18] fault_inject: Don't rely on "return value" from WRITE_ONCE() Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-11 20:41 ` [PATCH v5 09/18] arm64: csum: Disable KASAN for do_csum() Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-11 20:41 ` [PATCH v5 10/18] READ_ONCE: Simplify implementations of {READ,WRITE}_ONCE() Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-11 20:41 ` [PATCH v5 11/18] READ_ONCE: Enforce atomicity for {READ,WRITE}_ONCE() memory accesses Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-11 20:41 ` [PATCH v5 12/18] READ_ONCE: Drop pointer qualifiers when reading from scalar types Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-11 20:41 ` [PATCH v5 13/18] locking/barriers: Use '__unqual_scalar_typeof' for load-acquire macros Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-11 20:41 ` [PATCH v5 14/18] arm64: barrier: Use '__unqual_scalar_typeof' for acquire/release macros Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-11 20:41 ` [PATCH v5 15/18] gcov: Remove old GCC 3.4 support Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-11 20:41 ` [PATCH v5 16/18] kcsan: Rework data_race() so that it can be used by READ_ONCE() Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-11 20:41 ` [PATCH v5 17/18] READ_ONCE: Use data_race() to avoid KCSAN instrumentation Will Deacon
2020-05-12  8:23   ` Peter Zijlstra
2020-05-12  9:49     ` Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-20 22:17     ` Borislav Petkov
2020-05-20 22:30       ` Marco Elver
2020-05-21  7:25         ` Borislav Petkov
2020-05-21  9:37           ` Marco Elver
2020-05-21  3:30       ` Nathan Chancellor
2020-05-22 16:08       ` [tip: locking/kcsan] compiler.h: Avoid nested statement expression in data_race() tip-bot2 for Marco Elver
2020-05-11 20:41 ` [PATCH v5 18/18] linux/compiler.h: Remove redundant '#else' Will Deacon
2020-05-12 14:36   ` [tip: locking/kcsan] " tip-bot2 for Will Deacon
2020-05-12  8:18 ` [PATCH v5 00/18] Rework READ_ONCE() to improve codegen Peter Zijlstra
2020-05-12 17:53   ` Marco Elver
2020-05-12 18:55     ` Marco Elver
2020-05-12 19:07     ` Peter Zijlstra
2020-05-12 20:31       ` Marco Elver
2020-05-13 11:10         ` Peter Zijlstra
2020-05-13 11:14           ` Peter Zijlstra
2020-05-13 11:48           ` Marco Elver
2020-05-13 12:32             ` Peter Zijlstra
2020-05-13 12:40               ` Will Deacon
2020-05-13 13:15                 ` Marco Elver
2020-05-13 13:24                   ` Peter Zijlstra
2020-05-13 13:58                     ` Marco Elver
2020-05-14 11:21                       ` Peter Zijlstra
2020-05-14 11:24                         ` Peter Zijlstra
2020-05-14 11:35                         ` Peter Zijlstra
2020-05-14 12:01                         ` Will Deacon
2020-05-14 12:27                           ` Peter Zijlstra
2020-05-14 13:07                             ` Marco Elver
2020-05-14 13:14                               ` Peter Zijlstra
2020-05-14 12:20                         ` Peter Zijlstra
2020-05-14 14:13                       ` Peter Zijlstra
2020-05-14 14:20                         ` Marco Elver
2020-05-15  9:20                           ` Peter Zijlstra
2020-05-13 16:50                   ` Will Deacon
2020-05-13 17:32                     ` Marco Elver
2020-05-13 17:47                       ` Will Deacon [this message]
2020-05-13 18:54                         ` Marco Elver
2020-05-13 21:25                           ` Will Deacon
2020-05-14  7:31                             ` Marco Elver
2020-05-14 11:05                               ` Will Deacon
2020-05-14 13:35                                 ` Marco Elver
2020-05-14 13:47                                   ` Peter Zijlstra
2020-05-14 13:50                                   ` Peter Zijlstra
2020-05-14 13:56                                   ` Peter Zijlstra
2020-05-14 14:24                                   ` Peter Zijlstra
2020-05-14 15:09                                     ` Thomas Gleixner
2020-05-14 15:29                                       ` Marco Elver
2020-05-14 19:37                                         ` Thomas Gleixner
2020-05-15 13:55                                     ` David Laight
2020-05-15 14:04                                       ` Marco Elver
2020-05-15 14:07                                       ` Peter Zijlstra
2020-05-14 15:38                                   ` Paul E. McKenney
2020-05-22 16:08                                   ` [tip: locking/kcsan] kcsan: Restrict supported compilers tip-bot2 for Marco Elver
2020-06-03 18:52                                 ` [PATCH v5 00/18] Rework READ_ONCE() to improve codegen Borislav Petkov
2020-06-03 19:23                                   ` Marco Elver
2020-06-03 22:05                                     ` Borislav Petkov
2020-06-08 17:32                                     ` Martin Liška
2020-06-08 19:56                                       ` Marco Elver
2020-06-09 11:55                                         ` Martin Liška
2020-06-09 12:36                                           ` Martin Liška
2020-06-09 13:45                                             ` Marco Elver
2020-05-22 16:08                           ` [tip: locking/kcsan] kcsan: Remove 'noinline' from __no_kcsan_or_inline tip-bot2 for Marco Elver
2020-05-13 13:21                 ` [PATCH v5 00/18] Rework READ_ONCE() to improve codegen David Laight
2020-05-13 16:32                   ` Thomas Gleixner
2020-05-12 21:14       ` Will Deacon
2020-05-12 22:00         ` Marco Elver

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