From: Marco Elver <elver@google.com>
To: Will Deacon <will@kernel.org>
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: Thu, 14 May 2020 09:31:49 +0200 [thread overview]
Message-ID: <CANpmjNOAi2K6knC9OFUGjpMo-rvtLDzKMb==J=vTRkmaWctFaQ@mail.gmail.com> (raw)
In-Reply-To: <20200513212520.GC28594@willie-the-truck>
On Wed, 13 May 2020 at 23:25, Will Deacon <will@kernel.org> wrote:
>
> On Wed, May 13, 2020 at 08:54:03PM +0200, Marco Elver wrote:
> > On Wed, 13 May 2020 at 19:47, Will Deacon <will@kernel.org> wrote:
> > > On Wed, May 13, 2020 at 07:32:58PM +0200, Marco Elver wrote:
> > > > - 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.
> >
> > Right, so this is expected.
>
> Fair enough, I just found it weird since it's different to the usual
> "disable instrumentation/trace" function annotations.
>
> > We can definitely make __tsan_func_entry/exit disappear with Clang, with
> > GCC it's going to be a while if we want to fix it.
> >
> > If we remove 'noinline' from __no_kcsan_or_inline, we no longer get
> > the call to __read_once_word_nocheck above! But...
> >
> > For KCSAN we force 'noinline' because older compilers still inline and
> > then instrument small functions even if we just have the no_sanitize
> > attribute (without inline mentioned). The same is actually true for
> > KASAN, so KASAN's READ_ONCE_NOCHECK might be broken in a few places,
> > but nobody seems to have noticed [1]. KASAN's __no_kasan_or_inline
> > should also have a 'noinline' I think. I just tested
> > __no_kcsan_or_inline without 'noinline', and yes, GCC 9 still decided
> > to inline a small function and then instrument the accesses.
> >
> > [1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59600
> >
> > The good news is that Clang does the right thing when removing
> > 'noinline' from __no_kcsan_or_inline:
> > 1. doesn't inline into functions that are instrumented, and
> > 2. your above example doesn't do the call to __read_once_word_nocheck.
> >
> > The obvious solution to this is: restrict which compiler we want to support?
>
> I would be in favour of that, but I defer to the x86 folks since this
> affects them much more than it does me. On the arm64 side, we've got patches
> queued for 5.8 that require GCC 10.0.1 or later, and that thing is only a
> week old. I think it's reasonable to require a recent toolchain for optional
> features like this that inherently rely on compiler support.
>
> > > > 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/
> >
> > If you want a solution now, here is one way to get us closer to where
> > we want to be:
> >
> > 1. Peter's patch to add data_race around __READ_ONCE/__WRITE_ONCE.
> > 2. Patch to make __tsan_func_entry/exit disappear with Clang.
> > 3. Remove 'noinline' from __no_kcsan_or_inline.
> > 4. Patch to warn users that KCSAN may have problems with GCC and
> > should use Clang >= 7.
> >
> > But this is probably only half a solution.
>
> At this point, I think that if READ_ONCE_NOCHECK() works as expected, and
> calling __{READ,WRITE}_ONCE from functions tagged with __no_sanitize doesn't
> result in instrumentation, then we're good.
Ouch. With the __{READ,WRITE}_ONCE requirement, we're going to need
Clang 11 though.
Because without the data_race() around __*_ONCE,
arch_atomic_{read,set} will be broken for KCSAN, but we can't have
data_race() because it would still add
kcsan_{enable,disable}_current() calls to __no_sanitize functions (if
compilation unit is instrumented). We can't make arch_atomic functions
__no_sanitize_or_inline, because even in code that we want to
sanitize, they should remain __always_inline (so they work properly in
__no_sanitize functions). Therefore, Clang 11 with support for
distinguishing volatiles will be the compiler that will satisfy all
the constraints.
If this is what we want, let me prepare a series on top of
-tip/locking/kcsan with all the things I think we need.
Thanks,
-- Marco
next prev parent reply other threads:[~2020-05-14 7:32 UTC|newest]
Thread overview: 127+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-11 20:41 [PATCH v5 00/18] Rework READ_ONCE() to improve codegen 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
2020-05-13 18:54 ` Marco Elver
2020-05-13 21:25 ` Will Deacon
2020-05-14 7:31 ` Marco Elver [this message]
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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