From: Denys Vlasenko <dvlasenk@redhat.com>
To: Josh Poimboeuf <jpoimboe@redhat.com>,
James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Ingo Molnar <mingo@kernel.org>, Thomas Graf <tgraf@suug.ch>,
Peter Zijlstra <peterz@infradead.org>,
David Rientjes <rientjes@google.com>,
Andrew Morton <akpm@linux-foundation.org>,
linux-kernel@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
linux-scsi <linux-scsi@vger.kernel.org>
Subject: Re: This patch triggers a bad gcc bug (was Re: [PATCH] force inlining of some byteswap operations)
Date: Thu, 14 Apr 2016 17:29:06 +0200 [thread overview]
Message-ID: <570FB742.2010309@redhat.com> (raw)
In-Reply-To: <20160413171058.24tnvvquvnipnwnd@treble>
On 04/13/2016 07:10 PM, Josh Poimboeuf wrote:
>>>>>> From the disassembly of drivers/scsi/qla2xxx/qla_attr.o:
>>>>>>
>>>>>> 0000000000002f53 <qla2x00_get_host_fabric_name>:
>>>>>> 2f53: 55 push %rbp
>>>>>> 2f54: 48 89 e5 mov %rsp,%rbp
>>>>>>
>>>>>> 0000000000002f57 <qla2x00_get_fc_host_stats>:
>>>>>> 2f57: 55 push %rbp
>>>>>> 2f58: b9 e8 00 00 00 mov $0xe8,%ecx
>>>>>> 2f5d: 48 89 e5 mov %rsp,%rbp
>>>>>> ...
>>>>>>
>>>>>> Note that qla2x00_get_host_fabric_name() is inexplicably
>>>>>> truncated after
>>>>>> setting up the frame pointer. It falls through to the next
>>>>>> function, which is
>>>>>> very wrong.
>>>>>
>>>>> Wow, that's ... interesting.
>>>>>
>>>>>
>>>>>> I can recreate it with either gcc 5.3.1 or gcc 6.0 on
>>>>>> linus/master with
>>>>>> the .config from the above link.
>>>>>>
>>>>>> The call chain which appears to trigger the problem is:
>>>>>>
>>>>>> qla2x00_get_host_fabric_name()
>>>>>> wwn_to_u64()
>>>>>> get_unaligned_be64()
>>>>>> be64_to_cpup()
>>>>>> __be64_to_cpup() <- changed to __always_inline by this
>>>>>> patch
>>>>>>
>>>>>> It occurs with the combination of the following two recent
>>>>>> commits:
>>>>>>
>>>>>> - bc27fb68aaad ("include/uapi/linux/byteorder, swab: force
>>>>>> inlining of some byteswap operations")
>>>>>> - ef3fb2422ffe ("scsi: fc: use get/put_unaligned64 for wwn
>>>>>> access")
>>>>>>
>>>>>> I can confirm that reverting either patch makes the problem go
>>>>>> away.
>>>>>> I'm planning on opening a gcc bug tomorrow.
>>>>>
>>>>>
>>>>> Note that if CONFIG_OPTIMIZE_INLINING is not set, _all_ "inline"
>>>>> keywords are in fact __always_inline, so the bug must be
>>>>> triggering
>>>>> even without the patch.
>>>>
>>>> Makes sense in theory, but the bug doesn't actually trigger when I
>>>> revert the patch and set CONFIG_OPTIMIZE_INLINING=n.
>>>>
>>>> Perhaps even more surprising, it doesn't trigger *with* the patch
>>>> and
>>>> CONFIG_OPTIMIZE_INLINING=n.
>>>
>>> [ Adding James to CC since this bug affects scsi. ]
>>>
>>> Here's the gcc bug:
>>>
>>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70646
>>>
>>
>>
>> Actually, adding me doesn't help, I've added linux-scsi. The summary
>> is that there's a but in gcc-5.3.1 which is miscompiling qla_attr.c ...
>> this means we're going to have to ask the compiler version of reported
>> crashes.
>
> The bug isn't specific to a compiler version. I've seen it with gcc
> 5.3.1 and gcc 6.0. I haven't tried any older versions. And the gcc bug
> hasn't been resolved (or even investigated) yet.
>
> The bug is triggered by a combination of the above two commits from the
> 4.6 merge window, so presumably we'd need to revert one of them to avoid
> crashes in 4.6.
The bug is indeed in the compiler. 4.9 and all later versions are affected.
gcc bugzilla now has a reproducer. In abridged form:
static inline __attribute__((always_inline)) u64 __swab64p(const u64 *p)
{
return (__builtin_constant_p((u64)(*p)) ? ((u64)( (((u64)(*p) & (u64)0x00000000000000ffULL) << 56) | (((u64)(*p) & (u64)0x000000000000ff00ULL) << 40) | (((u64)(*p) & (u64)0x0000000000ff0000ULL) << 24) | (((u64)(*p) & (u64)0x00000000ff000000ULL) << 8) | (((u64)(*p) & (u64)0x000000ff00000000ULL) >> 8) | (((u64)(*p) & (u64)0x0000ff0000000000ULL) >> 24) | (((u64)(*p) & (u64)0x00ff000000000000ULL) >> 40) | (((u64)(*p) & (u64)0xff00000000000000ULL) >> 56))) : __builtin_bswap64(*p));
}
static inline u64 wwn_to_u64(void *wwn)
{
return __swab64p(wwn);
}
static void qla2x00_get_host_fabric_name(struct Scsi_Host *shost)
{
scsi_qla_host_t *vha = shost_priv(shost);
u8 node_name[8] = { 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF};
u64 fabric_name = wwn_to_u64(node_name);
if (vha->device_flags & 0x1)
fabric_name = wwn_to_u64(vha->fabric_node_name);
(((struct fc_host_attrs *)(shost)->shost_data)->fabric_name) = fabric_name;
}
Two (or more, there were more before simplification) levels of inlining
are necessary for bug to trigger in this example (folding to one level
makes it go away). "__attribute__((always_inline))" is necessary too.
Since we have lots of __always_inline anyway, this bug has a potential
to miscompile kernels regardless of CONFIG_OPTIMIZE_INLINING setting,
and with or without the patches mentioned above (they just happen
to create a reliable reproducer).
Since it was not detected for two years since gcc 4.9 release,
it must be triggering quite rarely.
next prev parent reply other threads:[~2016-04-14 15:29 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-02-04 19:45 [PATCH] asm-generic: force inlining of some atomic_long operations Denys Vlasenko
2016-02-04 19:45 ` [PATCH] force inlining of some byteswap operations Denys Vlasenko
2016-02-05 7:28 ` Ingo Molnar
2016-04-13 3:36 ` This patch triggers a bad gcc bug (was Re: [PATCH] force inlining of some byteswap operations) Josh Poimboeuf
2016-04-13 12:12 ` Denys Vlasenko
2016-04-13 12:36 ` Josh Poimboeuf
2016-04-13 15:15 ` Josh Poimboeuf
2016-04-13 16:55 ` James Bottomley
2016-04-13 17:10 ` Josh Poimboeuf
2016-04-14 15:29 ` Denys Vlasenko [this message]
2016-04-14 15:57 ` Josh Poimboeuf
2016-04-14 17:09 ` Denys Vlasenko
2016-04-15 5:45 ` Ingo Molnar
2016-04-15 13:47 ` Josh Poimboeuf
2016-04-15 22:20 ` Josh Poimboeuf
2016-04-16 9:03 ` Ingo Molnar
2016-04-18 13:39 ` Josh Poimboeuf
2016-04-18 14:07 ` Arnd Bergmann
2016-04-18 14:12 ` Josh Poimboeuf
2016-04-18 14:21 ` Arnd Bergmann
2016-04-19 8:52 ` Ingo Molnar
2016-04-19 13:56 ` [PATCH] scsi: fc: force inlining of wwn conversion functions Josh Poimboeuf
2016-04-22 23:17 ` Quinn Tran
2016-04-25 16:07 ` Josh Poimboeuf
2016-04-26 2:40 ` Martin K. Petersen
2016-04-26 3:37 ` James Bottomley
2016-04-26 7:22 ` Arnd Bergmann
2016-04-26 8:35 ` Christoph Hellwig
2016-04-26 10:05 ` Arnd Bergmann
2016-04-26 13:06 ` Martin K. Petersen
2016-04-26 15:58 ` Arnd Bergmann
2016-04-26 22:36 ` James Bottomley
2016-04-27 0:44 ` Martin K. Petersen
2016-04-27 11:05 ` Martin Jambor
2016-04-27 21:34 ` Arnd Bergmann
2016-04-28 14:58 ` Chris Metcalf
2016-04-28 15:23 ` Arnd Bergmann
2016-04-28 15:48 ` Chris Metcalf
2016-04-27 22:00 ` [PATCH, RFT] byteswap: try to avoid __builtin_constant_p gcc bug Arnd Bergmann
2016-04-27 22:11 ` Josh Poimboeuf
2016-04-28 16:27 ` Quinn Tran
2016-04-16 7:42 ` This patch triggers a bad gcc bug (was Re: [PATCH] force inlining of some byteswap operations) Arnd Bergmann
2016-04-18 13:22 ` Josh Poimboeuf
2016-02-04 19:45 ` [PATCH] force inlining of unaligned byteswap operations Denys Vlasenko
2016-02-05 7:28 ` Ingo Molnar
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