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Date: Wed, 6 Dec 2017 16:49:16 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Ding Tianhong <dingtianhong@huawei.com>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        LinuxArm <linuxarm@huawei.com>, David Laight <David.Laight@ACULAB.COM>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>
Subject: Re: [PATCH v2] ubsan: don't handle misaligned address when support
 unaligned access
Message-Id: <20171206164916.687d5006b7a49ef1149e7154@linux-foundation.org>
In-Reply-To: <5b905d56-609e-3822-096a-3b93b3eb7675@huawei.com>
References: <5b905d56-609e-3822-096a-3b93b3eb7675@huawei.com>
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(correcting Andrey's email address)


From: Ding Tianhong <dingtianhong@huawei.com>
Subject: lib/ubsan.c: don't handle misaligned address when kernel supports unaligned access

ubsan reports a warning like:

UBSAN: Undefined behaviour in ../include/linux/etherdevice.h:386:9
load of misaligned address ffffffc069ba0482 for type 'long unsigned int'
which requires 8 byte alignment
CPU: 0 PID: 901 Comm: sshd Not tainted 4.xx+ #1
Hardware name: linux,dummy-virt (DT)
Call trace:
[<ffffffc000093600>] dump_backtrace+0x0/0x348
[<ffffffc000093968>] show_stack+0x20/0x30
[<ffffffc001651664>] dump_stack+0x144/0x1b4
[<ffffffc0016519b0>] ubsan_epilogue+0x18/0x74
[<ffffffc001651bac>] __ubsan_handle_type_mismatch+0x1a0/0x25c
[<ffffffc00125d8a0>] dev_gro_receive+0x17d8/0x1830
[<ffffffc00125d928>] napi_gro_receive+0x30/0x158
[<ffffffc000f4f93c>] virtnet_receive+0xad4/0x1fa8

The reason is that when enabling the CONFIG_UBSAN_ALIGNMENT, ubsan will
report the unaligned access even if the system supports it
(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS=y).  This produces a lot of noise
in the log and causes confusion.

Prevent the detection of unaligned access when the system support
unaligned access.

Link: http://lkml.kernel.org/r/5b905d56-609e-3822-096a-3b93b3eb7675@huawei.com
Signed-off-by: Ding Tianhong <dingtianhong@huawei.com>
Cc: David Laight <David.Laight@ACULAB.COM>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 lib/ubsan.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff -puN lib/ubsan.c~ubsan-dont-handle-misaligned-address-when-support-unaligned-access lib/ubsan.c
--- a/lib/ubsan.c~ubsan-dont-handle-misaligned-address-when-support-unaligned-access
+++ a/lib/ubsan.c
@@ -322,7 +322,8 @@ void __ubsan_handle_type_mismatch(struct
 	if (!ptr)
 		handle_null_ptr_deref(data);
 	else if (data->alignment && !IS_ALIGNED(ptr, data->alignment))
-		handle_missaligned_access(data, ptr);
+		if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS))
+			handle_missaligned_access(data, ptr);
 	else
 		handle_object_size_mismatch(data, ptr);
 }
_