RE: [PATCH] ubsan: don't handle misaligned address when support unaligned access

[David Laight]

From: Ding Tianhong
> Sent: 01 December 2017 11:32
> To: akpm@xxxxxxxxxxxxxxxxxxxx; aryabinin@xxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; LinuxArm
> Subject: [PATCH] ubsan: don't handle misaligned address when support unaligned access
> 
> The ubsan always report Warning just 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 enable the CONFIG_UBSAN_ALIGNMENT, the ubsan
> will report the unaligned access even if the system support it
> (CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS=y), it will produce a lot
> of noise in the log and cause confusion.
> 
> This patch will close the detection of unaligned access when
> the system support unaligned access.
> 
> Signed-off-by: Ding Tianhong <dingtianhong@xxxxxxxxxx>
> ---
>  lib/ubsan.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/ubsan.c b/lib/ubsan.c
> index fb0409d..278b4c3 100644
> --- a/lib/ubsan.c
> +++ b/lib/ubsan.c
> @@ -321,7 +321,8 @@ void __ubsan_handle_type_mismatch(struct type_mismatch_data *data,
> 
>  	if (!ptr)
>  		handle_null_ptr_deref(data);
> -	else if (data->alignment && !IS_ALIGNED(ptr, data->alignment))
> +	else if (data->alignment && !IS_ALIGNED(ptr, data->alignment) &&
> +		 !IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS))
>  		handle_missaligned_access(data, ptr);
>  	else
>  		handle_object_size_mismatch(data, ptr);

Won't that report an object size error instead of actually
doing the required access?

Surely it shouldn't get into this function at all?

I guess 'alignment' is set to 4 or 8.
If it were set to 3 or 7 (or 0) then the tests on the pointer
would be much simpler - maybe at a slight extra cost in setup.

	David