linux-kernel - Re: [PATCH] ubsan: don't handle misaligned address when support unaligned access

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Date:   Fri, 1 Dec 2017 20:37:09 +0800
From:   Ding Tianhong <dingtianhong@...wei.com>
To:     David Laight <David.Laight@...LAB.COM>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "aryabinin@...tuozzo.co" <aryabinin@...tuozzo.co>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        LinuxArm <linuxarm@...wei.com>
Subject: Re: [PATCH] ubsan: don't handle misaligned address when support
 unaligned access



On 2017/12/1 19:47, David Laight wrote:
>> 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@...wei.com>
>> ---
>>  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?
>

Yes，I miss it.


> 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.
> 

Looks like we need to fix it in the handle_missaligned_access:

diff --git a/lib/ubsan.c b/lib/ubsan.c
index 278b4c3..040f8b2 100644
--- a/lib/ubsan.c
+++ b/lib/ubsan.c
@@ -289,6 +289,9 @@ static void handle_missaligned_access(struct type_mismatch_data *data,
        if (suppress_report(&data->location))
                return;

+       if (IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS))
+               return;
+
        ubsan_prologue(&data->location, &flags);

        pr_err("%s misaligned address %p for type %s\n",


Thanks
Ding


> 	David
>