Re: [PATCH] linux-user/syscall: Silence warning from the undefined behavior sanitizer

[Thomas Huth <thuth@redhat.com>]

On 11/02/2021 22.28, Laurent Vivier wrote:
> Le 11/02/2021 à 14:29, Thomas Huth a écrit :
>> When compiling QEMU with -fsanitize=undefined, there is a warning when
>> running "make check-tcg":
>>
>>    TEST    linux-test on m68k
>> ../linux-user/syscall.c:10499:34: runtime error: member access within
>>   misaligned address 0x00008006df3c for type 'struct linux_dirent64',
>>   which requires 8 byte alignment
>> 0x00008006df3c: note: pointer points here
>>    00 00 00 00 68 03 28 00  00 00 00 00 5b 96 3e e4  61 4b 05 26 18 00 04 2e  00 00 00 00 da 3f 18 00
>>                ^
>>
>> It's likely not an issue in reality, since I assume that on hosts where
>> the alignment really matters (like sparc64), the Linux kernel likely
>> adds the right padding. Anyway, let's use the stw_p() / stq_p() accessor
>> helpers here to silence the warning and thus to allow to compile the code
>> with -fsanitize=undefined, too.
>>
>> Signed-off-by: Thomas Huth <thuth@redhat.com>
>> ---
>>   linux-user/syscall.c | 16 +++++++++-------
>>   1 file changed, 9 insertions(+), 7 deletions(-)
>>
>> diff --git a/linux-user/syscall.c b/linux-user/syscall.c
>> index 34760779c8..50de535ade 100644
>> --- a/linux-user/syscall.c
>> +++ b/linux-user/syscall.c
>> @@ -10491,20 +10491,22 @@ static abi_long do_syscall1(void *cpu_env, int num, abi_long arg1,
>>                   return -TARGET_EFAULT;
>>               ret = get_errno(sys_getdents64(arg1, dirp, count));
>>               if (!is_error(ret)) {
>> -                struct linux_dirent64 *de;
>> +                char *de;
>>                   int len = ret;
>>                   int reclen;
>> -                de = dirp;
>> +                de = (char *)dirp;
>> +                #define de64(x) offsetof(struct linux_dirent64, x)
> 
> Do we really need the cast to the "(char *)"?
> 
> can't we use "&de->XXX" with the accessors?
> We don't access the memory, only read the address, the compiler should be happy.

That's what I thought and tried first, too. Unfortunately, it did not help 
to fix the issue, I had to take the detour via the char*

I guess the compiler also checks the alignment of the pointer when it gets 
assigned to the next record below ("de = ...").

  Thomas


> 
>>                   while (len > 0) {
>> -                    reclen = de->d_reclen;
>> +                    reclen = lduw_he_p(de + de64(d_reclen));
> 
> to avoid human error, it would be better to let the compiler take the good accessor:
> 
>   ldn_he_p(&de->d_reclen, sizeof(de->d_reclen))
> 
>>                       if (reclen > len)
>>                           break;
>> -                    de->d_reclen = tswap16(reclen);
>> -                    tswap64s((uint64_t *)&de->d_ino);
>> -                    tswap64s((uint64_t *)&de->d_off);
>> -                    de = (struct linux_dirent64 *)((char *)de + reclen);
>> +                    stw_p(de + de64(d_reclen), reclen);
>> +                    stq_p(de + de64(d_ino), ldq_he_p(de + de64(d_ino)));
>> +                    stq_p(de + de64(d_off), ldq_he_p(de + de64(d_off)));
> 
> and stwn_he_p() here too.
> 
>> +                    de += reclen;
>>                       len -= reclen;
>>                   }
>> +                #undef de64
>>               }
>>               unlock_user(dirp, arg2, ret);
>>           }
>>
> 
> Thank you Thomas for your help.
> 
> Laurent
>