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

[Laurent Vivier]

Le 11/02/2021 à 22:28, Laurent Vivier a écrit :
> 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.

Wait... if the alignment differs between m68k and the host, I guess the size of 
the structure differs?

In this case we cannot use the guest memory to call the host syscall, we must 
allocate a host
structure and copy the values into the guest structure.

Thanks,
Laurent


>> 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.
> 
> 
>>                  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
>