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

[Laurent Vivier <laurent@vivier.eu>]

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.


>                  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