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

[Thomas Huth <thuth@redhat.com>]

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)
                 while (len > 0) {
-                    reclen = de->d_reclen;
+                    reclen = lduw_he_p(de + de64(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)));
+                    de += reclen;
                     len -= reclen;
                 }
+                #undef de64
             }
             unlock_user(dirp, arg2, ret);
         }
-- 
2.27.0