linux: mips: Fix getdents64 fallback on mips64-n32

GCC mainline shows the following error:

../sysdeps/unix/sysv/linux/mips/mips64/getdents64.c: In function '__getdents64':
../sysdeps/unix/sysv/linux/mips/mips64/getdents64.c:121:7: error: 'memcpy' forming offset [4, 7] is out of the bounds [0, 4] [-Werror=array-bounds]
  121 |       memcpy (((char *) dp + offsetof (struct dirent64, d_ino)),
      |       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  122 |               KDP_MEMBER (kdp, d_ino), sizeof ((struct dirent64){0}.d_ino));
      |               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../sysdeps/unix/sysv/linux/mips/mips64/getdents64.c:123:7: error: 'memcpy' forming offset [4, 7] is out of the bounds [0, 4] [-Werror=array-bounds]
  123 |       memcpy (((char *) dp + offsetof (struct dirent64, d_off)),
      |       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  124 |               KDP_MEMBER (kdp, d_off), sizeof ((struct dirent64){0}.d_off));
      |               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The issue is due both d_ino and d_off fields for mips64-n32
kernel_dirent are 32-bits, while this is using memcpy to copy 64 bits
from it into the glibc dirent64.

The fix is to use a temporary buffer to read the correct type
from kernel_dirent.

Checked with a build-many-glibcs.py for mips64el-linux-gnu and I
also checked the tst-getdents64 on mips64el 4.1.4 kernel with
and without fallback enabled (by manually setting the
getdents64_supported).

sysdeps/unix/sysv/linux/mips/mips64/getdents64.c

@@ -90,17 +90,14 @@ __getdents64 (int fd, void *buf, size_t nbytes)
 
   while ((char *) kdp < (char *) skdp + r)
     {
-      /* This macro is used to avoid aliasing violation.  */
-#define KDP_MEMBER(src, member)			     			\
-    (__typeof__((struct kernel_dirent){0}.member) *)			\
-      memcpy (&((__typeof__((struct kernel_dirent){0}.member)){0}),	\
-	      ((char *)(src) + offsetof (struct kernel_dirent, member)),\
-	      sizeof ((struct kernel_dirent){0}.member))
-
       /* This is a conservative approximation, since some of size_diff might
 	 fit into the existing padding for alignment.  */
-      unsigned short int k_reclen = *KDP_MEMBER (kdp, d_reclen);
-      unsigned short int new_reclen = ALIGN_UP (k_reclen + size_diff,
+
+      /* Obtain the d_ino, d_off, and d_reclen from kernel filled buffer.  */
+      struct kernel_dirent kdirent;
+      memcpy (&kdirent, kdp, offsetof (struct kernel_dirent, d_name));
+
+      unsigned short int new_reclen = ALIGN_UP (kdirent.d_reclen + size_diff,
 						_Alignof (struct dirent64));
       if (nb + new_reclen > nbytes)
 	{
@@ -118,19 +115,21 @@ __getdents64 (int fd, void *buf, size_t nbytes)
 	}
       nb += new_reclen;
 
-      memcpy (((char *) dp + offsetof (struct dirent64, d_ino)),
-	      KDP_MEMBER (kdp, d_ino), sizeof ((struct dirent64){0}.d_ino));
-      memcpy (((char *) dp + offsetof (struct dirent64, d_off)),
-	      KDP_MEMBER (kdp, d_off), sizeof ((struct dirent64){0}.d_off));
-      last_offset = *KDP_MEMBER (kdp, d_off);
-      memcpy (((char *) dp + offsetof (struct dirent64, d_reclen)),
-	      &new_reclen, sizeof (new_reclen));
-      dp->d_type = *((char *) kdp + k_reclen - 1);
+      struct dirent64 d64;
+      d64.d_ino = kdirent.d_ino;
+      d64.d_off = kdirent.d_off;
+      d64.d_reclen = new_reclen;
+      d64.d_type = *((char *) kdp + kdirent.d_reclen - 1);
+      /* First copy only the header.  */
+      memcpy (dp, &d64, offsetof (struct dirent64, d_name));
+      /* And then the d_name.  */
       memcpy (dp->d_name, kdp->d_name,
-	      k_reclen - offsetof (struct kernel_dirent, d_name));
+	      kdirent.d_reclen - offsetof (struct kernel_dirent, d_name));
+
+      last_offset = kdirent.d_off;
 
       dp = (struct dirent64 *) ((char *) dp + new_reclen);
-      kdp = (struct kernel_dirent *) (((char *) kdp) + k_reclen);
+      kdp = (struct kernel_dirent *) (((char *) kdp) + kdirent.d_reclen);
     }
 
   return (char *) dp - (char *) buf;

sysdeps/unix/sysv/linux/tst-getdents64.c

@@ -76,8 +76,18 @@ large_buffer_checks (int fd)
     }
 }
 
-static int
-do_test (void)
+static void
+do_test_large_size (void)
+{
+  int fd = xopen (".", O_RDONLY | O_DIRECTORY, 0);
+  TEST_VERIFY (fd >= 0);
+  large_buffer_checks (fd);
+
+  xclose (fd);
+}
+
+static void
+do_test_by_size (size_t buffer_size)
 {
   /* The test compares the iteration order with readdir64.  */
   DIR *reference = opendir (".");
@@ -98,7 +108,7 @@ do_test (void)
              non-existing data.  */
           struct
           {
-            char buffer[1024];
+            char buffer[buffer_size];
             struct dirent64 pad;
           } data;
 
@@ -153,10 +163,19 @@ do_test (void)
       rewinddir (reference);
     }
 
-  large_buffer_checks (fd);
-
   xclose (fd);
   closedir (reference);
+}
+
+static int
+do_test (void)
+{
+  do_test_by_size (512);
+  do_test_by_size (1024);
+  do_test_by_size (4096);
+
+  do_test_large_size ();
+
   return 0;
 }