[PATCH] compat getdents64: Use correct offset for retry [BZ #23972]

[Florian Weimer <fweimer at redhat dot com>]

d_off is the offset of the *next* entry, not the offset of the current
entry.

2018-12-10  Florian Weimer  <fweimer@redhat.com>

	[BZ #23972]
	* sysdeps/unix/sysv/linux/getdents64.c (handle_overflow): Check
	offset instead of count for clarity.  Fix typo in comment.
	(__old_getdents64): Keep track of previous offset.  Use it to call
	handle_overflow.
	* sysdeps/unix/sysv/linux/tst-readdir64-compat.c (do_test): Check
	that d_off is never zero.

diff --git a/sysdeps/unix/sysv/linux/getdents64.c b/sysdeps/unix/sysv/linux/getdents64.c
index bc140b5a7f..46eb5f4419 100644
--- a/sysdeps/unix/sysv/linux/getdents64.c
+++ b/sysdeps/unix/sysv/linux/getdents64.c
@@ -41,14 +41,14 @@ handle_overflow (int fd, __off64_t offset, ssize_t count)
 {
   /* If this is the first entry in the buffer, we can report the
      error.  */
-  if (count == 0)
+  if (offset == 0)
     {
       __set_errno (EOVERFLOW);
       return -1;
     }
 
   /* Otherwise, seek to the overflowing entry, so that the next call
-     will report the error, and return the data read so far..  */
+     will report the error, and return the data read so far.  */
   if (__lseek64 (fd, offset, SEEK_SET) != 0)
     return -1;
   return count;
@@ -70,6 +70,15 @@ __old_getdents64 (int fd, char *buf, size_t nbytes)
   ssize_t retval = INLINE_SYSCALL_CALL (getdents64, fd, buf, nbytes);
   if (retval > 0)
     {
+      /* This is the marker for the first entry.  Offset 0 is reserved
+	 for the first entry (see rewinddir).  Here, we use it as a
+	 marker for the first entry in the buffer.  We never actually
+	 seek to offset 0 because handle_overflow reports the error
+	 directly, so it does not matter that the offset is incorrect
+	 if entries have been read from the descriptor before (so that
+	 the descriptor is not actually at offset 0).  */
+      __off64_t previous_offset = 0;
+
       char *p = buf;
       char *end = buf + retval;
       while (p < end)
@@ -84,7 +93,7 @@ __old_getdents64 (int fd, char *buf, size_t nbytes)
 
 	  /* Check for ino_t overflow.  */
 	  if (__glibc_unlikely (ino != source->d_ino))
-	    return handle_overflow (fd, offset, p - buf);
+	    return handle_overflow (fd, previous_offset, p - buf);
 
 	  /* Convert to the target layout.  Use a separate struct and
 	     memcpy to side-step aliasing issues.  */
@@ -107,6 +116,7 @@ __old_getdents64 (int fd, char *buf, size_t nbytes)
 		     reclen - offsetof (struct dirent64, d_name));
 
 	  p += reclen;
+	  previous_offset = offset;
 	}
      }
   return retval;
diff --git a/sysdeps/unix/sysv/linux/tst-readdir64-compat.c b/sysdeps/unix/sysv/linux/tst-readdir64-compat.c
index 43c4a8477c..cb78bc9be4 100644
--- a/sysdeps/unix/sysv/linux/tst-readdir64-compat.c
+++ b/sysdeps/unix/sysv/linux/tst-readdir64-compat.c
@@ -88,6 +88,10 @@ do_test (void)
       else
         TEST_VERIFY_EXIT (entry_test != NULL);
 
+      /* d_off is never zero because it is the offset of the next
+         entry (not the current entry).  */
+      TEST_VERIFY (entry_reference->d_off > 0);
+
       /* Check that the entries are the same.  */
       TEST_COMPARE_BLOB (entry_reference->d_name,
                          strlen (entry_reference->d_name),