xfs: avoid null *src in memcpy call in xlog_write

The gcc undefined behavior sanitizer caught this; surely
any sane memcpy implementation will no-op if size == 0,
but behavior with a *src of NULL is technically undefined
(declared nonnull), so avoid it here.

We are actually in this situation frequently via
xlog_commit_record(), because:

        struct xfs_log_iovec reg = {
                .i_addr = NULL,
                .i_len = 0,
                .i_type = XLOG_REG_TYPE_COMMIT,
        };

Reported-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
index 0c8ef76..af08326 100644 (file)
--- a/fs/xfs/xfs_log.c
+++ b/fs/xfs/xfs_log.c
@@ -2422,11 +2422,20 @@ xlog_write(
                                                     &partial_copy_len);
                        xlog_verify_dest_ptr(log, ptr);
 
-                       /* copy region */
+                       /*
+                        * Copy region.
+                        *
+                        * Unmount records just log an opheader, so can have
+                        * empty payloads with no data region to copy. Hence we
+                        * only copy the payload if the vector says it has data
+                        * to copy.
+                        */
                        ASSERT(copy_len >= 0);
-                       memcpy(ptr, reg->i_addr + copy_off, copy_len);
-                       xlog_write_adv_cnt(&ptr, &len, &log_offset, copy_len);
-
+                       if (copy_len > 0) {
+                               memcpy(ptr, reg->i_addr + copy_off, copy_len);
+                               xlog_write_adv_cnt(&ptr, &len, &log_offset,
+                                                  copy_len);
+                       }
                        copy_len += start_rec_copy + sizeof(xlog_op_header_t);
                        record_cnt++;
                        data_cnt += contwr ? copy_len : 0;