[PATCH v2 1/2] uapi: fcntl: guard AT_RENAME_* aliases

[Masaharu Noguchi <nogunix@gmail.com>]

Including <linux/fcntl.h> after libc headers such as stdio.h may leave
the renameat2() flag macros defined to libc's values.  That leaks the
wrong numbers into user space even though the kernel header tries to
provide its own aliases.

Check whether AT_RENAME_* is already defined and whether the value
matches what the uapi header expects.  If not, drop the old definition
and replace it with the kernel one so the exported flags stay stable
regardless of include order.

Signed-off-by: Masaharu Noguchi <nogunix@gmail.com>
---
 include/uapi/linux/fcntl.h | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index 3741ea1b73d8..8b667550e44a 100644
--- a/include/uapi/linux/fcntl.h
+++ b/include/uapi/linux/fcntl.h
@@ -156,10 +156,23 @@
  * as possible, so we can use them for generic bits in the future if necessary.
  */
 
-/* Flags for renameat2(2) (must match legacy RENAME_* flags). */
+/* Flags for renameat2(2) (must match legacy RENAME_* flags).
+ * stdio.h may define these differently, so check explicitly.
+ */
+#if !defined(AT_RENAME_NOREPLACE) || AT_RENAME_NOREPLACE != 0x0001
+#undef AT_RENAME_NOREPLACE
 #define AT_RENAME_NOREPLACE	0x0001
+#endif
+
+#if !defined(AT_RENAME_EXCHANGE) || AT_RENAME_EXCHANGE != 0x0002
+#undef AT_RENAME_EXCHANGE
 #define AT_RENAME_EXCHANGE	0x0002
+#endif
+
+#if !defined(AT_RENAME_WHITEOUT) || AT_RENAME_WHITEOUT != 0x0004
+#undef AT_RENAME_WHITEOUT
 #define AT_RENAME_WHITEOUT	0x0004
+#endif
 
 /* Flag for faccessat(2). */
 #define AT_EACCESS		0x200	/* Test access permitted for
-- 
2.51.1