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

[David Laight <david.laight.linux@gmail.com>]

On Mon, 10 Nov 2025 23:42:31 +0900
Masaharu Noguchi <nogunix@gmail.com> wrote:

> 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

I deliberately left out the #undef so you'd get a compile-time error
if the values of the definitions differ.

	David

>  #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