On March 31, 2026 1:25:03 PM PDT, Yann Droneaud <[email protected]> wrote: >Hi, > >Le 31/03/2026 à 19:19, Jori Koolstra a écrit : >> Currently there is no way to race-freely create and open a directory. >> For regular files we have open(O_CREAT) for creating a new file inode, >> and returning a pinning fd to it. The lack of such functionality for >> directories means that when populating a directory tree there's always >> a race involved: the inodes first need to be created, and then opened >> to adjust their permissions/ownership/labels/timestamps/acls/xattrs/..., >> but in the time window between the creation and the opening they might >> be replaced by something else. >> >> Addressing this race without proper APIs is possible (by immediately >> fstat()ing what was opened, to verify that it has the right inode type), >> but difficult to get right. Hence, mkdirat_fd() that creates a directory >> and returns an O_DIRECTORY fd is useful. >> >> This feature idea (and description) is taken from the UAPI group: >> https://github.com/uapi-group/kernel-features?tab=readme-ov-file#race-free-creation-and-opening-of-non-file-inodes >> >> Signed-off-by: Jori Koolstra <[email protected]> >> --- >> arch/x86/entry/syscalls/syscall_64.tbl | 1 + >> fs/internal.h | 1 + >> fs/namei.c | 26 ++++++++++++++++++++++++-- >> include/linux/fcntl.h | 2 ++ >> include/linux/syscalls.h | 2 ++ >> include/uapi/asm-generic/fcntl.h | 3 +++ >> include/uapi/asm-generic/unistd.h | 5 ++++- >> scripts/syscall.tbl | 1 + >> 8 files changed, 38 insertions(+), 3 deletions(-) > >> diff --git a/include/linux/fcntl.h b/include/linux/fcntl.h >> index a332e79b3207..d2f0fdb82847 100644 >> --- a/include/linux/fcntl.h >> +++ b/include/linux/fcntl.h >> @@ -25,6 +25,8 @@ >> #define force_o_largefile() (!IS_ENABLED(CONFIG_ARCH_32BIT_OFF_T)) >> #endif >> +#define VALID_MKDIRAT_FD_FLAGS (MKDIRAT_FD_NEED_FD) >> + > >I don't see support for O_CLOEXEC-ish flag, is the file descriptor in >close-on-exec mode by default ? If yes, it should be mentioned. > > >> diff --git a/include/uapi/asm-generic/fcntl.h >> b/include/uapi/asm-generic/fcntl.h >> index 613475285643..621458bf1fbf 100644 >> --- a/include/uapi/asm-generic/fcntl.h >> +++ b/include/uapi/asm-generic/fcntl.h >> @@ -95,6 +95,9 @@ >> #define O_NDELAY O_NONBLOCK >> #endif >> +/* Flags for mkdirat_fd */ >> +#define MKDIRAT_FD_NEED_FD 0x01 >> + > > >Regards. > >
And even if it is, POSIX already has O_CLOFORK and we should expect that that will be needed, too.
