On 2026-01-14, Andy Lutomirski [off-list ref] wrote:

On Wed, Jan 14, 2026 at 8:09 AM Christian Brauner [off-list ref] wrote:

quoted

On Tue, Jan 13, 2026 at 11:40:55PM +0100, Florian Weimer wrote:

quoted

In <linux/mount.h>, we have this:

#define OPEN_TREE_CLOEXEC      O_CLOEXEC       /* Close the file on execve() */

This causes a few pain points for us to on the glibc side when we mirror
this into <linux/mount.h> becuse O_CLOEXEC is defined in <fcntl.h>,
which is one of the headers that's completely incompatible with the UAPI
headers.

The reason why this is painful is because O_CLOEXEC has at least three
different values across architectures: 0x80000, 0x200000, 0x400000

Even for the UAPI this isn't ideal because it effectively burns three
open_tree flags, unless the flags are made architecture-specific, too.

I think that just got cargo-culted... A long time ago some API define as
O_CLOEXEC and now a lot of APIs have done the same. I'm pretty sure we
can't change that now but we can document that this shouldn't be ifdefed
and instead be a separate per-syscall bit. But I think that's the best
we can do right now.

How about, for future syscalls, we make CLOEXEC unconditional?  If
anyone wants an ofd to get inherited across exec, they can F_SETFD it
themselves.

I believe newer interfaces have already started doing that (e.g., all of
the pidfd stuff is O_CLOEXEC by default) but we should definitely update
the documentation in Documentation/process/adding-syscalls.rst to stop
recommending the inclusion of the O_CLOEXEC flag.

The funniest thing about open_tree(2) is that it actually borrows flag
bits from three distinct namespaces! It has an OPEN_TREE_* namespace,
the AT_* namespace (which now has a concept of "per-syscall flags"), and
O_CLOEXEC. What a fun interface!

-- 
Aleksa Sarai
https://www.cyphar.com/