Re: kern/59498: Add missing POSIX O_CLOFORK flag

[Robert Elz <kre%munnari.OZ.AU@localhost>]

    Date:        Tue, 1 Jul 2025 19:20:47 +0200
    From:        Ricardo Branco <rbranco%suse.de@localhost>
    Message-ID:  <78c4992b-b5b6-493b-8eb2-594df8990ad6%suse.de@localhost>

  | In this implementation, O_CLOFORK is cleared on exec,

That's fine, but not related to what I asked.

Typically O_CLOFORK is set by library functions to guard against
possible other threads forking while a temporary fd is open.
Those temporary fd's can last for noticeable time, and can be
revealed to the application code.

The application might want to see if close-on-exec has been set
for the fd (for some reason) and use fcntl(F_GETFD) to do it, and
never having heard of O_CLOFORK (or FD_CLOFORK) simply assumes that
the non-zero return means O_CLOEXEC is set on the fd.

I think we need an audit of applications (which includes library code
that they might call) to examine all fcntl(F_GETFD) (and fcntl(F_SETFD))
calls, and make sure they are doing the right thing, before the
O_CLOFORK mechanism is exposed to user space in any way (it doesn't hurt
to have it in the kernel, as long as nothing, tests excepted, ever
sets it).

kre