Re: [PATCH RFC POC 01/50] file: install files on syscall exit

So we've spoken about this idea quite a few times over the years that
we could reserve fds and files on a task and then install or a clean
them up on syscall success or error and get rid of most of the
complicated cleanup dance that we have.

My original approach to this had been to implement FD_PREPARE() based on
cleanup guards and use scopes to make this happen. The other idea was
what I'm illustrating here. It is overall equally robust and makes the
install fd and file pattern work even if it there's a ton of cleanup or
setup work happening in the middle. This is particular evident in the
conversions of various dma/drm code later in the series.

So this lets the syscall path do the install. fd_prepare() allocates a
descriptor like get_unused_fd_flags() does and records it in a slot on
the task. fd_stage() attaches the file to that slot and returns the
[ ... ]
fdtagble with outstanding reservations. Kernel threads never return to
userspace so nothing would commit. Anything left at exit is a bug and
gets warned about and dropped.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

Sashiko has reviewed this patch and found no issues. It looks great!

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Sashiko AI review · https://sashiko.dev/#/patchset/20260915-work-fd-reserve-unify-folded-v1-0-4d5217d6b246@kernel.org?part=1