Re: [PATCH RFC POC 00/50] file: handle files on syscall exit

[Linus Torvalds]

On Tue, 15 Sept 2026 at 04:31, Christian Brauner <brauner@xxxxxxxxxx> wrote:
>
> TL;DR, this lets arch code handle fd install and cleanup. 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 number.

Honestly, I am *not* a fan. At all.

I think this adds complexity rather than removing it.

The diffstat tells a story:

>  106 files changed, 962 insertions(+), 861 deletions(-)

in how this adds a hundred lines more than it deletes.

But to make things worse, it adds them in bad places: low-level
architecture assembly code.

Those extra lines are *not* making complicated code simpler. Quite the
reverse. They are taking fairly straightforward "just deal with errors
with normal cleanup in the place where it makes sense" and makes the
*straightforward* case simpler, while making the big picture more
complicated and subtle, and adding new code to places that are not
simple and just blindly do somethign that makes no sense in that
context.

IOW: it's more code, and it's more abstraction, and it DOES NOT HELP.

And yes, most of the well-maintained modern platforms (read: x86 and
arm64) use the generic infrastructure and they don't show that
low-level asm effect. So for those cases, it's mainly just  that added
conceptual complexity and a new odd rule - and makes a *successful*
system call go through that "extra work" and indirection phase that it
didn't use to go through.

So it just spreads out the work, and moves it to a less obvious place.

All for the very questionable advantage that you can now randomly add
a file descriptor without the few lines of fairly straightforward
rollback.

IOW, I do not see a single real upside to this, and I see immediate
downsides to it, and a more abstract complicated machinery.

I do *not* see why we woudl want to make this be a "every system call
exit" kind of thing. If there are places that think it's complicated
to do, they could have *their* local little stack of fd's pending. Why
force it onto the low-level system call entry for every system call
when there are only a small handful of actual real cases (the vfs
itself, and random ioctl's by odd driver subsystems).

              Linus