RE: [PATCH v1 0/7] Remove in-tree usage of MAP_DENYWRITE

David Laight
2021-08-27T08:22:14Z

From: Eric W. Biederman
> Sent: 26 August 2021 23:14
...
> I also ran into this issue not too long ago when I refactored the
> usermode_driver code. My challenge was not being in userspace
> the delayed fput was not happening in my kernel thread. Which meant
> that writing the file, then closing the file, then execing the file
> consistently reported -ETXTBSY.
>
> The kernel code wound up doing:
> /* Flush delayed fput so exec can open the file read-only */
> flush_delayed_fput();
> task_work_run();
>
> As I read the code the delay for userspace file descriptors is
> always done with task_work_add, so userspace should not hit
> that kind of silliness, and should be able to actually close
> the file descriptor before the exec.

If task_work_add ends up adding it to a task that is already
running on a different cpu, and that cpu takes a hardware
interrupt that takes some time and/or schedules the softint
code to run immediately the hardware interrupt completes
then it may well be possible for userspace to have 'issues'.

Any flags associated with O_DENY_WRITE would need to be cleared
synchronously in the close() rather then in any delayed fput().

David

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