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

[Eric W. Biederman]

David Laight <David.Laight@xxxxxxxxxx> writes:

> 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'.

It it task_work_add(current).  Which punts the work to the return to
userspace.

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

Eric