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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     David Laight <David.Laight@ACULAB.COM>
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Date:   Fri, 27 Aug 2021 10:58:32 -0500
In-Reply-To: <04e61e79ebad4a5d872d0a2b5be4c23d@AcuMS.aculab.com> (David
        Laight's message of "Fri, 27 Aug 2021 08:22:07 +0000")
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Subject: Re: [PATCH v1 0/7] Remove in-tree usage of MAP_DENYWRITE
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David Laight <David.Laight@ACULAB.COM> 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