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SubjectRe: Anonymous inode cleanup?
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Hi Al,

Thanks a lot! That was very helpful. I have one follow up question though.

On Thu, Nov 12, 2015 at 10:31 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
> On Thu, Nov 12, 2015 at 09:43:00PM -0800, Rajat Jain wrote:
>> Hello,
>>
>> I'm writing a module that wants to get anonymous fd [using
>> anon_inode_getfd()] and my code looks like this:
>>
>> fd = anon_inode_getfd(...)
>> if (fd < 0)
>> return -EINVAL;
>>
>> if (foobar_fail()) {
>> /* undo everything */
>> return -EINVAL;
>> }
>>
>> My question is that in case of a failure after the anon_inode_getfd(),
>> I want to cleanup and undo whatever needs to be done w.r.t. anodnymous
>> fd I just allocated. (May be put a reference, or return the fd to the
>> free pool or whatever). Can some one please let me know what cleanup
>> needs to be done?
>>
>> However neither I see a cleanup function, nor I see any of the drivers
>> attempting
>> to free the fd in case of failure.
>
> You can't. As soon as it's in descriptor table, you'd better be *done*
> with it. No "I need more setup done", no "I just need to do one final
> check" - the moment it hits the descriptor table, another thread might
> be issuing syscalls on it. Including dup2(), so there's no way to take
> it back. Moreover, another thread might've done dup2() over your
> descriptor, so you can't even decide to close the one you'd just installed.
> Yes, even in cases when the failed action would be to report the resulting
> descriptor to userland. Generally you should try to return descriptors to
> userland only via the syscall return value.
>
> _If_ you are returning them via a sucky API, the right sequence is
> reserve the descriptor(s)
> set the file(s) up
> fill whatever structure you'll be using to report descriptors to
> userland and copy it to userland memory

If this step fails, what is the cleanup needed for "set the files up"
anon_inode_getfile() step? is it fput()?

Thanks,

Rajat

> use fd_install() to put files into descriptor table.
>
> See e.g. fs/pipe.c and look for pipe2 in there for example of dealing with
> such APIs.
>
> "Set the file up" primitive in case of anon_inode is anon_inode_getfile();
> grep and you shall see...
>
> Again, fd_install() is the equivalent of hitting "send" - there's no way
> to make what you've published disappear. It's the point of no return.


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