linux-kernel - Re: [PATCH 1/2] open: add close

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Date:   Tue, 21 May 2019 15:23:23 +0200
From:   Christian Brauner <christian@...uner.io>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     viro@...iv.linux.org.uk, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
        jannh@...gle.com, oleg@...hat.com, tglx@...utronix.de,
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Subject: Re: [PATCH 1/2] open: add close_range()

On Tue, May 21, 2019 at 03:10:11PM +0200, Florian Weimer wrote:
> * Christian Brauner:
> 
> >> Solaris has an fdwalk function:
> >> 
> >>   <https://docs.oracle.com/cd/E88353_01/html/E37843/closefrom-3c.html>
> >> 
> >> So a different way to implement this would expose a nextfd system call
> >
> > Meh. If nextfd() then I would like it to be able to:
> > - get the nextfd(fd) >= fd
> > - get highest open fd e.g. nextfd(-1)
> 
> The highest open descriptor isn't istering for fdwalk because nextfd
> would just fail.

Sure. I was thinking about other usecases. For example, sometimes in
userspace you want to do the following:
save_fd = dup(fd, <well-known-number-at-the-end-of-the-range);
close_range(3, (save_fd - 1));

Which brings me to another point. So even if we don't do close_range() I
would like libc to maybe give us something like close_range() for such
scenarios.

> 
> > But then I wonder if nextfd() needs to be a syscall and isn't just
> > either:
> > fcntl(fd, F_GET_NEXT)?
> > or
> > prctl(PR_GET_NEXT)?
> 
> I think the fcntl route is a bit iffy because you might need it to get
> the *first* valid descriptor.
> 
> >> to userspace, so that we can use that to implement both fdwalk and
> >> closefrom.  But maybe fdwalk is just too obscure, given the existence of
> >> /proc.
> >
> > Yeah we probably don't need fdwalk.
> 
> Agreed.  Just wanted to bring it up for completeness.  I certainly don't
> want to derail the implementation of close_range.
> 
> Thanks,
> Florian