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

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Message-ID: <20190521131849.2mguu5sszhbxhvgu@brauner.io>
Date:   Tue, 21 May 2019 15:18:50 +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,
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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.
> 
> > 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.

Oh, how would that be difficult? Maybe I'm missing context.
Couldn't you just do

fcntl(0, F_GET_NEXT)

> 
> >> 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.

No, that's perfectly fine. I mean, you clearly need this and are one of
the major stakeholders. For example, Rust (probably also Python) will
call down into libc and not use the syscall directly. They kinda do this
with getfdtable<sm> rn already.
So what you say makes sense for libc has some relevance for the other
tools as well.

Christian