linux-kernel - Re: [PATCH v5 1/3] open: add close

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Message-ID: <20200602233749.mo65o2enokiypwiz@wittgenstein>
Date:   Wed, 3 Jun 2020 01:37:49 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Florian Weimer <fw@...eb.enyo.de>
Cc:     torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
        Kyle Evans <self@...e-evans.net>,
        Victor Stinner <victor.stinner@...il.com>,
        viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
        linux-api@...r.kernel.org, fweimer@...hat.com, jannh@...gle.com,
        oleg@...hat.com, arnd@...db.de, shuah@...nel.org,
        dhowells@...hat.com, ldv@...linux.org
Subject: Re: [PATCH v5 1/3] open: add close_range()

On Wed, Jun 03, 2020 at 01:30:57AM +0200, Florian Weimer wrote:
> * Christian Brauner:
> 
> > The performance is striking. For good measure, comparing the following
> > simple close_all_fds() userspace implementation that is essentially just
> > glibc's version in [6]:
> >
> > static int close_all_fds(void)
> > {
> >         int dir_fd;
> >         DIR *dir;
> >         struct dirent *direntp;
> >
> >         dir = opendir("/proc/self/fd");
> >         if (!dir)
> >                 return -1;
> >         dir_fd = dirfd(dir);
> >         while ((direntp = readdir(dir))) {
> >                 int fd;
> >                 if (strcmp(direntp->d_name, ".") == 0)
> >                         continue;
> >                 if (strcmp(direntp->d_name, "..") == 0)
> >                         continue;
> >                 fd = atoi(direntp->d_name);
> >                 if (fd == dir_fd || fd == 0 || fd == 1 || fd == 2)
> >                         continue;
> >                 close(fd);
> >         }
> >         closedir(dir);
> >         return 0;
> > }
> >
> 
> > [6]: https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/grantpt.c;h=2030e07fa6e652aac32c775b8c6e005844c3c4eb;hb=HEAD#l17
> >      Note that this is an internal implementation that is not exported.
> >      Currently, libc seems to not provide an exported version of this
> >      because of missing kernel support to do this.
> 
> Just to be clear, this code is not compiled into glibc anymore in
> typical configurations.  I have posted a patch to turn grantpt into a
> no-op: <https://sourceware.org/pipermail/libc-alpha/2020-May/114379.html>

That's great! (I remember commenting on that thread.)