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Date: Wed, 15 Jul 2015 16:39:22 -0700
From: Calvin Owens <calvinowens@fb.com>
To: Andrew Morton <akpm@linux-foundation.org>
CC: Alexey Dobriyan <adobriyan@gmail.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Al Viro <viro@zeniv.linux.org.uk>, Miklos Szeredi <miklos@szeredi.hu>,
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        <linux-kernel@vger.kernel.org>, <kernel-team@fb.com>,
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        Cyrill Gorcunov <gorcunov@openvz.org>,
        "Kirill A. Shutemov" <kirill@shutemov.name>
Subject: Re: [PATCH v7] procfs: Always expose /proc/<pid>/map_files/ and make
 it readable
Message-ID: <20150715233922.GA3509821@mail.thefacebook.com>
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On Wednesday 07/15 at 15:21 -0700, Andrew Morton wrote:
> On Thu, 18 Jun 2015 19:32:18 -0700 Calvin Owens <calvinowens@fb.com> wrote:
> 
> > Currently, /proc/<pid>/map_files/ is restricted to CAP_SYS_ADMIN, and
> > is only exposed if CONFIG_CHECKPOINT_RESTORE is set.
> > 
> > Each mapped file region gets a symlink in /proc/<pid>/map_files/
> > corresponding to the virtual address range at which it is mapped. The
> > symlinks work like the symlinks in /proc/<pid>/fd/, so you can follow
> > them to the backing file even if that backing file has been unlinked.
> > 
> > Currently, files which are mapped, unlinked, and closed are impossible
> > to stat() from userspace. Exposing /proc/<pid>/map_files/ closes this
> > functionality "hole".
> > 
> > Not being able to stat() such files makes noticing and explicitly
> > accounting for the space they use on the filesystem impossible. You can
> > work around this by summing up the space used by every file in the
> > filesystem and subtracting that total from what statfs() tells you, but
> > that obviously isn't great, and it becomes unworkable once your
> > filesystem becomes large enough.
> > 
> > This patch moves map_files/ out from behind CONFIG_CHECKPOINT_RESTORE,
> > and adjusts the permissions enforced on it as follows:
> 
> proc_pid_follow_link() got changed while you weren't looking, causing
> 
> fs/proc/base.c: In function 'proc_map_files_follow_link':
> fs/proc/base.c:1963: warning: passing argument 2 of 'proc_pid_follow_link' from incompatible pointer type
> fs/proc/base.c:1578: note: expected 'void **' but argument is of type 'struct nameidata *'
> fs/proc/base.c:1963: warning: return discards qualifiers from pointer target type
> fs/proc/base.c: At top level:
> fs/proc/base.c:1971: warning: initialization from incompatible pointer type
> 
> I just changed it to pass NULL:

Thanks for cleaning this up, I'll make sure to check outstanding patches
against new -rcs and -nexts in the future.

Thanks,
Calvin

> --- a/fs/proc/base.c~procfs-always-expose-proc-pid-map_files-and-make-it-readable-fix
> +++ a/fs/proc/base.c
> @@ -1955,12 +1955,13 @@ struct map_files_info {
>   * symlinks may be used to bypass permissions on ancestor directories in the
>   * path to the file in question.
>   */
> -static void *proc_map_files_follow_link(struct dentry *dentry, struct nameidata *nd)
> +static void *
> +proc_map_files_follow_link(struct dentry *dentry, struct nameidata *nd)
>  {
>  	if (!capable(CAP_SYS_ADMIN))
>  		return ERR_PTR(-EPERM);
>  
> -	return proc_pid_follow_link(dentry, nd);
> +	return proc_pid_follow_link(dentry, NULL);
>  }
>  
>  /*
> _
>