Re: [PATCH v7] procfs: Always expose /proc/<pid>/map_files/ and make it readable

[Andrew Morton <akpm@linux-foundation.org>]

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:

--- 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);
 }
 
 /*
_