procfs: expose umask in /proc/<PID>/status

It's not possible to read the process umask without also modifying it,
which is what umask(2) does.  A library cannot read umask safely,
especially if the main program might be multithreaded.

Add a new status line ("Umask") in /proc/<PID>/status.  It contains the
file mode creation mask (umask) in octal.  It is only shown for tasks
which have task->fs.

This patch is adapted from one originally written by Pierre Carrier.

The use case is that we have endless trouble with people setting weird
umask() values (usually on the grounds of "security"), and then
everything breaking.  I'm on the hook to fix these.  We'd like to add
debugging to our program so we can dump out the umask in debug reports.

Previous versions of the patch used a syscall so you could only read
your own umask.  That's all I need.  However there was quite a lot of
push-back from those, so this new version exports it in /proc.

See:
  https://lkml.org/lkml/2016/4/13/704 [umask2]
  https://lkml.org/lkml/2016/4/13/487 [getumask]

Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
Acked-by: Konstantin Khlebnikov <koct9i@gmail.com>
Acked-by: Jerome Marchand <jmarchan@redhat.com>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Pierre Carrier <pierre@spotify.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index 7f5607a089b447eb2f741ff544db9eefe30f44fc..e8d00759bfa50fc34dd088e3be9af77ff1ad55c0 100644 (file)
--- a/Documentation/filesystems/proc.txt
+++ b/Documentation/filesystems/proc.txt
@@ -225,6 +225,7 @@ Table 1-2: Contents of the status files (as of 4.1)
  TracerPid                   PID of process tracing this process (0 if not)
  Uid                         Real, effective, saved set, and  file system UIDs
  Gid                         Real, effective, saved set, and  file system GIDs
+ Umask                       file mode creation mask
  FDSize                      number of file descriptor slots currently allocated
  Groups                      supplementary group list
  NStgid                      descendant namespace thread group ID hierarchy

diff --git a/fs/proc/array.c b/fs/proc/array.c
index b6c00ce0e29e3563037c16fe7bbc754b396f606f..88c7de12197bd763c452b7770fcd19d15dcb3df1 100644 (file)
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -83,6 +83,7 @@
 #include <linux/tracehook.h>
 #include <linux/string_helpers.h>
 #include <linux/user_namespace.h>
+#include <linux/fs_struct.h>
 
 #include <asm/pgtable.h>
 #include <asm/processor.h>
@@ -139,12 +140,25 @@ static inline const char *get_task_state(struct task_struct *tsk)
        return task_state_array[fls(state)];
 }
 
+static inline int get_task_umask(struct task_struct *tsk)
+{
+       struct fs_struct *fs;
+       int umask = -ENOENT;
+
+       task_lock(tsk);
+       fs = tsk->fs;
+       if (fs)
+               umask = fs->umask;
+       task_unlock(tsk);
+       return umask;
+}
+
 static inline void task_state(struct seq_file *m, struct pid_namespace *ns,
                                struct pid *pid, struct task_struct *p)
 {
        struct user_namespace *user_ns = seq_user_ns(m);
        struct group_info *group_info;
-       int g;
+       int g, umask;
        struct task_struct *tracer;
        const struct cred *cred;
        pid_t ppid, tpid = 0, tgid, ngid;
@@ -162,6 +176,10 @@ static inline void task_state(struct seq_file *m, struct pid_namespace *ns,
        ngid = task_numa_group_id(p);
        cred = get_task_cred(p);
 
+       umask = get_task_umask(p);
+       if (umask >= 0)
+               seq_printf(m, "Umask:\t%#04o\n", umask);
+
        task_lock(p);
        if (p->files)
                max_fds = files_fdtable(p->files)->max_fds;