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Subject[PATCH v2] procfs: expose umask in /proc/<PID>/status
Date
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.

Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
---
Documentation/filesystems/proc.txt | 1 +
fs/proc/array.c | 20 +++++++++++++++++++-
2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index 7f5607a..e8d0075 100644
--- 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 b6c00ce..88c7de1 100644
--- 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;
--
2.7.4
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