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Subject: Re: [PATCH v2] procfs: expose umask in /proc/<PID>/status
From: Kees Cook <keescook@chromium.org>
To: "Richard W.M. Jones" <rjones@redhat.com>
Cc: LKML <linux-kernel@vger.kernel.org>, Jonathan Corbet <corbet@lwn.net>,
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On Thu, Apr 14, 2016 at 4:08 AM, Richard W.M. Jones <rjones@redhat.com> wrote:
> 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>

Acked-by: Kees Cook <keescook@chromium.org>

-Kees

> ---
>  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
>



-- 
Kees Cook
Chrome OS & Brillo Security