linux-kernel - Re: [PATCH v2] procfs: expose umask in /proc/<PID>/status

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Date:	Fri, 15 Apr 2016 15:13:10 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	"Richard W.M. Jones" <rjones@...hat.com>
Cc:	linux-kernel@...r.kernel.org, corbet@....net,
	akpm@...ux-foundation.org, vbabka@...e.cz, hughd@...gle.com,
	koct9i@...il.com, chenhanxiao@...fujitsu.com,
	n-horiguchi@...jp.nec.com, ross.zwisler@...ux.intel.com,
	john.stultz@...aro.org, minchan@...nel.org, jmarchan@...hat.com,
	hannes@...xchg.org, nathans@...hat.com,
	andriy.shevchenko@...ux.intel.com, keescook@...omium.org,
	gorcunov@...nvz.org, joe@...ches.com, linux@...musvillemoes.dk,
	mingo@...nel.org, cmetcalf@...hip.com, iago@...ocode.com,
	luto@...nel.org, linux-doc@...r.kernel.org, gorcunov@...il.com,
	fw@...eb.enyo.de, walters@...bum.org
Subject: Re: [PATCH v2] procfs: expose umask in /proc/<PID>/status

On Thu 14-04-16 12:08:15, Richard W.M. Jones 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.

It would be helpful to describe the usecase a bit better. Who needs to
read the umask and what for (e.g. what if the umask changes right after
it has been read by the 3rd party?).

I am not opposing the patch as is but this exports a new information to
the userspace and we will have to maintain it for ever, so we should
better describe the usecase.
 
> 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@...hat.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

-- 
Michal Hocko
SUSE Labs