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Subject: Re: [PATCH] procfs: expose umask in /proc/<PID>/status
To: "Richard W.M. Jones" <rjones@redhat.com>, corbet@lwn.net
References: <1460626489-31932-1-git-send-email-rjones@redhat.com>
 <1460626489-31932-2-git-send-email-rjones@redhat.com>
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From: Jerome Marchand <jmarchan@redhat.com>
Message-ID: <570F6F37.50209@redhat.com>
Date: Thu, 14 Apr 2016 12:21:43 +0200
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From: Jerome Marchand <jmarchan@redhat.com>
To: "Richard W.M. Jones" <rjones@redhat.com>, corbet@lwn.net
Cc: akpm@linux-foundation.org, vbabka@suse.cz, mhocko@suse.com,
 hughd@google.com, koct9i@gmail.com, chenhanxiao@cn.fujitsu.com,
 n-horiguchi@ah.jp.nec.com, ross.zwisler@linux.intel.com,
 john.stultz@linaro.org, minchan@kernel.org, hannes@cmpxchg.org,
 nathans@redhat.com, andriy.shevchenko@linux.intel.com,
 keescook@chromium.org, gorcunov@openvz.org, joe@perches.com,
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Message-ID: <570F6F37.50209@redhat.com>
Subject: Re: [PATCH] procfs: expose umask in /proc/<PID>/status
References: <1460626489-31932-1-git-send-email-rjones@redhat.com>
 <1460626489-31932-2-git-send-email-rjones@redhat.com>
In-Reply-To: <1460626489-31932-2-git-send-email-rjones@redhat.com>

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On 04/14/2016 11:34 AM, 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.
>=20
> 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.
>=20
> This patch is adapted from one originally written by Pierre Carrier.
>=20
> 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(-)
>=20
> diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesys=
tems/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 sys=
tem UIDs
>   Gid                         Real, effective, saved set, and  file sys=
tem GIDs
> + Umask                       file mode creation mask
>   FDSize                      number of file descriptor slots currently=
 allocated
>   Groups                      supplementary group list
>   NStgid                      descendant namespace thread group ID hier=
archy
> diff --git a/fs/proc/array.c b/fs/proc/array.c
> index b6c00ce..03e8d3f 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>
> =20
>  #include <asm/pgtable.h>
>  #include <asm/processor.h>
> @@ -139,12 +140,25 @@ static inline const char *get_task_state(struct t=
ask_struct *tsk)
>  	return task_state_array[fls(state)];
>  }
> =20
> +static inline int get_task_umask(struct task_struct *tsk)
> +{
> +	struct fs_struct *fs;
> +	int umask =3D -ENOENT;
> +
> +	task_lock(tsk);
> +	fs =3D tsk->fs;
> +	if (fs)
> +		umask =3D 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 =3D 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 =3D 0, tgid, ngid;
> @@ -162,6 +176,10 @@ static inline void task_state(struct seq_file *m, =
struct pid_namespace *ns,
>  	ngid =3D task_numa_group_id(p);
>  	cred =3D get_task_cred(p);
> =20
> +	umask =3D get_task_umask(p);
> +	if (umask >=3D 0)
> +		seq_printf(m, "Umask:\t0%o\n", umask);

It seems to me that umasks are usually displayed in the form 0XXX, such
as the output of umask command. So what about:

seq_printf(m, "Umask:\t%#04o\n", umask);

Provided printk() supports those flags, of course.

Thanks,
Jerome

> +
>  	task_lock(p);
>  	if (p->files)
>  		max_fds =3D files_fdtable(p->files)->max_fds;
>=20



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