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

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

[Richard W.M. Jones]

v1 -> v2:

 - Change printf format to %#04o.

 - Retest and update examples accordingly.

----------

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.

For the library this allows me to read the umask from
/proc/self/status.

This patch is adapted from one originally written by Pierre Carrier:
https://lkml.org/lkml/2012/5/4/451

Example usage:

$ grep Umask /proc/1/status 
Umask:	0022
$ grep Umask /proc/2/status 
Umask:	0022
$ grep Umask /proc/self/status 
Umask:	0022
$ umask 002
$ grep Umask /proc/self/status 
Umask:	0002

Rich.

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

[Richard W.M. Jones]

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

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

[Konstantin Khlebnikov]

On Thu, Apr 14, 2016 at 2:08 PM, 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: Konstantin Khlebnikov <koct9i@gmail.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
>

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

[Jerome Marchand]

[-- Attachment #1.1: Type: text/plain, Size: 3093 bytes --]

On 04/14/2016 01:08 PM, 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.
> 
> 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: Jerome Marchand <jmarchan@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;
> 



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

[Michal Hocko]

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@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

-- 
Michal Hocko
SUSE Labs

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

[Richard W.M. Jones]

On Fri, Apr 15, 2016 at 03:13:10PM +0200, Michal Hocko wrote:
> 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.

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]

Rich.

> > 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
> 
> -- 
> Michal Hocko
> SUSE Labs

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
Fedora Windows cross-compiler. Compile Windows programs, test, and
build Windows installers. Over 100 libraries supported.
http://fedoraproject.org/wiki/MinGW

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

[Theodore Ts'o]

On Fri, Apr 15, 2016 at 02:29:52PM +0100, Richard W.M. Jones wrote:
> 
> 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]

I think /proc is better because it's already pretty well understood
that programs like "ps" returns information which is subject to
change, even before the ps command has had a chance to print the
values.  Where as I could easily see programmers who read umask via a
system call could more easily get confused about the expectations of
stability of the returned value.  (This is not a strong argument,
admittedly, because the same stupid application programmer could get
confused about the stability of getuid() as well.)

The stronger argument IMHO is that you're not at the mercy of glibc
developers to add it to glibc.

					- Ted

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

[Kees Cook]

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