[PATCH] sched: share task comm length value to userspace

[PATCH] sched: share task comm length value to userspace

[Vladimir Zapolskiy]

This change allows to get a defined length of task comm in
userspace. For a moment this value is implied at least in two cases:
* max length of comm value got from /proc/$pid/stat
* max string length argument of prctl(PR_[SG]ET_NAME, ...)

For these reasons it becomes desirable to impart task comm max length
value to userspace.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
---
 include/linux/sched.h |    6 +++---
 1 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4ac2c05..21d300b 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -42,6 +42,9 @@
 /* Can be ORed in to make sure the process is reverted back to SCHED_NORMAL on fork */
 #define SCHED_RESET_ON_FORK     0x40000000
 
+/* Task command name length */
+#define TASK_COMM_LEN 16
+
 #ifdef __KERNEL__
 
 struct sched_param {
@@ -242,9 +245,6 @@ extern char ___assert_task_state[1 - 2*!!(
 #define set_current_state(state_value)		\
 	set_mb(current->state, (state_value))
 
-/* Task command name length */
-#define TASK_COMM_LEN 16
-
 #include <linux/spinlock.h>
 
 /*
-- 
1.7.5.4

Re: [PATCH] sched: share task comm length value to userspace

[Arjan van de Ven]

On Mon,  5 Sep 2011 21:38:01 +0300
Vladimir Zapolskiy <vz@mleia.com> wrote:

> This change allows to get a defined length of task comm in
> userspace. For a moment this value is implied at least in two cases:
> * max length of comm value got from /proc/$pid/stat
> * max string length argument of prctl(PR_[SG]ET_NAME, ...)
> 
> For these reasons it becomes desirable to impart task comm max length
> value to userspace.

I'm sorry but I don't like doing this.

The moment we do this, we never can increase it anymore since userspace
might be using it for its own uses and we'd break it if we changed
this value.

If we need to export this, we should export it in /proc or /sys
somewhere as a dynamic value.....



-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org

Re: [PATCH] sched: share task comm length value to userspace

[Vladimir Zapolskiy]

On 05.09.2011 22:47, Arjan van de Ven wrote:
> On Mon,  5 Sep 2011 21:38:01 +0300
> Vladimir Zapolskiy<vz@mleia.com>  wrote:
>
>> This change allows to get a defined length of task comm in
>> userspace. For a moment this value is implied at least in two cases:
>> * max length of comm value got from /proc/$pid/stat
>> * max string length argument of prctl(PR_[SG]ET_NAME, ...)
>>
>> For these reasons it becomes desirable to impart task comm max length
>> value to userspace.
>
> I'm sorry but I don't like doing this.
>
> The moment we do this, we never can increase it anymore since userspace
> might be using it for its own uses and we'd break it if we changed
> this value.
Thanks for a comment. Your objections are quite clear, and I don't like 
this solution as well, but eventually having that value in userspace is 
really desirable. However in assumption that this value is preserved 
fixed and won't be changed in reasonable perspective, the presented 
approach becomes the most evident and simple.

>
> If we need to export this, we should export it in /proc or /sys
> somewhere as a dynamic value.....
>
Let's do it in a proper way. What's the common practice to export such 
limit values? I suppose via /proc/sys/kernel, please correct me, if I'm 
wrong.

--
With best wishes,
Vladimir

Re: [PATCH] sched: share task comm length value to userspace

[Arjan van de Ven]

On Mon, 05 Sep 2011 23:17:05 +0300
Vladimir Zapolskiy <vz@mleia.com> wrote:

> but eventually having that value in
> userspace is really desirable. However in assumption that this value
> is preserved fixed and won't be changed in reasonable perspective,
> the presented approach becomes the most evident and simple.

yeah if it would never change, your change would be the most simple.
but I really don't want to pin that thing down....

> 
> >
> > If we need to export this, we should export it in /proc or /sys
> > somewhere as a dynamic value.....
> >
> Let's do it in a proper way. What's the common practice to export
> such limit values? I suppose via /proc/sys/kernel, please correct me,
> if I'm wrong.

that or /sys/kernel

personally I like /proc/sys/kernel for this kind of thing more, but...
... others might disagree.


-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org

Re: [PATCH] sched: share task comm length value to userspace

[Andrew Morton]

On Mon, 5 Sep 2011 13:39:03 -0700
Arjan van de Ven <arjan@infradead.org> wrote:

> On Mon, 05 Sep 2011 23:17:05 +0300
> Vladimir Zapolskiy <vz@mleia.com> wrote:
> 
> > but eventually having that value in
> > userspace is really desirable. However in assumption that this value
> > is preserved fixed and won't be changed in reasonable perspective,
> > the presented approach becomes the most evident and simple.
> 
> yeah if it would never change, your change would be the most simple.
> but I really don't want to pin that thing down....
> 
> > 
> > >
> > > If we need to export this, we should export it in /proc or /sys
> > > somewhere as a dynamic value.....
> > >
> > Let's do it in a proper way. What's the common practice to export
> > such limit values? I suppose via /proc/sys/kernel, please correct me,
> > if I'm wrong.
> 
> that or /sys/kernel
> 
> personally I like /proc/sys/kernel for this kind of thing more, but...
> ... others might disagree.
> 

Run this:

#include <sys/prctl.h>
#include <errno.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>

main()
{
	char comm[512];
	int len;

	memset(comm, 0, sizeof(comm));
	for (len = 1; len < sizeof(comm) - 1; len++) {
		char comm2[512];
	
		comm[len - 1] = 'a';
		if (prctl(PR_SET_NAME, (unsigned long)comm, 0, 0, 0) < 0) {
			fprintf(stderr, "PR_SET_NAME failed with: %s\n",
					strerror(errno));
			exit(1);
		}
		if (prctl(PR_GET_NAME, (unsigned long)comm2, 0, 0, 0) < 0) {
			fprintf(stderr, "PR_SET_NAME failed with: %s\n",
					strerror(errno));
			exit(1);
		}
		if (strlen(comm2) != len) {
			printf("%d\n", len);
			break;
		}
	}
	exit(0);
}
			

once per boot, echoing the result into /etc/sizeof-comm.  Sell resulting
package to all vendors ;)

Re: [PATCH] sched: share task comm length value to userspace

[Andrew Morton]

On Fri, 9 Sep 2011 16:41:30 -0700
Andrew Morton <akpm@linux-foundation.org> wrote:

> 		if (prctl(PR_GET_NAME, (unsigned long)comm2, 0, 0, 0) < 0) {

Actually, if we were ever to increase sizeof(task_struct.comm),
existing code which does

	char comm[16];
	prctl(PR_GET_NAME, comm, ...)

will get its stack scribbled on.

So we _could_ increase it, but we'd need to retain the old 16-char
interfaces and add new prctl() modes.  And if we did that, we'd design
them in such a way that userspace wouldn't need to assume a fixed
size..