[PATCH] Add prctl to modify current->comm

[PATCH] Add prctl to modify current->comm

[Andi Kleen]

This patch adds a prctl to modify current->comm as shown in /proc.
This feature was requested by KDE developers. In KDE most programs
are started by forking from a kdeinit program that already has the 
libraries loaded and some other state. 

Problem is to give these forked programs the proper name.
It already writes the command line in the environment (as seen in ps),
but top uses a different field in /proc/pid/status that reports
current->comm. And that was always "kdeinit" instead of the
real command name. So you ended up with lots of kdeinits
in your top listing, which was not very useful.

This patch adds a new prctl PR_SET_NAME to allow a program to change its 
comm field. 

I considered the potential security issues of a program obscuring
itself with this interface, but I don't think it matters much
because a program can already obscure itself when the admin uses
ps instead of top. In case of a KDE desktop calling everything
kdeinit is much more obfuscation than the alternative.

diff -u linux-2.6.8-5/kernel/sys.c-o linux-2.6.8-5/kernel/sys.c
--- linux-2.6.8-5/kernel/sys.c-o	2004-08-14 07:36:16.000000000 +0200
+++ linux-2.6.8-5/kernel/sys.c	2004-09-07 11:34:07.000000000 +0200
@@ -1660,6 +1660,13 @@
 			}
 			current->keep_capabilities = arg2;
 			break;
+		case PR_SET_NAME: {
+			struct task_struct *me = current;
+			me->comm[sizeof(me->comm)-1] = 0;
+			if (strncpy_from_user(me->comm, (char *)arg2, sizeof(me->comm)-1) < 0)
+				return -EFAULT;
+			return 0;
+		}
 		default:
 			error = -EINVAL;
 			break;
diff -u linux-2.6.8-5/include/linux/prctl.h-o linux-2.6.8-5/include/linux/prctl.h
--- linux-2.6.8-5/include/linux/prctl.h-o	2004-08-14 07:37:14.000000000 +0200
+++ linux-2.6.8-5/include/linux/prctl.h	2004-09-07 11:35:02.000000000 +0200
@@ -49,5 +49,6 @@
 # define PR_TIMING_TIMESTAMP    1       /* Accurate timestamp based
                                                    process timing */
 
+#define PR_SET_NAME    15		/* Set process name. */
 
 #endif /* _LINUX_PRCTL_H */

Re: [PATCH] Add prctl to modify current->comm

[Alan Cox]

On Maw, 2004-09-07 at 15:27, Andi Kleen wrote:
> I considered the potential security issues of a program obscuring
> itself with this interface, but I don't think it matters much
> because a program can already obscure itself when the admin uses
> ps instead of top. In case of a KDE desktop calling everything
> kdeinit is much more obfuscation than the alternative.

Actually its a lot simpler than that.  I long ago as a student wrong a
shell which simply did ln [binarytorun] banana; execve(.."banana", ...)

So I agree its not security related. You just need to fix the
implementation

> +		case PR_SET_NAME: {
> +			struct task_struct *me = current;
> +			me->comm[sizeof(me->comm)-1] = 0;
> +			if (strncpy_from_user(me->comm, (char *)arg2, sizeof(me->comm)-1) < 0)
> +				return -EFAULT;
> +			return 0;
> +		}

If the strncpy_from_user faults the state of current->comm is undefined.
This strikes me as bad design.

Alan

Re: [PATCH] Add prctl to modify current->comm

[Andi Kleen]

On Tue, Sep 07, 2004 at 06:12:48PM +0100, Alan Cox wrote:
> > +		case PR_SET_NAME: {
> > +			struct task_struct *me = current;
> > +			me->comm[sizeof(me->comm)-1] = 0;
> > +			if (strncpy_from_user(me->comm, (char *)arg2, sizeof(me->comm)-1) < 0)
> > +				return -EFAULT;
> > +			return 0;
> > +		}
> 
> If the strncpy_from_user faults the state of current->comm is undefined.

I didn't see it as a big issue because the result will be 0 terminated
anyways. 

> This strikes me as bad design.

Here's is a new patch with this fixed. 

-Andi

--------------------------------------------------------------------

Allow a program to change its current->comm

Useful for KDE & kdeinit. 


diff -u linux-2.6.8-5/kernel/sys.c-o linux-2.6.8-5/kernel/sys.c
--- linux-2.6.8-5/kernel/sys.c-o	2004-08-14 07:36:16.000000000 +0200
+++ linux-2.6.8-5/kernel/sys.c	2004-09-07 23:09:56.000000000 +0200
@@ -1660,6 +1660,15 @@
 			}
 			current->keep_capabilities = arg2;
 			break;
+		case PR_SET_NAME: {
+			struct task_struct *me = current;
+			unsigned char ncomm[sizeof(me->comm)];
+			ncomm[sizeof(me->comm)-1] = 0;
+			if (strncpy_from_user(ncomm, (char *)arg2, sizeof(me->comm)-1) < 0)
+				return -EFAULT;
+			memcpy(me->comm, ncomm, sizeof(me->comm));
+			return 0;
+		}
 		default:
 			error = -EINVAL;
 			break;
diff -u linux-2.6.8-5/include/linux/prctl.h-o linux-2.6.8-5/include/linux/prctl.h
--- linux-2.6.8-5/include/linux/prctl.h-o	2004-08-14 07:37:14.000000000 +0200
+++ linux-2.6.8-5/include/linux/prctl.h	2004-09-07 23:09:44.000000000 +0200
@@ -49,5 +49,6 @@
 # define PR_TIMING_TIMESTAMP    1       /* Accurate timestamp based
                                                    process timing */
 
+#define PR_SET_NAME    15		/* Set process name. */
 
 #endif /* _LINUX_PRCTL_H */