[PATCH v4] Added PR_SET_PROCTITLE_AREA option for prctl()

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

ChangeLog
  v3 -> v4
    - Use mutex instead seq_lock as akpm requested.

========================================

From: Timo Sirainen <tss@iki.fi>

Currently glibc2 doesn't have setproctitle(3), so several userland
daemons attempt to emulate it by doing some brutal stack modifications.
This works most of the time, but it has problems. For example:

 % ps -ef |grep avahi-daemon
 avahi     1679     1  0 09:20 ?        00:00:00 avahi-daemon: running [kosadesk.local]

 # cat /proc/1679/cmdline
 avahi-daemon: running [kosadesk.local]

This looks good, but the process has also overwritten its environment
area and made the environ file useless:

 # cat /proc/1679/environ
 adesk.local]

Another problem is that the process title length is limited by the size of
the environment. Security conscious people try to avoid potential information
leaks by clearing most of the environment before running a daemon:

 # env - MINIMUM_NEEDED_VAR=foo /path/to/daemon

The resulting environment size may be too small to fit the wanted process
titles.

This patch makes it possible for userspace to implement setproctitle()
cleanly. It adds a new PR_SET_PROCTITLE_AREA option for prctl(), which
updates task's mm_struct->arg_start and arg_end to the given area.

 test_setproctitle.c
 ================================================
 #include <string.h>
 #include <stdlib.h>
 #include <unistd.h>
 #include <stdio.h>
 #include <sys/prctl.h>

 #define ERR(str) (perror(str), exit(1))

 void settitle(char* title){
         int err;

         err = prctl(34, title, strlen(title)+1);
         if (err < 0)
                 ERR("prctl ");
 }

 void main(void){
         long i;
         char buf[1024];

         for (i = 0; i < 10000000000LL; i++){
                 sprintf(buf, "loooooooooooooooooooooooong string %d",i);
                 settitle(buf);
         }
 }
 ==================================================

Cc: Bryan Donlan <bdonlan@gmail.com>
Cc: Ulrich Drepper <drepper@redhat.com>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Timo Sirainen <tss@iki.fi>
---
 fs/proc/base.c           |   31 ++++++++++++++++++++++---------
 include/linux/mm_types.h |    2 ++
 include/linux/prctl.h    |    3 +++
 kernel/fork.c            |    1 +
 kernel/sys.c             |   22 ++++++++++++++++++++++
 5 files changed, 50 insertions(+), 9 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 837469a..ac800b4 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -255,32 +255,45 @@ static int proc_pid_cmdline(struct task_struct *task, char * buffer)
 	int res = 0;
 	unsigned int len;
 	struct mm_struct *mm = get_task_mm(task);
+
 	if (!mm)
 		goto out;
+
+	/* The process was not constructed yet? */
 	if (!mm->arg_end)
 		goto out_mm;	/* Shh! No looking before we're done */
 
- 	len = mm->arg_end - mm->arg_start;
- 
+	mutex_lock(&mm->arg_lock);
+	len = mm->arg_end - mm->arg_start;
 	if (len > PAGE_SIZE)
 		len = PAGE_SIZE;
- 
+
 	res = access_process_vm(task, mm->arg_start, buffer, len, 0);
+	if (mm->arg_end != mm->env_start)
+		/* prctl(PR_SET_PROCTITLE_AREA) used */
+		goto out_unlock;
 
-	// If the nul at the end of args has been overwritten, then
-	// assume application is using setproctitle(3).
+	/*
+	 * If the nul at the end of args has been overwritten, then assume
+	 * application is using sendmail's SPT_REUSEARGV style argv override.
+	 */
 	if (res > 0 && buffer[res-1] != '\0' && len < PAGE_SIZE) {
 		len = strnlen(buffer, res);
-		if (len < res) {
-		    res = len;
-		} else {
+		if (len < res)
+			res = len;
+		else {
 			len = mm->env_end - mm->env_start;
 			if (len > PAGE_SIZE - res)
 				len = PAGE_SIZE - res;
-			res += access_process_vm(task, mm->env_start, buffer+res, len, 0);
+			res += access_process_vm(task, mm->env_start,
+						 buffer+res, len, 0);
 			res = strnlen(buffer, res);
 		}
 	}
+
+out_unlock:
+	mutex_unlock(&mm->arg_lock);
+
 out_mm:
 	mmput(mm);
 out:
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 84a524a..3e2a346 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -12,6 +12,7 @@
 #include <linux/completion.h>
 #include <linux/cpumask.h>
 #include <linux/page-debug-flags.h>
+#include <linux/mutex.h>
 #include <asm/page.h>
 #include <asm/mmu.h>
 
@@ -236,6 +237,7 @@ struct mm_struct {
 	unsigned long stack_vm, reserved_vm, def_flags, nr_ptes;
 	unsigned long start_code, end_code, start_data, end_data;
 	unsigned long start_brk, brk, start_stack;
+	struct mutex arg_lock;
 	unsigned long arg_start, arg_end, env_start, env_end;
 
 	unsigned long saved_auxv[AT_VECTOR_SIZE]; /* for /proc/PID/auxv */
diff --git a/include/linux/prctl.h b/include/linux/prctl.h
index 9311505..da47542 100644
--- a/include/linux/prctl.h
+++ b/include/linux/prctl.h
@@ -90,4 +90,7 @@
 
 #define PR_MCE_KILL	33
 
+/* Set process title memory area for setproctitle() */
+#define PR_SET_PROCTITLE_AREA 34
+
 #endif /* _LINUX_PRCTL_H */
diff --git a/kernel/fork.c b/kernel/fork.c
index 4c20fff..881a6b4 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -459,6 +459,7 @@ static struct mm_struct * mm_init(struct mm_struct * mm, struct task_struct *p)
 	mm->cached_hole_size = ~0UL;
 	mm_init_aio(mm);
 	mm_init_owner(mm, p);
+	mutex_init(&mm->arg_lock);
 
 	if (likely(!mm_alloc_pgd(mm))) {
 		mm->def_flags = 0;
diff --git a/kernel/sys.c b/kernel/sys.c
index 255475d..434ea13 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1564,6 +1564,28 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 			error = 0;
 			break;
 
+		case PR_SET_PROCTITLE_AREA: {
+			struct mm_struct *mm = current->mm;
+			unsigned long addr = arg2;
+			unsigned long len = arg3;
+			unsigned long end = arg2 + arg3;
+
+			if (len > PAGE_SIZE)
+				return -EINVAL;
+
+			if (addr >= end)
+				return -EINVAL;
+
+			if (!access_ok(VERIFY_READ, addr, len))
+				return -EFAULT;
+
+			mutex_lock(&mm->arg_lock);
+			mm->arg_start = addr;
+			mm->arg_end = addr + len;
+			mutex_unlock(&mm->arg_lock);
+
+			return 0;
+		}
 		default:
 			error = -EINVAL;
 			break;
-- 
1.6.2.5