[PATCH] replace cad_pid by a struct pid

There are a few places in the kernel where the init task is signaled.  The
ctrl+alt+del sequence is one them.  It kills a task, usually init, using a
cached pid (cad_pid).

This patch replaces the pid_t by a struct pid to avoid pid wrap around
problem.  The struct pid is initialized at boot time in init() and can be
modified through systctl with

	/proc/sys/kernel/cad_pid

[ I haven't found any distro using it ? ]

It also introduces a small helper routine kill_cad_pid() which is used
where it seemed ok to use cad_pid instead of pid 1.

[akpm@osdl.org: cleanups, build fix]
Signed-off-by: Cedric Le Goater <clg@fr.ibm.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 27 insertions, 3 deletions

diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index a79ccf9d113b..8020fb273c4f 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -68,7 +68,6 @@ extern int sysrq_enabled;
 extern int core_uses_pid;
 extern int suid_dumpable;
 extern char core_pattern[];
-extern int cad_pid;
 extern int pid_max;
 extern int min_free_kbytes;
 extern int printk_ratelimit_jiffies;
@@ -137,6 +136,9 @@ static int parse_table(int __user *, int, void __user *, size_t __user *,
 static int proc_do_uts_string(ctl_table *table, int write, struct file *filp,
                   void __user *buffer, size_t *lenp, loff_t *ppos);
 
+static int proc_do_cad_pid(ctl_table *table, int write, struct file *filp,
+                  void __user *buffer, size_t *lenp, loff_t *ppos);
+
 static ctl_table root_table[];
 static struct ctl_table_header root_table_header =
         { root_table, LIST_HEAD_INIT(root_table_header.ctl_entry) };
@@ -543,10 +545,10 @@ static ctl_table kern_table[] = {
         {
                 .ctl_name       = KERN_CADPID,
                 .procname       = "cad_pid",
-                .data           = &cad_pid,
+                .data           = NULL,
                 .maxlen         = sizeof (int),
                 .mode           = 0600,
-                .proc_handler   = &proc_dointvec,
+                .proc_handler   = &proc_do_cad_pid,
         },
         {
                 .ctl_name       = KERN_MAX_THREADS,
@@ -2427,6 +2429,28 @@ proc_minmax:
 }
 #endif
 
+static int proc_do_cad_pid(ctl_table *table, int write, struct file *filp,
+                           void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+        struct pid *new_pid;
+        pid_t tmp;
+        int r;
+
+        tmp = pid_nr(cad_pid);
+
+        r = __do_proc_dointvec(&tmp, table, write, filp, buffer,
+                               lenp, ppos, NULL, NULL);
+        if (r || !write)
+                return r;
+
+        new_pid = find_get_pid(tmp);
+        if (!new_pid)
+                return -ESRCH;
+
+        put_pid(xchg(&cad_pid, new_pid));
+        return 0;
+}
+
 #else /* CONFIG_PROC_FS */
 
 int proc_dostring(ctl_table *table, int write, struct file *filp,