[PATCH 1/7] pid: change task_struct::pid to read-only

[Richard Guy Briggs <rgb@redhat.com>]

task->pid is only ever assigned once (well ok, twice).  For system health and
secure logging confidence, make it const to make it much more intentional when
it is being changed.

Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
---
 arch/x86/kernel/process.c |    2 +-
 fs/exec.c                 |    4 +++-
 include/linux/sched.h     |    2 +-
 kernel/fork.c             |    8 ++++++--
 4 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 3fb8d95..bab0730 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -66,7 +66,7 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
 {
 	int ret;
 
-	*dst = *src;
+	memcpy(dst, src, sizeof(struct task_struct));
 	if (fpu_allocated(&src->thread.fpu)) {
 		memset(&dst->thread.fpu, 0, sizeof(dst->thread.fpu));
 		ret = fpu_alloc(&dst->thread.fpu);
diff --git a/fs/exec.c b/fs/exec.c
index 7ea097f..a6b585e 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -906,6 +906,8 @@ static int de_thread(struct task_struct *tsk)
 	 */
 	if (!thread_group_leader(tsk)) {
 		struct task_struct *leader = tsk->group_leader;
+		/* tast_struct::pid is const pid_t, hence the ugly cast */
+		pid_t *pid_p = (pid_t *)&(tsk->pid);
 
 		sig->notify_count = -1;	/* for exit_notify() */
 		for (;;) {
@@ -948,7 +950,7 @@ static int de_thread(struct task_struct *tsk)
 		 * Note: The old leader also uses this pid until release_task
 		 *       is called.  Odd but simple and correct.
 		 */
-		tsk->pid = leader->pid;
+		*pid_p = leader->pid;
 		change_pid(tsk, PIDTYPE_PID, task_pid(leader));
 		transfer_pid(leader, tsk, PIDTYPE_PGID);
 		transfer_pid(leader, tsk, PIDTYPE_SID);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 53f97eb..195f6bd 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1128,7 +1128,7 @@ struct task_struct {
 	unsigned sched_reset_on_fork:1;
 	unsigned sched_contributes_to_load:1;
 
-	pid_t pid;
+	const pid_t pid;
 	pid_t tgid;
 
 #ifdef CONFIG_CC_STACKPROTECTOR
diff --git a/kernel/fork.c b/kernel/fork.c
index dfa736c..296e564 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -286,7 +286,7 @@ void __init fork_init(unsigned long mempages)
 int __attribute__((weak)) arch_dup_task_struct(struct task_struct *dst,
 					       struct task_struct *src)
 {
-	*dst = *src;
+	memcpy(dst, src, sizeof(struct task_struct));
 	return 0;
 }
 
@@ -1135,6 +1135,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 {
 	int retval;
 	struct task_struct *p;
+	pid_t *pid_p;
 
 	if ((clone_flags & (CLONE_NEWNS|CLONE_FS)) == (CLONE_NEWNS|CLONE_FS))
 		return ERR_PTR(-EINVAL);
@@ -1389,7 +1390,10 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 	clear_all_latency_tracing(p);
 
 	/* ok, now we should be set up.. */
-	p->pid = pid_nr(pid);
+
+	/* tast_struct::pid is const pid_t, hence the ugly cast */
+	pid_p = (pid_t *)&(p->pid);
+	*pid_p = pid_nr(pid);
 	if (clone_flags & CLONE_THREAD) {
 		p->exit_signal = -1;
 		p->group_leader = current->group_leader;
-- 
1.7.1