Command name has been restrict to only 16 bytes, which is too limiting,
specially when debugging and tracing complex software with thousands of
threads and the need to differentiate them.

Just as it was done with kthreads in commit 6b59808bfe48 ("workqueue:
Show the latest workqueue name in /proc/PID/{comm,stat,status}"), support
long names for userspace threads as well.

To avoid buffer overflows, cap all existing userspace APIs to
TASK_COMM_LEN, and leave the full extended name for a new interface.

Signed-off-by: André Almeida <andrealmeid@igalia.com>
---
 fs/proc/array.c          |  2 +-
 include/linux/sched.h    |  3 ++-
 kernel/sys.c             | 10 +++++-----
 lib/tests/string_kunit.c |  2 +-
 4 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/fs/proc/array.c b/fs/proc/array.c
index 905d910b598f..0490b9ef2788 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -110,7 +110,7 @@ void proc_task_name(struct seq_file *m, struct task_struct *p, bool escape)
 	else if (p->flags & PF_KTHREAD)
 		get_kthread_comm(tcomm, sizeof(tcomm), p);
 	else
-		strscpy_pad(tcomm, p->comm);
+		strscpy_pad(tcomm, p->comm, TASK_COMM_LEN);
 
 	if (escape)
 		seq_escape_str(m, tcomm, ESCAPE_SPACE | ESCAPE_SPECIAL, "\n\\");
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 9b6a413f6618..6a48517f01cc 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -323,6 +323,7 @@ struct user_event_mm;
  */
 enum {
 	TASK_COMM_LEN = 16,
+	TASK_COMM_EXT_LEN = 64,
 };
 
 extern void sched_tick(void);
@@ -1178,7 +1179,7 @@ struct task_struct {
 	 * - set it with set_task_comm() to ensure it is always
 	 *   NUL-terminated and zero-padded
 	 */
-	char				comm[TASK_COMM_LEN];
+	char				comm[TASK_COMM_EXT_LEN];
 
 	struct nameidata		*nameidata;
 
diff --git a/kernel/sys.c b/kernel/sys.c
index 74c22d6dded0..3c37b5c1c072 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2535,7 +2535,7 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 		unsigned long, arg4, unsigned long, arg5)
 {
 	struct task_struct *me = current;
-	unsigned char comm[sizeof(me->comm)];
+	unsigned char comm[TASK_COMM_LEN];
 	long error;
 
 	error = security_task_prctl(option, arg2, arg3, arg4, arg5);
@@ -2601,16 +2601,16 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 			error = -EINVAL;
 		break;
 	case PR_SET_NAME:
-		comm[sizeof(me->comm) - 1] = 0;
+		comm[TASK_COMM_LEN - 1] = 0;
 		if (strncpy_from_user(comm, (char __user *)arg2,
-				      sizeof(me->comm) - 1) < 0)
+				      TASK_COMM_LEN - 1) < 0)
 			return -EFAULT;
 		set_task_comm(me, comm);
 		proc_comm_connector(me);
 		break;
 	case PR_GET_NAME:
-		strscpy_pad(comm, me->comm);
-		if (copy_to_user((char __user *)arg2, comm, sizeof(comm)))
+		strscpy_pad(comm, me->comm, TASK_COMM_LEN);
+		if (copy_to_user((char __user *)arg2, comm, TASK_COMM_LEN))
 			return -EFAULT;
 		break;
 	case PR_GET_ENDIAN:
diff --git a/lib/tests/string_kunit.c b/lib/tests/string_kunit.c
index b64d7f0e54a3..5d26029d2d01 100644
--- a/lib/tests/string_kunit.c
+++ b/lib/tests/string_kunit.c
@@ -883,7 +883,7 @@ static void string_bench_strrchr(struct kunit *test)
 
 #define TASK_NAME "task_name"
 #define TASK_NAME_LEN 9
-#define TASK_MAX_LEN TASK_COMM_LEN
+#define TASK_MAX_LEN TASK_COMM_EXT_LEN
 #define SMALLER_LEN TASK_NAME_LEN - 3
 #define BIGGER_LEN TASK_MAX_LEN + 3
 

-- 
2.55.0