[PATCH v6 3/3] include: Set tsk->comm length to 64 bytes

[Bhupesh]

Historically due to the 16-byte length of TASK_COMM_LEN, the
users of 'tsk->comm' are restricted to use a fixed-size target
buffer also of TASK_COMM_LEN for 'memcpy()' like use-cases.

To fix the same, we now use a 64-byte TASK_COMM_EXT_LEN and
set the comm element inside 'task_struct' to the same length:
       struct task_struct {
	       .....
               char    comm[TASK_COMM_EXT_LEN];
	       .....
       };

       where TASK_COMM_EXT_LEN is 64-bytes.

Now, in order to maintain existing ABI, we ensure that:

- Existing users of 'get_task_comm'/ 'set_task_comm' will get 'tsk->comm'
  truncated to a maximum of 'TASK_COMM_LEN' (16-bytes) to maintain ABI,
- New / Modified users of 'get_task_comm'/ 'set_task_comm' will get
 'tsk->comm' supported up to the maximum of 'TASK_COMM_EXT_LEN' (64-bytes).

Note, that the existing users have not been modified to migrate to
'TASK_COMM_EXT_LEN', in case they have hard-coded expectations of
dealing with only a 'TASK_COMM_LEN' long 'tsk->comm'.

Signed-off-by: Bhupesh <bhupesh@xxxxxxxxxx>
---
 include/linux/sched.h | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 8bbd03f1b978..b6abb759292c 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -317,6 +317,7 @@ struct user_event_mm;
  */
 enum {
 	TASK_COMM_LEN = 16,
+	TASK_COMM_EXT_LEN = 64,
 };
 
 extern void sched_tick(void);
@@ -1159,7 +1160,7 @@ struct task_struct {
 	 *   - logic inside set_task_comm() will ensure it is always NUL-terminated and
 	 *     zero-padded
 	 */
-	char				comm[TASK_COMM_LEN];
+	char				comm[TASK_COMM_EXT_LEN];
 
 	struct nameidata		*nameidata;
 
@@ -1954,7 +1955,7 @@ extern void kick_process(struct task_struct *tsk);
 
 extern void __set_task_comm(struct task_struct *tsk, const char *from, bool exec);
 #define set_task_comm(tsk, from) ({			\
-	BUILD_BUG_ON(sizeof(from) != TASK_COMM_LEN);	\
+	BUILD_BUG_ON(sizeof(from) < TASK_COMM_LEN);	\
 	__set_task_comm(tsk, from, false);		\
 })
 
@@ -1974,6 +1975,10 @@ extern void __set_task_comm(struct task_struct *tsk, const char *from, bool exec
 #define get_task_comm(buf, tsk) ({			\
 	BUILD_BUG_ON(sizeof(buf) < TASK_COMM_LEN);	\
 	strscpy_pad(buf, (tsk)->comm);			\
+	if ((sizeof(buf)) == TASK_COMM_LEN)		\
+		buf[TASK_COMM_LEN - 1] = '\0';		\
+	else						\
+		buf[TASK_COMM_EXT_LEN - 1] = '\0';	\
 	buf;						\
 })
 
-- 
2.38.1