[PATCH v2 1/3] exec: Dynamically allocate memory to store task's full name

[Bhupesh <bhupesh@xxxxxxxxxx>]

Provide a parallel implementation for get_task_comm() called
get_task_full_name() which allows the dynamically allocated
and filled-in task's full name to be passed to interested
users such as 'gdb'.

Currently while running 'gdb', the 'task->comm' value of a long
task name is truncated due to the limitation of TASK_COMM_LEN.

For example using gdb to debug a simple app currently which generate
threads with long task names:
  # gdb ./threadnames -ex "run info thread" -ex "detach" -ex "quit" > log
  # cat log

  NameThatIsTooLo

This patch does not touch 'TASK_COMM_LEN' at all, i.e.
'TASK_COMM_LEN' and the 16-byte design remains untouched. Which means
that all the legacy / existing ABI, continue to work as before using
'/proc/$pid/task/$tid/comm'.

This patch only adds a parallel, dynamically-allocated
'task->full_name' which can be used by interested users
via '/proc/$pid/task/$tid/full_name'.

After this change, gdb is able to show full name of the task:
  # gdb ./threadnames -ex "run info thread" -ex "detach" -ex "quit" > log
  # cat log

  NameThatIsTooLongForComm[4662]

Signed-off-by: Bhupesh <bhupesh@xxxxxxxxxx>
---
 fs/exec.c             | 21 ++++++++++++++++++---
 include/linux/sched.h |  9 +++++++++
 2 files changed, 27 insertions(+), 3 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index f45859ad13ac..4219d77a519c 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1208,6 +1208,9 @@ int begin_new_exec(struct linux_binprm * bprm)
 {
 	struct task_struct *me = current;
 	int retval;
+	va_list args;
+	char *name;
+	const char *fmt;
 
 	/* Once we are committed compute the creds */
 	retval = bprm_creds_from_file(bprm);
@@ -1348,11 +1351,22 @@ int begin_new_exec(struct linux_binprm * bprm)
 		 * detecting a concurrent rename and just want a terminated name.
 		 */
 		rcu_read_lock();
-		__set_task_comm(me, smp_load_acquire(&bprm->file->f_path.dentry->d_name.name),
-				true);
+		fmt = smp_load_acquire(&bprm->file->f_path.dentry->d_name.name);
+		name = kvasprintf(GFP_KERNEL, fmt, args);
+		if (!name)
+			return -ENOMEM;
+
+		me->full_name = name;
+		__set_task_comm(me, fmt, true);
 		rcu_read_unlock();
 	} else {
-		__set_task_comm(me, kbasename(bprm->filename), true);
+		fmt = kbasename(bprm->filename);
+		name = kvasprintf(GFP_KERNEL, fmt, args);
+		if (!name)
+			return -ENOMEM;
+
+		me->full_name = name;
+		__set_task_comm(me, fmt, true);
 	}
 
 	/* An exec changes our domain. We are no longer part of the thread
@@ -1399,6 +1413,7 @@ int begin_new_exec(struct linux_binprm * bprm)
 	return 0;
 
 out_unlock:
+	kfree(me->full_name);
 	up_write(&me->signal->exec_update_lock);
 	if (!bprm->cred)
 		mutex_unlock(&me->signal->cred_guard_mutex);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 56ddeb37b5cd..053b52606652 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1166,6 +1166,9 @@ struct task_struct {
 	 */
 	char				comm[TASK_COMM_LEN];
 
+	/* To store the full name if task comm is truncated. */
+	char				*full_name;
+
 	struct nameidata		*nameidata;
 
 #ifdef CONFIG_SYSVIPC
@@ -2007,6 +2010,12 @@ extern void __set_task_comm(struct task_struct *tsk, const char *from, bool exec
 	buf;						\
 })
 
+#define get_task_full_name(buf, buf_size, tsk) ({	\
+	BUILD_BUG_ON(sizeof(buf) < TASK_COMM_LEN);	\
+	strscpy_pad(buf, (tsk)->full_name, buf_size);	\
+	buf;						\
+})
+
 #ifdef CONFIG_SMP
 static __always_inline void scheduler_ipi(void)
 {
-- 
2.38.1