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

Bhupesh
2025-03-31T12:18:47Z

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@igalia.com>
---
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