[PATCH RFC 1/2] exec: Dynamically allocate memory to store task's full name

Bhupesh
2025-03-14T05:27:40Z

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 'ps'.

Currently while running 'ps', the 'task->comm' value of a long
task name is truncated due to the limitation of TASK_COMM_LEN.
For example:
# ./create_very_long_name_user_space_script.sh&
# ps
PID TTY TIME CMD
332 ttyAMA0 00:00:00 create_very_lon

This leads to the names passed from userland via 'pthread_setname_np()'
being truncated.

Now, during debug tracing, seeing truncated names is not very useful.
(for example for debug applications invoking 'pthread_getname_np()') to
debug task names.

One possible way to fix this issue is extending the task comm size, but
as 'task->comm' is used in lots of places, that may cause some potential
buffer overflows. Another more conservative approach is introducing a new
pointer to store task's full name, which won't introduce too much overhead
as it is in the non-critical path.

After this change, the full name of these truncated tasks will be shown
in 'ps'. For example:
# ps
PID TTY TIME CMD
305 ttyAMA0 00:00:00 create_very_long_name_user_space_script.sh

Here is the proposed flow now:
1. 'pthread_setname_np()' like userspace API sets thread name.
2. This will set 'task->full_name' in addition to default 16-byte
truncated 'task->comm'.
3. And 'pthread_getname_np()' will retrieve 'task->full_name' by
default from the same '/proc/self/task/[tid]/full_name'

Step 3 implementation is achieved via the subsequent patch in this
patchset.

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 506cd411f4ac2..43d0a0d81d44e 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1210,6 +1210,9 @@ 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);
@@ -1350,11 +1353,22 @@ 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
@@ -1401,6 +1415,7 @@ 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 9c15365a30c08..ebf121768d951 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1144,6 +1144,9 @@ 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
@@ -1984,6 +1987,12 @@ 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