Sorry for the delay in reply, I was out for a couple of days.
On 4/6/25 7:58 AM, Yafang Shao wrote:
On Fri, Apr 4, 2025 at 2:35 PM Bhupesh Sharma <bhsharma@xxxxxxxxxx> wrote:
On 4/1/25 7:37 AM, Yafang Shao wrote:
On Mon, Mar 31, 2025 at 8:18 PM Bhupesh <bhupesh@xxxxxxxxxx> wrote:
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;
+
Adding another field to store the task name isn’t ideal. What about
combining them into a single field, as Linus suggested [0]?
[0]. https://lore.kernel.org/all/CAHk-=wjAmmHUg6vho1KjzQi2=psR30+CogFd4aXrThr2gsiS4g@xxxxxxxxxxxxxx/
Thanks for sharing Linus's suggestion. I went through the suggested
changes in the related threads and came up with the following set of points:
1. struct task_struct would contain both 'comm' and 'full_name',
Correct.
2. Remove the task_lock() inside __get_task_comm(),
This has been implemented in the patch series titled "Improve the copy
of task comm". For details, please refer to:
https://lore.kernel.org/linux-mm/20240828030321.20688-1-laoar.shao@xxxxxxxxx/.
3. Users of task->comm will be affected in the following ways:
Correct.
(a). Printing with '%s' and tsk->comm would just continue to
work,but will get a longer max string.
(b). For users of memcpy.*->comm\>', we should change 'memcpy()' to
'copy_comm()' which would look like:
memcpy(dst, src, TASK_COMM_LEN);
dst[TASK_COMM_LEN-1] = 0;
(c). Users which use "sizeof(->comm)" will continue to get the old value because of the hacky union.
Using a separate pointer rather than a union could simplify the
implementation. I’m open to introducing a new pointer if you believe
it’s the better approach.
Right, that's what I was thinking of earlier as well, i.e. having a new
pointer like tsk->full_name, however
allocating it outside the exec() hot-path may be tricky.
Let me try that though and come up with a v3, that addresses (a), (b) as
mentioned above and (c) with a pointer instead of union.
Thanks,
Bhupesh
