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

[Yafang Shao <laoar.shao@gmail.com>]

On Mon, Mar 31, 2025 at 8:18 PM Bhupesh <bhupesh@igalia.com> 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@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;
> +

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@mail.gmail.com/

-- 
Regards
Yafang