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

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

On Fri, Apr 4, 2025 at 2:35 PM Bhupesh Sharma <bhsharma@igalia.com> wrote:
>
>
> On 4/1/25 7:37 AM, Yafang Shao wrote:
> > 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/
> >
>
> 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@gmail.com/.

> 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.

>
> Am I missing something here. Please let me know your views.


-- 
Regards
Yafang