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

[Bhupesh Sharma]

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',
2. Remove the task_lock() inside __get_task_comm(),
3. Users of task->comm will be affected in the following ways:
    (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.

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

Thanks,
Bhupesh