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

[Bhupesh Sharma]

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