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

[Bhupesh Sharma]

On 4/1/25 9:32 AM, Harry Yoo wrote:
> On Mon, Mar 31, 2025 at 05:48:18PM +0530, Bhupesh 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;
> Is it safe to return error here, instead of jumping to 'out_unlock' label
> and then releasing locks?

Ok, let me modify this in v3 to ensure that locks are released properly.

> > +		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;
> > +
> >   	struct nameidata		*nameidata;
> >   
> >   #ifdef CONFIG_SYSVIPC
> > @@ -2007,6 +2010,12 @@ extern void __set_task_comm(struct task_struct *tsk, const char *from, bool exec
> >   	buf;						\
> >   })
> >   
> > +#define get_task_full_name(buf, buf_size, tsk) ({	\
> > +	BUILD_BUG_ON(sizeof(buf) < TASK_COMM_LEN);	\
> > +	strscpy_pad(buf, (tsk)->full_name, buf_size);	\
> > +	buf;						\
> > +})
> > +
> >   #ifdef CONFIG_SMP
> >   static __always_inline void scheduler_ipi(void)
> >   {
> > --
> > 2.38.1

Thanks.