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

Andrii Nakryiko
2025-04-03T16:17:20Z

On Mon, Mar 31, 2025 at 5:18 AM 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);

this `args` argument, it's not initialized anywhere, right? It's not
clear where it's coming from, but you are passing it directly into
kvasprintf(), I can't convince myself that this is correct. Can you
please explain what is happening here?

Also, instead of allocating a buffer unconditionally, maybe check that
comm is longer than 16, and if not, just use the old-schoold 16-byte
comm array?

> + 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;
> +
> 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
>