Re: [PATCH 2/5] kernel/fork: allocate task->comm dynamicly

[Yafang Shao]

On Wed, Sep 29, 2021 at 7:51 PM Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
>
> task->comm is defined as an array embedded in struct task_struct before.
> This patch changes it to a char pointer. It will be allocated in the fork
> and freed when the task is freed.
>
> Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
> ---
>  include/linux/sched.h |  2 +-
>  kernel/fork.c         | 19 +++++++++++++++++++
>  2 files changed, 20 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index e12b524426b0..b387b5943db4 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1051,7 +1051,7 @@ struct task_struct {
>          * - access it with [gs]et_task_comm()
>          * - lock it with task_lock()
>          */
> -       char                            comm[TASK_COMM_LEN];
> +       char                            *comm;
>
>         struct nameidata                *nameidata;
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 38681ad44c76..227aec240501 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -721,6 +721,20 @@ static void mmdrop_async(struct mm_struct *mm)
>         }
>  }
>
> +static int task_comm_alloc(struct task_struct *p)
> +{
> +       p->comm = kzalloc(TASK_COMM_LEN, GFP_KERNEL);
> +       if (!p->comm)
> +               return -ENOMEM;
> +
> +       return 0;
> +}
> +
> +static void task_comm_free(struct task_struct *p)
> +{
> +       kfree(p->comm);
> +}
> +
>  static inline void free_signal_struct(struct signal_struct *sig)
>  {
>         taskstats_tgid_free(sig);
> @@ -753,6 +767,7 @@ void __put_task_struct(struct task_struct *tsk)
>         bpf_task_storage_free(tsk);
>         exit_creds(tsk);
>         delayacct_tsk_free(tsk);
> +       task_comm_free(tsk);
>         put_signal_struct(tsk->signal);
>         sched_core_free(tsk);
>
> @@ -2076,6 +2091,10 @@ static __latent_entropy struct task_struct *copy_process(
>         if (data_race(nr_threads >= max_threads))
>                 goto bad_fork_cleanup_count;
>
> +       retval = task_comm_alloc(p);
> +       if (retval)
> +               goto bad_fork_cleanup_count;
> +
>         delayacct_tsk_init(p);  /* Must remain after dup_task_struct() */
>         p->flags &= ~(PF_SUPERPRIV | PF_WQ_WORKER | PF_IDLE | PF_NO_SETAFFINITY);
>         p->flags |= PF_FORKNOEXEC;
> --
> 2.17.1
>

After sending the series, I find that I forget to move the
task_comm_alloc() to the end of fork().  Below is the updated one,

---
 include/linux/sched.h |  2 +-
 kernel/fork.c         | 19 +++++++++++++++++++
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index e12b524426b0..b387b5943db4 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1051,7 +1051,7 @@ struct task_struct {
         * - access it with [gs]et_task_comm()
         * - lock it with task_lock()
         */
-       char                            comm[TASK_COMM_LEN];
+       char                            *comm;

        struct nameidata                *nameidata;

diff --git a/kernel/fork.c b/kernel/fork.c
index 38681ad44c76..d1e0c38464ed 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -721,6 +721,20 @@ static void mmdrop_async(struct mm_struct *mm)
        }
 }

+static int task_comm_alloc(struct task_struct *p)
+{
+       p->comm = kzalloc(TASK_COMM_LEN, GFP_KERNEL);
+       if (!p->comm)
+               return -ENOMEM;
+
+       return 0;
+}
+
+static void task_comm_free(struct task_struct *p)
+{
+       kfree(p->comm);
+}
+
 static inline void free_signal_struct(struct signal_struct *sig)
 {
        taskstats_tgid_free(sig);
@@ -753,6 +767,7 @@ void __put_task_struct(struct task_struct *tsk)
        bpf_task_storage_free(tsk);
        exit_creds(tsk);
        delayacct_tsk_free(tsk);
+       task_comm_free(tsk);
        put_signal_struct(tsk->signal);
        sched_core_free(tsk);

@@ -2352,6 +2367,10 @@ static __latent_entropy struct task_struct *copy_process(
                goto bad_fork_cancel_cgroup;
        }

+       retval = task_comm_alloc(p);
+       if (retval)
+               goto bad_fork_cancel_cgroup;
+
        /* past the last point of failure */
        if (pidfile)
                fd_install(pidfd, pidfile);


-- 
Thanks
Yafang