Re: [PATCH v6 1/9] Get rid of __get_task_comm()

[Alejandro Colomar <alx@kernel.org>]

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Hi Yafang,

On Mon, Aug 12, 2024 at 10:29:25AM GMT, Yafang Shao wrote:
> We want to eliminate the use of __get_task_comm() for the following
> reasons:
> 
> - The task_lock() is unnecessary
>   Quoted from Linus [0]:
>   : Since user space can randomly change their names anyway, using locking
>   : was always wrong for readers (for writers it probably does make sense
>   : to have some lock - although practically speaking nobody cares there
>   : either, but at least for a writer some kind of race could have
>   : long-term mixed results
> 
> - The BUILD_BUG_ON() doesn't add any value
>   The only requirement is to ensure that the destination buffer is a valid
>   array.
> 
> - Zeroing is not necessary in current use cases
>   To avoid confusion, we should remove it. Moreover, not zeroing could
>   potentially make it easier to uncover bugs. If the caller needs a
>   zero-padded task name, it should be explicitly handled at the call site.
> 
> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> Link: https://lore.kernel.org/all/CAHk-=wivfrF0_zvf+oj6==Sh=-npJooP8chLPEfaFV0oNYTTBA@mail.gmail.com [0]
> Link: https://lore.kernel.org/all/CAHk-=whWtUC-AjmGJveAETKOMeMFSTwKwu99v7+b6AyHMmaDFA@mail.gmail.com/
> Suggested-by: Alejandro Colomar <alx@kernel.org>
> Link: https://lore.kernel.org/all/2jxak5v6dfxlpbxhpm3ey7oup4g2lnr3ueurfbosf5wdo65dk4@srb3hsk72zwq
> Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Cc: Christian Brauner <brauner@kernel.org>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Eric Biederman <ebiederm@xmission.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>
> Cc: Matus Jokay <matus.jokay@stuba.sk>
> Cc: Alejandro Colomar <alx@kernel.org>
> Cc: "Serge E. Hallyn" <serge@hallyn.com>
> ---
>  fs/exec.c             | 10 ----------
>  fs/proc/array.c       |  2 +-
>  include/linux/sched.h | 31 +++++++++++++++++++++++++------
>  kernel/kthread.c      |  2 +-
>  4 files changed, 27 insertions(+), 18 deletions(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index a47d0e4c54f6..2e468ddd203a 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1264,16 +1264,6 @@ static int unshare_sighand(struct task_struct *me)
>  	return 0;
>  }
>  
> -char *__get_task_comm(char *buf, size_t buf_size, struct task_struct *tsk)
> -{
> -	task_lock(tsk);
> -	/* Always NUL terminated and zero-padded */
> -	strscpy_pad(buf, tsk->comm, buf_size);

This comment is correct (see other comments below).

(Except that pedantically, I'd write it as NUL-terminated with a hyphen,
 just like zero-padded.)

> -	task_unlock(tsk);
> -	return buf;
> -}
> -EXPORT_SYMBOL_GPL(__get_task_comm);
> -
>  /*
>   * These functions flushes out all traces of the currently running executable
>   * so that a new one can be started
> diff --git a/fs/proc/array.c b/fs/proc/array.c
> index 34a47fb0c57f..55ed3510d2bb 100644
> --- a/fs/proc/array.c
> +++ b/fs/proc/array.c
> @@ -109,7 +109,7 @@ void proc_task_name(struct seq_file *m, struct task_struct *p, bool escape)
>  	else if (p->flags & PF_KTHREAD)
>  		get_kthread_comm(tcomm, sizeof(tcomm), p);
>  	else
> -		__get_task_comm(tcomm, sizeof(tcomm), p);
> +		get_task_comm(tcomm, p);

LGTM.  (This would have been good even if not removing the helper.)

>  
>  	if (escape)
>  		seq_escape_str(m, tcomm, ESCAPE_SPACE | ESCAPE_SPECIAL, "\n\\");
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 33dd8d9d2b85..e0e26edbda61 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1096,9 +1096,11 @@ struct task_struct {
>  	/*
>  	 * executable name, excluding path.
>  	 *
> -	 * - normally initialized setup_new_exec()
> -	 * - access it with [gs]et_task_comm()
> -	 * - lock it with task_lock()
> +	 * - normally initialized begin_new_exec()
> +	 * - set it with set_task_comm()
> +	 *   - strscpy_pad() to ensure it is always NUL-terminated

The comment above is inmprecise.
It should say either
"strscpy() to ensure it is always NUL-terminated", or
"strscpy_pad() to ensure it is NUL-terminated and zero-padded".

> +	 *   - task_lock() to ensure the operation is atomic and the name is
> +	 *     fully updated.
>  	 */
>  	char				comm[TASK_COMM_LEN];
>  
> @@ -1912,10 +1914,27 @@ static inline void set_task_comm(struct task_struct *tsk, const char *from)
>  	__set_task_comm(tsk, from, false);
>  }
>  
> -extern char *__get_task_comm(char *to, size_t len, struct task_struct *tsk);
> +/*
> + * - Why not use task_lock()?
> + *   User space can randomly change their names anyway, so locking for readers
> + *   doesn't make sense. For writers, locking is probably necessary, as a race
> + *   condition could lead to long-term mixed results.
> + *   The strscpy_pad() in __set_task_comm() can ensure that the task comm is
> + *   always NUL-terminated.

This comment has the same imprecission that I noted above.

> Therefore the race condition between reader and
> + *   writer is not an issue.
> + *
> + * - Why not use strscpy_pad()?
> + *   While strscpy_pad() prevents writing garbage past the NUL terminator, which
> + *   is useful when using the task name as a key in a hash map, most use cases
> + *   don't require this. Zero-padding might confuse users if it’s unnecessary,
> + *   and not zeroing might even make it easier to expose bugs. If you need a
> + *   zero-padded task name, please handle that explicitly at the call site.
> + *
> + * - ARRAY_SIZE() can help ensure that @buf is indeed an array.
> + */
>  #define get_task_comm(buf, tsk) ({			\
> -	BUILD_BUG_ON(sizeof(buf) != TASK_COMM_LEN);	\
> -	__get_task_comm(buf, sizeof(buf), tsk);		\
> +	strscpy(buf, (tsk)->comm, ARRAY_SIZE(buf));	\
> +	buf;						\
>  })
>  
>  #ifdef CONFIG_SMP
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index f7be976ff88a..7d001d033cf9 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -101,7 +101,7 @@ void get_kthread_comm(char *buf, size_t buf_size, struct task_struct *tsk)
>  	struct kthread *kthread = to_kthread(tsk);
>  
>  	if (!kthread || !kthread->full_name) {
> -		__get_task_comm(buf, buf_size, tsk);
> +		strscpy(buf, tsk->comm, buf_size);
>  		return;
>  	}

Other than that, LGTM.

-- 
<https://www.alejandro-colomar.es/>

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