linux-kernel - Re: [PATCH v6 3/3] include: Set tsk->comm length to 64 bytes

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Message-ID: <202507241634.C0346AC@keescook>
Date: Thu, 24 Jul 2025 16:38:02 -0700
From: Kees Cook <kees@...nel.org>
To: Bhupesh <bhupesh@...lia.com>
Cc: akpm@...ux-foundation.org, kernel-dev@...lia.com,
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
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	torvalds@...ux-foundation.org
Subject: Re: [PATCH v6 3/3] include: Set tsk->comm length to 64 bytes

On Thu, Jul 24, 2025 at 06:06:12PM +0530, Bhupesh wrote:
> Historically due to the 16-byte length of TASK_COMM_LEN, the
> users of 'tsk->comm' are restricted to use a fixed-size target
> buffer also of TASK_COMM_LEN for 'memcpy()' like use-cases.
> 
> To fix the same, we now use a 64-byte TASK_COMM_EXT_LEN and
> set the comm element inside 'task_struct' to the same length:
>        struct task_struct {
> 	       .....
>                char    comm[TASK_COMM_EXT_LEN];
> 	       .....
>        };
> 
>        where TASK_COMM_EXT_LEN is 64-bytes.
> 
> Now, in order to maintain existing ABI, we ensure that:
> 
> - Existing users of 'get_task_comm'/ 'set_task_comm' will get 'tsk->comm'
>   truncated to a maximum of 'TASK_COMM_LEN' (16-bytes) to maintain ABI,
> - New / Modified users of 'get_task_comm'/ 'set_task_comm' will get
>  'tsk->comm' supported up to the maximum of 'TASK_COMM_EXT_LEN' (64-bytes).
> 
> Note, that the existing users have not been modified to migrate to
> 'TASK_COMM_EXT_LEN', in case they have hard-coded expectations of
> dealing with only a 'TASK_COMM_LEN' long 'tsk->comm'.
> 
> Signed-off-by: Bhupesh <bhupesh@...lia.com>
> ---
>  include/linux/sched.h | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 8bbd03f1b978..b6abb759292c 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -317,6 +317,7 @@ struct user_event_mm;
>   */
>  enum {
>  	TASK_COMM_LEN = 16,
> +	TASK_COMM_EXT_LEN = 64,
>  };
>  
>  extern void sched_tick(void);
> @@ -1159,7 +1160,7 @@ struct task_struct {
>  	 *   - logic inside set_task_comm() will ensure it is always NUL-terminated and
>  	 *     zero-padded
>  	 */
> -	char				comm[TASK_COMM_LEN];
> +	char				comm[TASK_COMM_EXT_LEN];
>  
>  	struct nameidata		*nameidata;
>  
> @@ -1954,7 +1955,7 @@ extern void kick_process(struct task_struct *tsk);
>  
>  extern void __set_task_comm(struct task_struct *tsk, const char *from, bool exec);
>  #define set_task_comm(tsk, from) ({			\
> -	BUILD_BUG_ON(sizeof(from) != TASK_COMM_LEN);	\
> +	BUILD_BUG_ON(sizeof(from) < TASK_COMM_LEN);	\
>  	__set_task_comm(tsk, from, false);		\
>  })
>  
> @@ -1974,6 +1975,10 @@ extern void __set_task_comm(struct task_struct *tsk, const char *from, bool exec
>  #define get_task_comm(buf, tsk) ({			\
>  	BUILD_BUG_ON(sizeof(buf) < TASK_COMM_LEN);	\
>  	strscpy_pad(buf, (tsk)->comm);			\
> +	if ((sizeof(buf)) == TASK_COMM_LEN)		\
> +		buf[TASK_COMM_LEN - 1] = '\0';		\
> +	else						\
> +		buf[TASK_COMM_EXT_LEN - 1] = '\0';	\

strscpy_pad() will already make sure buf is NUL-terminated, so I don't
see why there is a need for explicit final byte termination? (And even
if we do need it, then it should just be buf[sizeof(buf) - 1] otherwise
using a buf that is < TASK_COMM_EXT_LEN but > TASK_COMM_LEN will have
a spurious NUL byte write beyond its buffer.)

-- 
Kees Cook