linux-kernel - Re: [PATCH v8 4/5] treewide: Switch memcpy() users of 'task->comm' to a more safer implementation

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Message-ID: <202508250656.9D56526@keescook>
Date: Mon, 25 Aug 2025 07:01:49 -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,
	linux-perf-users@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, oliver.sang@...el.com, lkp@...el.com,
	laoar.shao@...il.com, pmladek@...e.com, rostedt@...dmis.org,
	mathieu.desnoyers@...icios.com, arnaldo.melo@...il.com,
	alexei.starovoitov@...il.com, andrii.nakryiko@...il.com,
	mirq-linux@...e.qmqm.pl, peterz@...radead.org, willy@...radead.org,
	david@...hat.com, viro@...iv.linux.org.uk, ebiederm@...ssion.com,
	brauner@...nel.org, jack@...e.cz, mingo@...hat.com,
	juri.lelli@...hat.com, bsegall@...gle.com, mgorman@...e.de,
	vschneid@...hat.com, linux-trace-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH v8 4/5] treewide: Switch memcpy() users of 'task->comm'
 to a more safer implementation

On Thu, Aug 21, 2025 at 03:51:51PM +0530, Bhupesh wrote:
> As Linus mentioned in [1], currently we have several memcpy() use-cases
> which use 'current->comm' to copy the task name over to local copies.
> For an example:
> 
>  ...
>  char comm[TASK_COMM_LEN];
>  memcpy(comm, current->comm, TASK_COMM_LEN);
>  ...
> 
> These should be rather calling a wrappper like "get_task_array()",
> which is implemented as:
> 
>    static __always_inline void
>        __cstr_array_copy(char *dst,
>             const char *src, __kernel_size_t size)
>    {
>         memcpy(dst, src, size);
>         dst[size] = 0;
>    }
> 
>    #define get_task_array(dst,src) \
>       __cstr_array_copy(dst, src, __must_be_array(dst))
> 
> The relevant 'memcpy()' users were identified using the following search
> pattern:
>  $ git grep 'memcpy.*->comm\>'
> 
> Link: https://lore.kernel.org/all/CAHk-=wi5c=_-FBGo_88CowJd_F-Gi6Ud9d=TALm65ReN7YjrMw@mail.gmail.com/ #1
> 
> Signed-off-by: Bhupesh <bhupesh@...lia.com>
> ---
>  include/linux/coredump.h                      |  2 +-
>  include/linux/sched.h                         | 32 +++++++++++++++++++
>  include/linux/tracepoint.h                    |  4 +--
>  include/trace/events/block.h                  | 10 +++---
>  include/trace/events/oom.h                    |  2 +-
>  include/trace/events/osnoise.h                |  2 +-
>  include/trace/events/sched.h                  | 13 ++++----
>  include/trace/events/signal.h                 |  2 +-
>  include/trace/events/task.h                   |  4 +--
>  tools/bpf/bpftool/pids.c                      |  6 ++--
>  .../bpf/test_kmods/bpf_testmod-events.h       |  2 +-
>  11 files changed, 54 insertions(+), 25 deletions(-)
> 
> diff --git a/include/linux/coredump.h b/include/linux/coredump.h
> index 68861da4cf7c..bcee0afc5eaf 100644
> --- a/include/linux/coredump.h
> +++ b/include/linux/coredump.h
> @@ -54,7 +54,7 @@ extern void vfs_coredump(const kernel_siginfo_t *siginfo);
>  	do {	\
>  		char comm[TASK_COMM_LEN];	\
>  		/* This will always be NUL terminated. */ \
> -		memcpy(comm, current->comm, sizeof(comm)); \
> +		get_task_array(comm, current->comm); \
>  		printk_ratelimited(Level "coredump: %d(%*pE): " Format "\n",	\
>  			task_tgid_vnr(current), (int)strlen(comm), comm, ##__VA_ARGS__);	\
>  	} while (0)	\
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 5a58c1270474..d26d1dfb9904 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1960,12 +1960,44 @@ extern void wake_up_new_task(struct task_struct *tsk);
>  
>  extern void kick_process(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 logic inside __set_task_comm() should ensure that the task comm is
> + *   always NUL-terminated and zero-padded. Therefore the race condition between
> + *   reader and writer is not an issue.
> + */
> +
>  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);	\
>  	__set_task_comm(tsk, from, false);		\
>  })
>  
> +/*
> + * 'get_task_array' can be 'data-racy' in the destination and
> + * should not be used for cases where a 'stable NUL at the end'
> + * is needed. Its better to use strscpy and friends for such
> + * use-cases.
> + *
> + * It is suited mainly for a 'just copy comm to a constant-sized
> + * array' case - especially in performance sensitive use-cases,
> + * like tracing.
> + */
> +
> +static __always_inline void
> +	__cstr_array_copy(char *dst, const char *src,
> +			  __kernel_size_t size)
> +{
> +	memcpy(dst, src, size);
> +	dst[size] = 0;
> +}

Please don't reinvent the wheel. :) We already have memtostr, please use
that (or memtostr_pad).

> +
> +#define get_task_array(dst, src) \
> +	__cstr_array_copy(dst, src, __must_be_array(dst))

Uh, __must_be_array(dst) returns 0 on success. :P Are you sure you
tested this?

-- 
Kees Cook