On Mon, 1 Sep 2025 10:58:17 +0530
Bhupesh Sharma [off-list ref] wrote:

Hi Kees,

On 8/25/25 7:31 PM, Kees Cook wrote:

quoted

On Thu, Aug 21, 2025 at 03:51:51PM +0530, Bhupesh wrote:  

quoted

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/ (local)  #1

Signed-off-by: Bhupesh<redacted>
---
  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).  

Sure, but wouldn't we get a static assertion failure: "must be array" 
for memtostr() usage, because of the following:

#define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]) + 
__must_be_array(arr))

I think it would be easier just to set:

   memcpy(dst, src, size);
   dst[size -1] = 0;

instead as Linus and Steven suggested.

The compiler is still likely to make a mess of it.
You really want:
	*(u64 *)dst = *(u64 *)src;
	*(u64 *)(dst + 8) = *(u64 *)(src + 8) & ~htobe64(0xff);
That may need something to force 8 byte alignment.
Or force 4 byte alignment and use a u64 type with 4 byte alignment.

	David

Thanks,
Bhupesh

quoted

quoted

+
+#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?