Patch "exec: avoid gcc-8 warning for get_task_comm" has been added to the 4.14-stable tree

This is a note to let you know that I've just added the patch titled

exec: avoid gcc-8 warning for get_task_comm

to the 4.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
exec-avoid-gcc-8-warning-for-get_task_comm.patch
and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Arnd Bergmann <[email protected]>
Date: Thu, 14 Dec 2017 15:32:41 -0800
Subject: exec: avoid gcc-8 warning for get_task_comm

From: Arnd Bergmann <[email protected]>

[ Upstream commit 3756f6401c302617c5e091081ca4d26ab604bec5 ]

gcc-8 warns about using strncpy() with the source size as the limit:

fs/exec.c:1223:32: error: argument to 'sizeof' in 'strncpy' call is the same expression as the source; did you mean to use the size of the destination? [-Werror=sizeof-pointer-memaccess]

This is indeed slightly suspicious, as it protects us from source arguments without NUL-termination, but does not guarantee that the destination is terminated.

This keeps the strncpy() to ensure we have properly padded target buffer, but ensures that we use the correct length, by passing the actual length of the destination buffer as well as adding a build-time check to ensure it is exactly TASK_COMM_LEN.

There are only 23 callsites which I all reviewed to ensure this is currently the case. We could get away with doing only the check or passing the right length, but it doesn't hurt to do both.

---
 fs/exec.c             |    7 +++----
 include/linux/sched.h |    6 +++++-
 2 files changed, 8 insertions(+), 5 deletions(-)

@@ -1216,15 +1216,14 @@ killed:
 	return -EAGAIN;
 }
 
-char *get_task_comm(char *buf, struct task_struct *tsk)
+char *__get_task_comm(char *buf, size_t buf_size, struct task_struct *tsk)
 {
-	/* buf must be at least sizeof(tsk->comm) in size */
 	task_lock(tsk);
-	strncpy(buf, tsk->comm, sizeof(tsk->comm));
+	strncpy(buf, tsk->comm, buf_size);
 	task_unlock(tsk);
 	return buf;
 }
-EXPORT_SYMBOL_GPL(get_task_comm);
+EXPORT_SYMBOL_GPL(__get_task_comm);
 
 /*
  * These functions flushes out all traces of the currently running executable

@@ -1502,7 +1502,11 @@ static inline void set_task_comm(struct
 	__set_task_comm(tsk, from, false);
 }
 
-extern char *get_task_comm(char *to, struct task_struct *tsk);
+extern char *__get_task_comm(char *to, size_t len, struct task_struct *tsk);
+#define get_task_comm(buf, tsk) ({			\
+	BUILD_BUG_ON(sizeof(buf) != TASK_COMM_LEN);	\
+	__get_task_comm(buf, sizeof(buf), tsk);		\
+})
 
 #ifdef CONFIG_SMP
 void scheduler_ipi(void);

Patches currently in stable-queue which might be from [email protected] are

queue-4.14/exec-avoid-gcc-8-warning-for-get_task_comm.patch queue-4.14/x86-asm-allow-again-using-asm.h-when-building-for-the-bpf-clang-target.patch queue-4.14/arm-dts-ls1021a-fix-incorrect-clock-references.patch