[PATCH v2] exec: avoid gcc-8 warning for get_task_comm

[Arnd Bergmann <arnd@arndb.de>]

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 callsights 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.

Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 fs/exec.c             | 7 +++----
 include/linux/sched.h | 6 +++++-
 2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 6be2aa0ab26f..156f56acfe8e 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1216,15 +1216,14 @@ static int de_thread(struct task_struct *tsk)
 	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
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 21991d668d35..5124ba709830 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1503,7 +1503,11 @@ 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, 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);
-- 
2.9.0