[PATCH AUTOSEL for 4.14 002/110] exec: avoid gcc-8 warning for get_task_comm

[Sasha Levin]

From: Arnd Bergmann <arnd@xxxxxxxx>

[ 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.

Link: http://lkml.kernel.org/r/20171205151724.1764896-1-arnd@xxxxxxxx
Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
Suggested-by: Kees Cook <keescook@xxxxxxxxxxxx>
Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>
Acked-by: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Serge Hallyn <serge@xxxxxxxxxx>
Cc: James Morris <james.l.morris@xxxxxxxxxx>
Cc: Aleksa Sarai <asarai@xxxxxxx>
Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
---
 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 acec119fcc31..0da4d748b4e6 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -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
diff --git a/include/linux/sched.h b/include/linux/sched.h
index fdf74f27acf1..41354690e4e3 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1502,7 +1502,11 @@ 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);
-- 
2.11.0