[PATCH v7 03/11] sched.h: use __must_be_array instead of BUILD_BUG_ON in get_task_comm

[Yafang Shao]

Now that __get_task_comm() will truncate and pad, it's safe to use on both
too-small and too-big arrays. So it is not needed to check array length
any more. For the original goal of making sure get_task_comm() is being
used on a char array, we can use __must_be_array().

Below is the verification when I changed the dest buffer of
get_task_comm() in a driver code,

  CC [M]  drivers/infiniband/hw/qib/qib_file_ops.o
In file included from ./include/linux/bits.h:22:0,
                 from ./include/linux/ioport.h:13,
                 from ./include/linux/pci.h:31,
                 from drivers/infiniband/hw/qib/qib_file_ops.c:35:
drivers/infiniband/hw/qib/qib_file_ops.c: In function ‘setup_ctxt’:
./include/linux/build_bug.h:16:51: error: negative width in bit-field ‘<anonymous>’
 #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); })))
                                                   ^
./include/linux/compiler.h:258:28: note: in expansion of macro ‘BUILD_BUG_ON_ZERO’
 #define __must_be_array(a) BUILD_BUG_ON_ZERO(__same_type((a), &(a)[0]))
                            ^~~~~~~~~~~~~~~~~
./include/linux/sched.h:1941:23: note: in expansion of macro ‘__must_be_array’
  __get_task_comm(buf, __must_be_array(buf) + sizeof(buf), tsk)
                       ^~~~~~~~~~~~~~~
drivers/infiniband/hw/qib/qib_file_ops.c:1325:2: note: in expansion of macro ‘get_task_comm’
  get_task_comm(test, current);

It hit this warnig as expected.

Suggested-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <arnaldo.melo@xxxxxxxxx>
Cc: Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx>
Cc: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Petr Mladek <pmladek@xxxxxxxx>
---
 include/linux/sched.h | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index c1a927ddec64..b9c85c52fed0 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1931,10 +1931,8 @@ static inline void set_task_comm(struct task_struct *tsk, const char *from)
 }
 
 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);		\
-})
+#define get_task_comm(buf, tsk)		\
+	__get_task_comm(buf, __must_be_array(buf) + sizeof(buf), tsk)
 
 #ifdef CONFIG_SMP
 static __always_inline void scheduler_ipi(void)
-- 
2.17.1