[PATCH v2] drm: fix indirect goto into statement expression UB

[ndesaulniers@google.com]

A new diagnostic in clang-17 now produces the following build error:

drivers/gpu/drm/tests/drm_exec_test.c:41:3: error: cannot jump from this
indirect goto statement to one of its possible targets
   41 |                 drm_exec_retry_on_contention(&exec);
      |                 ^
include/drm/drm_exec.h:96:4: note: expanded from macro
'drm_exec_retry_on_contention'
   96 |                         goto *__drm_exec_retry_ptr;
      |                         ^
drivers/gpu/drm/tests/drm_exec_test.c:39:2: note: possible target of
indirect goto statement
   39 |         drm_exec_until_all_locked(&exec) {
      |         ^
include/drm/drm_exec.h:79:33: note: expanded from macro
'drm_exec_until_all_locked'
   79 |                 __label__ __drm_exec_retry;
drivers/gpu/drm/tests/drm_exec_test.c:39:2: note: jump enters a
statement expression

The GCC manually currently states that:
>> Jumping into a statement expression with a computed goto (see Labels
>> as Values) has undefined behavior.

So the diagnostic appears correct, even if codegen happened to produce
working code.

Looking closer at this code, while the original combination of statement
expression, local label, and computed/indirect goto GNU C expressions
were clever, a simple while loop and continue block might have sufficed.

This approach might not work as expected if drm_exec_until_all_locked
"loops" can be nested, but that doesn't appear to be an existing use
case in the codebase.

Fixes: commit 09593216bff1 ("drm: execution context for GEM buffers v7")
Link: https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html
Link: https://github.com/ClangBuiltLinux/linux/issues/1890
Link: https://github.com/llvm/llvm-project/commit/20219106060208f0c2f5d096eb3aed7b712f5067
Reported-by: Nathan Chancellor <nathan@kernel.org>
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
Changes in v2:
Fix the continue to be outside of the do while
- Link to v1: https://lore.kernel.org/r/20230727-amdgpu-v1-1-a95690e75388@google.com
---
 include/drm/drm_exec.h | 21 +++++----------------
 1 file changed, 5 insertions(+), 16 deletions(-)

diff --git a/include/drm/drm_exec.h b/include/drm/drm_exec.h
index 73205afec162..fa1cc5c3d021 100644
--- a/include/drm/drm_exec.h
+++ b/include/drm/drm_exec.h
@@ -70,18 +70,8 @@ struct drm_exec {
  * Core functionality of the drm_exec object. Loops until all GEM objects are
  * locked and no more contention exists. At the beginning of the loop it is
  * guaranteed that no GEM object is locked.
- *
- * Since labels can't be defined local to the loops body we use a jump pointer
- * to make sure that the retry is only used from within the loops body.
  */
-#define drm_exec_until_all_locked(exec)				\
-	for (void *__drm_exec_retry_ptr; ({			\
-		__label__ __drm_exec_retry;			\
-__drm_exec_retry:						\
-		__drm_exec_retry_ptr = &&__drm_exec_retry;	\
-		(void)__drm_exec_retry_ptr;			\
-		drm_exec_cleanup(exec);				\
-	});)
+#define drm_exec_until_all_locked(exec)	while(drm_exec_cleanup(exec))
 
 /**
  * drm_exec_retry_on_contention - restart the loop to grap all locks
@@ -90,11 +80,10 @@ __drm_exec_retry:						\
  * Control flow helper to continue when a contention was detected and we need to
  * clean up and re-start the loop to prepare all GEM objects.
  */
-#define drm_exec_retry_on_contention(exec)			\
-	do {							\
-		if (unlikely(drm_exec_is_contended(exec)))	\
-			goto *__drm_exec_retry_ptr;		\
-	} while (0)
+#define drm_exec_retry_on_contention(exec)		\
+	if (unlikely(drm_exec_is_contended(exec)))	\
+		continue;				\
+	do {} while (0)
 
 /**
  * drm_exec_is_contended - check for contention

---
base-commit: 451cc82bd11eb6a374f4dbcfc1cf007eafea91ab
change-id: 20230727-amdgpu-93c0e5302951

Best regards,
-- 
Nick Desaulniers <ndesaulniers@google.com>