nathanchance added a comment.
The error added by this patch triggers on some recently added code to the Linux
kernel's -next tree, which I failed to test above due to its generally unstable
nature:
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
https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/include/drm/drm_exec.h
https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/drivers/gpu/drm/tests/drm_exec_test.c
A standalone reproducer from the kernel sources:
struct drm_exec {};
int drm_exec_cleanup(struct drm_exec *);
void drm_exec_fini(struct drm_exec *);
void drm_exec_init(struct drm_exec *);
int drm_exec_is_contended(struct drm_exec *);
void drm_exec_lock_obj(struct drm_exec *);
void test_lock(void)
{
struct drm_exec exec;
drm_exec_init(&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);
});) {
drm_exec_lock_obj(&exec);
do {
if (__builtin_expect(!!(drm_exec_is_contended(&exec)),
0))
goto *__drm_exec_retry_ptr;
} while (0);
}
drm_exec_fini(&exec);
}
$ clang -fsyntax-only test.c
test.c:24:5: error: cannot jump from this indirect goto statement to one of
its possible targets
24 | goto *__drm_exec_retry_ptr;
| ^
test.c:15:6: note: possible target of indirect goto statement
15 | __drm_exec_retry:
| ^
test.c:13:35: note: jump enters a statement expression
13 | for (void *__drm_exec_retry_ptr; ({
| ^
1 error generated.
I am not sure this is a problem with the patch, since if I am reading GCC's
documentation on statement expressions correctly
(https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html), the construct the
kernel is using with labels as values to jump into a statement expression with
a computed goto is undefined behavior, but I figured I would bring it up just
in case there is a problem.
> Jumping into a statement expression with goto or using a switch statement
> outside the statement expression with a case or default label inside the
> statement expression is not permitted. Jumping into a statement expression
> with a computed goto (see Labels as Values) has undefined behavior.
If this is a problem on the kernel side, I can bring this up to them. GCC
13.1.0 at least does not error on this but if it is documented as UB, the
construct should still be avoided.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D154696/new/
https://reviews.llvm.org/D154696
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