The macro list_for_each_entry is defined in bpf_arena_list.h as
follows:
#define list_for_each_entry(pos, head, member) \
for (void * ___tmp = (pos = list_entry_safe((head)->first, \
typeof(*(pos)), member), \
(void *)0); \
pos && ({ ___tmp = (void *)pos->member.next; 1; }); \
cond_break, \
pos = list_entry_safe((void __arena *)___tmp, typeof(*(pos)), member))
The macro cond_break, in turn, expands to a statement expression that
contains a `break' statement. Compound statement expressions, and the
subsequent ability of placing statements in the header of a `for'
loop, are GNU extensions.
Unfortunately, clang implements this GNU extension differently than
GCC:
- In GCC the `break' statement is bound to the containing "breakable"
context in which the defining `for' appears. If there is no such
context, GCC emits a warning: break statement without enclosing `for'
o `switch' statement.
- In clang the `break' statement is bound to the defining `for'. If
the defining `for' is itself inside some breakable construct, then
clang emits a -Wgcc-compat warning.
This patch makes it possible to use the cond_break macro with GCC by
adding an outer breakable context __compat_break that expands to a
one-iteration `for' loop when compiling with GCC, and to nothing when
compiling with clang. It is important to note that the __compat_break
one-iteration loop gets optimized away by GCC with -O1 and higher.
The list_for_each_entry macro is adapted to use __compat_break, as are
the pertinent loops in the few tests that use cond_break directly.
Tested in bpf-next master.
No regressions.
Signed-off-by: Jose E. Marchesi <jose.marchesi@xxxxxxxxxx>
Cc: david.faust@xxxxxxxxxx
Cc: cupertino.miranda@xxxxxxxxxx
Cc: Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx>
---
tools/testing/selftests/bpf/bpf_arena_list.h | 2 ++
tools/testing/selftests/bpf/bpf_experimental.h | 11 +++++++++++
tools/testing/selftests/bpf/progs/arena_list.c | 1 +
.../bpf/progs/verifier_iterating_callbacks.c | 3 +++
4 files changed, 17 insertions(+)
diff --git a/tools/testing/selftests/bpf/bpf_arena_list.h b/tools/testing/selftests/bpf/bpf_arena_list.h
index b99b9f408eff..5659c715a8a0 100644
--- a/tools/testing/selftests/bpf/bpf_arena_list.h
+++ b/tools/testing/selftests/bpf/bpf_arena_list.h
@@ -29,10 +29,12 @@ static inline void *bpf_iter_num_new(struct bpf_iter_num *it, int i, int j) { re
static inline void bpf_iter_num_destroy(struct bpf_iter_num *it) {}
static inline bool bpf_iter_num_next(struct bpf_iter_num *it) { return true; }
#define cond_break ({})
+#define __compat_break
#endif
/* Safely walk link list elements. Deletion of elements is allowed. */
#define list_for_each_entry(pos, head, member) \
+ __compat_break \
for (void * ___tmp = (pos = list_entry_safe((head)->first, \
typeof(*(pos)), member), \
(void *)0); \
diff --git a/tools/testing/selftests/bpf/bpf_experimental.h b/tools/testing/selftests/bpf/bpf_experimental.h
index 8b9cc87be4c4..7f03570638a6 100644
--- a/tools/testing/selftests/bpf/bpf_experimental.h
+++ b/tools/testing/selftests/bpf/bpf_experimental.h
@@ -326,6 +326,17 @@ l_true: \
})
#endif
+/* A `break' executed in the head of a `for' loop statement is bound
+ to the current loop in clang, but it is bound to the enclosing loop
+ in GCC. Note both compilers optimize the outer loop out with -O1
+ and higher. This macro shall be used to annotate any loop that
+ uses cond_break within its header. */
+#ifdef __clang__
+#define __compat_break
+#else
+#define __compat_break for (int __control = 1; __control; --__control)
+#endif
+
#ifdef __BPF_FEATURE_MAY_GOTO
#define cond_break \
({ __label__ l_break, l_continue; \
diff --git a/tools/testing/selftests/bpf/progs/arena_list.c b/tools/testing/selftests/bpf/progs/arena_list.c
index c0422c58cee2..570c1e043257 100644
--- a/tools/testing/selftests/bpf/progs/arena_list.c
+++ b/tools/testing/selftests/bpf/progs/arena_list.c
@@ -49,6 +49,7 @@ int arena_list_add(void *ctx)
list_head = &global_head;
+ __compat_break
for (i = zero; i < cnt; cond_break, i++) {
struct elem __arena *n = bpf_alloc(sizeof(*n));
diff --git a/tools/testing/selftests/bpf/progs/verifier_iterating_callbacks.c b/tools/testing/selftests/bpf/progs/verifier_iterating_callbacks.c
index 99e561f18f9b..e0437609af21 100644
--- a/tools/testing/selftests/bpf/progs/verifier_iterating_callbacks.c
+++ b/tools/testing/selftests/bpf/progs/verifier_iterating_callbacks.c
@@ -318,6 +318,7 @@ int cond_break1(const void *ctx)
unsigned long i;
unsigned int sum = 0;
+ __compat_break
for (i = zero; i < ARR_SZ; cond_break, i++)
sum += i;
for (i = zero; i < ARR_SZ; i++) {
@@ -336,6 +337,7 @@ int cond_break2(const void *ctx)
int i, j;
int sum = 0;
+ __compat_break
for (i = zero; i < 1000; cond_break, i++)
for (j = zero; j < 1000; j++) {
sum += i + j;
@@ -349,6 +351,7 @@ static __noinline int loop(void)
{
int i, sum = 0;
+ __compat_break
for (i = zero; i <= 1000000; i++, cond_break)
sum += i;
--
2.30.2
