bpf: make list_for_each_entry portable

[Changes from V1:
- The __compat_break has been abandoned in favor of
  a more readable can_loop macro that can be used anywhere, including
  loop conditions.]

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 adds a new macro can_loop to bpf_experimental, that
implements the same logic than cond_break but evaluates to a boolean
expression.  The patch also changes all the current instances of usage
of cond_break withing the header of loop accordingly.

Tested in bpf-next master.
No regressions.

Signed-off-by: Jose E. Marchesi <jose.marchesi@oracle.com>
Cc: david.faust@oracle.com
Cc: cupertino.miranda@oracle.com
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Link: https://lore.kernel.org/r/20240511212243.23477-1-jose.marchesi@oracle.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/bpf_arena_list.h

@@ -29,6 +29,7 @@ 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 can_loop true
 #endif
 
 /* Safely walk link list elements. Deletion of elements is allowed. */
@@ -36,8 +37,7 @@ static inline bool bpf_iter_num_next(struct bpf_iter_num *it) { return true; }
 	for (void * ___tmp = (pos = list_entry_safe((head)->first,		\
 						    typeof(*(pos)), member),	\
 			      (void *)0);					\
-	     pos && ({ ___tmp = (void *)pos->member.next; 1; });		\
-	     cond_break,							\
+	     pos && ({ ___tmp = (void *)pos->member.next; 1; }) && can_loop;    \
 	     pos = list_entry_safe((void __arena *)___tmp, typeof(*(pos)), member))
 
 static inline void list_add_head(arena_list_node_t *n, arena_list_head_t *h)

tools/testing/selftests/bpf/bpf_experimental.h

@@ -326,19 +326,48 @@ l_true:												\
        })
 #endif
 
+/*
+ * Note that cond_break can only be portably used in the body of a breakable
+ * construct, whereas can_loop can be used anywhere.
+ */
 #ifdef __BPF_FEATURE_MAY_GOTO
+#define can_loop					\
+	({ __label__ l_break, l_continue;		\
+	bool ret = true;				\
+	asm volatile goto("may_goto %l[l_break]"	\
+		      :::: l_break);			\
+	goto l_continue;				\
+	l_break: ret = false;				\
+	l_continue:;					\
+	ret;						\
+	})
+
 #define cond_break					\
 	({ __label__ l_break, l_continue;		\
-	 asm volatile goto("may_goto %l[l_break]"	\
+	asm volatile goto("may_goto %l[l_break]"	\
 		      :::: l_break);			\
 	goto l_continue;				\
 	l_break: break;					\
 	l_continue:;					\
 	})
 #else
+#define can_loop					\
+	({ __label__ l_break, l_continue;		\
+	bool ret = true;				\
+	asm volatile goto("1:.byte 0xe5;		\
+		      .byte 0;				\
+		      .long ((%l[l_break] - 1b - 8) / 8) & 0xffff;	\
+		      .short 0"				\
+		      :::: l_break);			\
+	goto l_continue;				\
+	l_break: ret = false;				\
+	l_continue:;					\
+	ret;						\
+	})
+
 #define cond_break					\
 	({ __label__ l_break, l_continue;		\
-	 asm volatile goto("1:.byte 0xe5;			\
+	asm volatile goto("1:.byte 0xe5;		\
 		      .byte 0;				\
 		      .long ((%l[l_break] - 1b - 8) / 8) & 0xffff;	\
 		      .short 0"				\

tools/testing/selftests/bpf/progs/arena_list.c

@@ -49,7 +49,7 @@ int arena_list_add(void *ctx)
 
 	list_head = &global_head;
 
-	for (i = zero; i < cnt; cond_break, i++) {
+	for (i = zero; i < cnt && can_loop; i++) {
 		struct elem __arena *n = bpf_alloc(sizeof(*n));
 
 		test_val++;

tools/testing/selftests/bpf/progs/verifier_iterating_callbacks.c

@@ -318,7 +318,7 @@ int cond_break1(const void *ctx)
 	unsigned long i;
 	unsigned int sum = 0;
 
-	for (i = zero; i < ARR_SZ; cond_break, i++)
+	for (i = zero; i < ARR_SZ && can_loop; i++)
 		sum += i;
 	for (i = zero; i < ARR_SZ; i++) {
 		barrier_var(i);
@@ -336,12 +336,11 @@ int cond_break2(const void *ctx)
 	int i, j;
 	int sum = 0;
 
-	for (i = zero; i < 1000; cond_break, i++)
+	for (i = zero; i < 1000 && can_loop; i++)
 		for (j = zero; j < 1000; j++) {
 			sum += i + j;
 			cond_break;
-		}
-
+	}
 	return sum;
 }
 
@@ -349,7 +348,7 @@ static __noinline int loop(void)
 {
 	int i, sum = 0;
 
-	for (i = zero; i <= 1000000; i++, cond_break)
+	for (i = zero; i <= 1000000 && can_loop; i++)
 		sum += i;
 
 	return sum;