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 <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: Alexei Starovoitov <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

1 files changed, 4 insertions, 5 deletions

diff --git a/tools/testing/selftests/bpf/progs/verifier_iterating_callbacks.c b/tools/testing/selftests/bpf/progs/verifier_iterating_callbacks.c
index 99e561f18f9b..bd676d7e615f 100644
--- a/tools/testing/selftests/bpf/progs/verifier_iterating_callbacks.c
+++ b/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;