Merge branch 'Provide bpf_for() and bpf_for_each() by libbpf'

Andrii Nakryiko says:

====================

This patch set moves bpf_for(), bpf_for_each(), and bpf_repeat() macros from
selftests-internal bpf_misc.h header to libbpf-provided bpf_helpers.h header.
To do this in a way to allow users to feature-detect and guard such
bpf_for()/bpf_for_each() uses on old kernels we also extend libbpf to improve
unresolved kfunc calls handling and reporting. This lets us mark
bpf_iter_num_{new,next,destroy}() declarations as __weak, and thus not fail
program loading outright if such kfuncs are missing on the host kernel.

Patches #1 and #2 do some simple clean ups and logging improvements. Patch #3
adds kfunc call poisoning and log fixup logic and is the hear of this patch
set, effectively. Patch #4 adds selftest for this logic. Patches #4 and #5
move bpf_for()/bpf_for_each()/bpf_repeat() into bpf_helpers.h header and mark
kfuncs as __weak to allow users to feature-detect and guard their uses.
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>

2 files changed, 10 insertions, 103 deletions

diff --git a/tools/testing/selftests/bpf/progs/bpf_misc.h b/tools/testing/selftests/bpf/progs/bpf_misc.h
index 6e3b4903c541..3b307de8dab9 100644
--- a/tools/testing/selftests/bpf/progs/bpf_misc.h
+++ b/tools/testing/selftests/bpf/progs/bpf_misc.h
@@ -121,107 +121,4 @@
 /* make it look to compiler like value is read and written */
 #define __sink(expr) asm volatile("" : "+g"(expr))
 
-struct bpf_iter_num;
-
-extern int bpf_iter_num_new(struct bpf_iter_num *it, int start, int end) __ksym;
-extern int *bpf_iter_num_next(struct bpf_iter_num *it) __ksym;
-extern void bpf_iter_num_destroy(struct bpf_iter_num *it) __ksym;
-
-#ifndef bpf_for_each
-/* bpf_for_each(iter_type, cur_elem, args...) provides generic construct for
- * using BPF open-coded iterators without having to write mundane explicit
- * low-level loop logic. Instead, it provides for()-like generic construct
- * that can be used pretty naturally. E.g., for some hypothetical cgroup
- * iterator, you'd write:
- *
- * struct cgroup *cg, *parent_cg = <...>;
- *
- * bpf_for_each(cgroup, cg, parent_cg, CG_ITER_CHILDREN) {
- *     bpf_printk("Child cgroup id = %d", cg->cgroup_id);
- *     if (cg->cgroup_id == 123)
- *         break;
- * }
- *
- * I.e., it looks almost like high-level for each loop in other languages,
- * supports continue/break, and is verifiable by BPF verifier.
- *
- * For iterating integers, the difference betwen bpf_for_each(num, i, N, M)
- * and bpf_for(i, N, M) is in that bpf_for() provides additional proof to
- * verifier that i is in [N, M) range, and in bpf_for_each() case i is `int
- * *`, not just `int`. So for integers bpf_for() is more convenient.
- *
- * Note: this macro relies on C99 feature of allowing to declare variables
- * inside for() loop, bound to for() loop lifetime. It also utilizes GCC
- * extension: __attribute__((cleanup(<func>))), supported by both GCC and
- * Clang.
- */
-#define bpf_for_each(type, cur, args...) for (							\
-	/* initialize and define destructor */							\
-	struct bpf_iter_##type ___it __attribute__((aligned(8), /* enforce, just in case */,	\
-						    cleanup(bpf_iter_##type##_destroy))),	\
-	/* ___p pointer is just to call bpf_iter_##type##_new() *once* to init ___it */		\
-			       *___p __attribute__((unused)) = (				\
-					bpf_iter_##type##_new(&___it, ##args),			\
-	/* this is a workaround for Clang bug: it currently doesn't emit BTF */			\
-	/* for bpf_iter_##type##_destroy() when used from cleanup() attribute */		\
-					(void)bpf_iter_##type##_destroy, (void *)0);		\
-	/* iteration and termination check */							\
-	(((cur) = bpf_iter_##type##_next(&___it)));						\
-)
-#endif /* bpf_for_each */
-
-#ifndef bpf_for
-/* bpf_for(i, start, end) implements a for()-like looping construct that sets
- * provided integer variable *i* to values starting from *start* through,
- * but not including, *end*. It also proves to BPF verifier that *i* belongs
- * to range [start, end), so this can be used for accessing arrays without
- * extra checks.
- *
- * Note: *start* and *end* are assumed to be expressions with no side effects
- * and whose values do not change throughout bpf_for() loop execution. They do
- * not have to be statically known or constant, though.
- *
- * Note: similarly to bpf_for_each(), it relies on C99 feature of declaring for()
- * loop bound variables and cleanup attribute, supported by GCC and Clang.
- */
-#define bpf_for(i, start, end) for (								\
-	/* initialize and define destructor */							\
-	struct bpf_iter_num ___it __attribute__((aligned(8), /* enforce, just in case */	\
-						 cleanup(bpf_iter_num_destroy))),		\
-	/* ___p pointer is necessary to call bpf_iter_num_new() *once* to init ___it */		\
-			    *___p __attribute__((unused)) = (					\
-				bpf_iter_num_new(&___it, (start), (end)),			\
-	/* this is a workaround for Clang bug: it currently doesn't emit BTF */			\
-	/* for bpf_iter_num_destroy() when used from cleanup() attribute */			\
-				(void)bpf_iter_num_destroy, (void *)0);				\
-	({											\
-		/* iteration step */								\
-		int *___t = bpf_iter_num_next(&___it);						\
-		/* termination and bounds check */						\
-		(___t && ((i) = *___t, (i) >= (start) && (i) < (end)));				\
-	});											\
-)
-#endif /* bpf_for */
-
-#ifndef bpf_repeat
-/* bpf_repeat(N) performs N iterations without exposing iteration number
- *
- * Note: similarly to bpf_for_each(), it relies on C99 feature of declaring for()
- * loop bound variables and cleanup attribute, supported by GCC and Clang.
- */
-#define bpf_repeat(N) for (									\
-	/* initialize and define destructor */							\
-	struct bpf_iter_num ___it __attribute__((aligned(8), /* enforce, just in case */	\
-						 cleanup(bpf_iter_num_destroy))),		\
-	/* ___p pointer is necessary to call bpf_iter_num_new() *once* to init ___it */		\
-			    *___p __attribute__((unused)) = (					\
-				bpf_iter_num_new(&___it, 0, (N)),				\
-	/* this is a workaround for Clang bug: it currently doesn't emit BTF */			\
-	/* for bpf_iter_num_destroy() when used from cleanup() attribute */			\
-				(void)bpf_iter_num_destroy, (void *)0);				\
-	bpf_iter_num_next(&___it);								\
-	/* nothing here  */									\
-)
-#endif /* bpf_repeat */
-
 #endif
diff --git a/tools/testing/selftests/bpf/progs/test_log_fixup.c b/tools/testing/selftests/bpf/progs/test_log_fixup.c
index 60450cb0e72e..1bd48feaaa42 100644
--- a/tools/testing/selftests/bpf/progs/test_log_fixup.c
+++ b/tools/testing/selftests/bpf/progs/test_log_fixup.c
@@ -61,4 +61,14 @@ int use_missing_map(const void *ctx)
 	return value != NULL;
 }
 
+extern int bpf_nonexistent_kfunc(void) __ksym __weak;
+
+SEC("?raw_tp/sys_enter")
+int use_missing_kfunc(const void *ctx)
+{
+	bpf_nonexistent_kfunc();
+
+	return 0;
+}
+
 char _license[] SEC("license") = "GPL";