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>

Author: Alexei Starovoitov

tools/lib/bpf/bpf_helpers.h

@@ -291,4 +291,107 @@ enum libbpf_tristate {
 /* Helper macro to print out debug messages */
 #define bpf_printk(fmt, args...) ___bpf_pick_printk(args)(fmt, ##args)
 
+struct bpf_iter_num;
+
+extern int bpf_iter_num_new(struct bpf_iter_num *it, int start, int end) __weak __ksym;
+extern int *bpf_iter_num_next(struct bpf_iter_num *it) __weak __ksym;
+extern void bpf_iter_num_destroy(struct bpf_iter_num *it) __weak __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

tools/lib/bpf/libbpf.c

@@ -333,6 +333,7 @@ struct reloc_desc {
 		struct {
 			int map_idx;
 			int sym_off;
+			int ext_idx;
 		};
 	};
 };
@@ -4042,7 +4043,7 @@ static int bpf_program__record_reloc(struct bpf_program *prog,
 		else
 			reloc_desc->type = RELO_EXTERN_LD64;
 		reloc_desc->insn_idx = insn_idx;
-		reloc_desc->sym_off = i; /* sym_off stores extern index */
+		reloc_desc->ext_idx = i;
 		return 0;
 	}
 
@@ -5811,8 +5812,8 @@ bpf_object__relocate_core(struct bpf_object *obj, const char *targ_btf_path)
 }
 
 /* base map load ldimm64 special constant, used also for log fixup logic */
-#define MAP_LDIMM64_POISON_BASE 2001000000
-#define MAP_LDIMM64_POISON_PFX "200100"
+#define POISON_LDIMM64_MAP_BASE 2001000000
+#define POISON_LDIMM64_MAP_PFX "200100"
 
 static void poison_map_ldimm64(struct bpf_program *prog, int relo_idx,
 			       int insn_idx, struct bpf_insn *insn,
@@ -5834,12 +5835,36 @@ static void poison_map_ldimm64(struct bpf_program *prog, int relo_idx,
 		 * invalid func unknown#2001000123
 		 * where lower 123 is map index into obj->maps[] array
 		 */
-		insn->imm = MAP_LDIMM64_POISON_BASE + map_idx;
+		insn->imm = POISON_LDIMM64_MAP_BASE + map_idx;
 
 		insn++;
 	}
 }
 
+/* unresolved kfunc call special constant, used also for log fixup logic */
+#define POISON_CALL_KFUNC_BASE 2002000000
+#define POISON_CALL_KFUNC_PFX "2002"
+
+static void poison_kfunc_call(struct bpf_program *prog, int relo_idx,
+			      int insn_idx, struct bpf_insn *insn,
+			      int ext_idx, const struct extern_desc *ext)
+{
+	pr_debug("prog '%s': relo #%d: poisoning insn #%d that calls kfunc '%s'\n",
+		 prog->name, relo_idx, insn_idx, ext->name);
+
+	/* we turn kfunc call into invalid helper call with identifiable constant */
+	insn->code = BPF_JMP | BPF_CALL;
+	insn->dst_reg = 0;
+	insn->src_reg = 0;
+	insn->off = 0;
+	/* if this instruction is reachable (not a dead code),
+	 * verifier will complain with something like:
+	 * invalid func unknown#2001000123
+	 * where lower 123 is extern index into obj->externs[] array
+	 */
+	insn->imm = POISON_CALL_KFUNC_BASE + ext_idx;
+}
+
 /* Relocate data references within program code:
  *  - map references;
  *  - global variable references;
@@ -5885,7 +5910,7 @@ bpf_object__relocate_data(struct bpf_object *obj, struct bpf_program *prog)
 			}
 			break;
 		case RELO_EXTERN_LD64:
-			ext = &obj->externs[relo->sym_off];
+			ext = &obj->externs[relo->ext_idx];
 			if (ext->type == EXT_KCFG) {
 				if (obj->gen_loader) {
 					insn[0].src_reg = BPF_PSEUDO_MAP_IDX_VALUE;
@@ -5907,14 +5932,14 @@ bpf_object__relocate_data(struct bpf_object *obj, struct bpf_program *prog)
 			}
 			break;
 		case RELO_EXTERN_CALL:
-			ext = &obj->externs[relo->sym_off];
+			ext = &obj->externs[relo->ext_idx];
 			insn[0].src_reg = BPF_PSEUDO_KFUNC_CALL;
 			if (ext->is_set) {
 				insn[0].imm = ext->ksym.kernel_btf_id;
 				insn[0].off = ext->ksym.btf_fd_idx;
-			} else { /* unresolved weak kfunc */
-				insn[0].imm = 0;
-				insn[0].off = 0;
+			} else { /* unresolved weak kfunc call */
+				poison_kfunc_call(prog, i, relo->insn_idx, insn,
+						  relo->ext_idx, ext);
 			}
 			break;
 		case RELO_SUBPROG_ADDR:
@@ -7022,13 +7047,13 @@ static void fixup_log_missing_map_load(struct bpf_program *prog,
 				       char *buf, size_t buf_sz, size_t log_sz,
 				       char *line1, char *line2, char *line3)
 {
-	/* Expected log for failed and not properly guarded CO-RE relocation:
+	/* Expected log for failed and not properly guarded map reference:
 	 * line1 -> 123: (85) call unknown#2001000345
 	 * line2 -> invalid func unknown#2001000345
 	 * line3 -> <anything else or end of buffer>
 	 *
 	 * "123" is the index of the instruction that was poisoned.
-	 * "345" in "2001000345" are map index in obj->maps to fetch map name.
+	 * "345" in "2001000345" is a map index in obj->maps to fetch map name.
 	 */
 	struct bpf_object *obj = prog->obj;
 	const struct bpf_map *map;
@@ -7038,7 +7063,7 @@ static void fixup_log_missing_map_load(struct bpf_program *prog,
 	if (sscanf(line1, "%d: (%*d) call unknown#%d\n", &insn_idx, &map_idx) != 2)
 		return;
 
-	map_idx -= MAP_LDIMM64_POISON_BASE;
+	map_idx -= POISON_LDIMM64_MAP_BASE;
 	if (map_idx < 0 || map_idx >= obj->nr_maps)
 		return;
 	map = &obj->maps[map_idx];
@@ -7051,6 +7076,39 @@ static void fixup_log_missing_map_load(struct bpf_program *prog,
 	patch_log(buf, buf_sz, log_sz, line1, line3 - line1, patch);
 }
 
+static void fixup_log_missing_kfunc_call(struct bpf_program *prog,
+					 char *buf, size_t buf_sz, size_t log_sz,
+					 char *line1, char *line2, char *line3)
+{
+	/* Expected log for failed and not properly guarded kfunc call:
+	 * line1 -> 123: (85) call unknown#2002000345
+	 * line2 -> invalid func unknown#2002000345
+	 * line3 -> <anything else or end of buffer>
+	 *
+	 * "123" is the index of the instruction that was poisoned.
+	 * "345" in "2002000345" is an extern index in obj->externs to fetch kfunc name.
+	 */
+	struct bpf_object *obj = prog->obj;
+	const struct extern_desc *ext;
+	int insn_idx, ext_idx;
+	char patch[128];
+
+	if (sscanf(line1, "%d: (%*d) call unknown#%d\n", &insn_idx, &ext_idx) != 2)
+		return;
+
+	ext_idx -= POISON_CALL_KFUNC_BASE;
+	if (ext_idx < 0 || ext_idx >= obj->nr_extern)
+		return;
+	ext = &obj->externs[ext_idx];
+
+	snprintf(patch, sizeof(patch),
+		 "%d: <invalid kfunc call>\n"
+		 "kfunc '%s' is referenced but wasn't resolved\n",
+		 insn_idx, ext->name);
+
+	patch_log(buf, buf_sz, log_sz, line1, line3 - line1, patch);
+}
+
 static void fixup_verifier_log(struct bpf_program *prog, char *buf, size_t buf_sz)
 {
 	/* look for familiar error patterns in last N lines of the log */
@@ -7070,23 +7128,33 @@ static void fixup_verifier_log(struct bpf_program *prog, char *buf, size_t buf_s
 		if (!cur_line)
 			return;
 
-		/* failed CO-RE relocation case */
 		if (str_has_pfx(cur_line, "invalid func unknown#195896080\n")) {
 			prev_line = find_prev_line(buf, cur_line);
 			if (!prev_line)
 				continue;
 
+			/* failed CO-RE relocation case */
 			fixup_log_failed_core_relo(prog, buf, buf_sz, log_sz,
 						   prev_line, cur_line, next_line);
 			return;
-		} else if (str_has_pfx(cur_line, "invalid func unknown#"MAP_LDIMM64_POISON_PFX)) {
+		} else if (str_has_pfx(cur_line, "invalid func unknown#"POISON_LDIMM64_MAP_PFX)) {
 			prev_line = find_prev_line(buf, cur_line);
 			if (!prev_line)
 				continue;
 
+			/* reference to uncreated BPF map */
 			fixup_log_missing_map_load(prog, buf, buf_sz, log_sz,
 						   prev_line, cur_line, next_line);
 			return;
+		} else if (str_has_pfx(cur_line, "invalid func unknown#"POISON_CALL_KFUNC_PFX)) {
+			prev_line = find_prev_line(buf, cur_line);
+			if (!prev_line)
+				continue;
+
+			/* reference to unresolved kfunc */
+			fixup_log_missing_kfunc_call(prog, buf, buf_sz, log_sz,
+						     prev_line, cur_line, next_line);
+			return;
 		}
 	}
 }
@@ -7098,7 +7166,7 @@ static int bpf_program_record_relos(struct bpf_program *prog)
 
 	for (i = 0; i < prog->nr_reloc; i++) {
 		struct reloc_desc *relo = &prog->reloc_desc[i];
-		struct extern_desc *ext = &obj->externs[relo->sym_off];
+		struct extern_desc *ext = &obj->externs[relo->ext_idx];
 		int kind;
 
 		switch (relo->type) {
@@ -7536,8 +7604,9 @@ static int bpf_object__resolve_ksym_func_btf_id(struct bpf_object *obj,
 	ret = bpf_core_types_are_compat(obj->btf, local_func_proto_id,
 					kern_btf, kfunc_proto_id);
 	if (ret <= 0) {
-		pr_warn("extern (func ksym) '%s': func_proto [%d] incompatible with kernel [%d]\n",
-			ext->name, local_func_proto_id, kfunc_proto_id);
+		pr_warn("extern (func ksym) '%s': func_proto [%d] incompatible with %s [%d]\n",
+			ext->name, local_func_proto_id,
+			mod_btf ? mod_btf->name : "vmlinux", kfunc_proto_id);
 		return -EINVAL;
 	}
 
@@ -7571,8 +7640,8 @@ static int bpf_object__resolve_ksym_func_btf_id(struct bpf_object *obj,
 	 * {kernel_btf_id, kernel_btf_obj_fd} -> fixup ld_imm64.
 	 */
 	ext->ksym.kernel_btf_obj_fd = mod_btf ? mod_btf->fd : 0;
-	pr_debug("extern (func ksym) '%s': resolved to kernel [%d]\n",
-		 ext->name, kfunc_id);
+	pr_debug("extern (func ksym) '%s': resolved to %s [%d]\n",
+		 ext->name, mod_btf ? mod_btf->name : "vmlinux", kfunc_id);
 
 	return 0;
 }

tools/testing/selftests/bpf/prog_tests/log_fixup.c

@@ -135,6 +135,35 @@ static void missing_map(void)
 	test_log_fixup__destroy(skel);
 }
 
+static void missing_kfunc(void)
+{
+	char log_buf[8 * 1024];
+	struct test_log_fixup* skel;
+	int err;
+
+	skel = test_log_fixup__open();
+	if (!ASSERT_OK_PTR(skel, "skel_open"))
+		return;
+
+	bpf_program__set_autoload(skel->progs.use_missing_kfunc, true);
+	bpf_program__set_log_buf(skel->progs.use_missing_kfunc, log_buf, sizeof(log_buf));
+
+	err = test_log_fixup__load(skel);
+	if (!ASSERT_ERR(err, "load_fail"))
+		goto cleanup;
+
+	ASSERT_HAS_SUBSTR(log_buf,
+			  "0: <invalid kfunc call>\n"
+			  "kfunc 'bpf_nonexistent_kfunc' is referenced but wasn't resolved\n",
+			  "log_buf");
+
+	if (env.verbosity > VERBOSE_NONE)
+		printf("LOG:   \n=================\n%s=================\n", log_buf);
+
+cleanup:
+	test_log_fixup__destroy(skel);
+}
+
 void test_log_fixup(void)
 {
 	if (test__start_subtest("bad_core_relo_trunc_none"))
@@ -147,4 +176,6 @@ void test_log_fixup(void)
 		bad_core_relo_subprog();
 	if (test__start_subtest("missing_map"))
 		missing_map();
+	if (test__start_subtest("missing_kfunc"))
+		missing_kfunc();
 }