[PATCH dwarves] btf_encoder: Infer arena kfunc arguments from suffixes

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

The kernel verifier recognizes __arena and __arena_nullable parameter
suffixes for registered kfuncs. These arguments need the matching
address_space(1) BTF type attribute so bpftool emits usable declarations.

Extend the existing KF_ARENA_ARG1/2 handling to select arguments by either
the legacy flag or either suffix. Iterate over all parameters, allowing the
suffix convention at any argument position and avoiding duplicate tags when
a flag and suffix select the same argument.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 btf_encoder.c | 38 ++++++++++++++++++++++++++++----------
 dutil.h       | 13 +++++++++++++
 2 files changed, 41 insertions(+), 10 deletions(-)

diff --git a/btf_encoder.c b/btf_encoder.c
index 4b422e09800f..07ca4f41ac32 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -42,6 +42,8 @@
 #define BTF_KFUNC_TYPE_TAG	"bpf_kfunc"
 #define BTF_FASTCALL_TAG       "bpf_fastcall"
 #define BPF_ARENA_ATTR         "address_space(1)"
+#define BPF_ARENA_SUFFIX       "__arena"
+#define BPF_ARENA_NULLABLE_SUFFIX "__arena_nullable"
 
 /* kfunc flags, see include/linux/btf.h in the kernel source */
 #define KF_FASTCALL   (1 << 12)
@@ -808,12 +810,32 @@ static int btf__tag_bpf_arena_arg(struct btf *btf, struct btf_encoder_func_state
 	return id;
 }
 
-/* Modifies state->ret_type_id and state->parms[i].type_id for flagged kfuncs */
+static bool btf__is_bpf_arena_arg(const struct btf *btf,
+				  const struct btf_encoder_func_state *state, int idx)
+{
+	uint32_t flags = state->elf->kfunc_flags;
+	const char *name;
+	size_t name_len;
+
+	if ((idx == 0 && (flags & KF_ARENA_ARG1)) ||
+	    (idx == 1 && (flags & KF_ARENA_ARG2)))
+		return true;
+
+	name = btf__name_by_offset(btf, state->parms[idx].name_off);
+	if (!name)
+		return false;
+	name_len = strlen(name);
+	return (name_len > sizeof(BPF_ARENA_SUFFIX) - 1 && strends(name, BPF_ARENA_SUFFIX)) ||
+	       (name_len > sizeof(BPF_ARENA_NULLABLE_SUFFIX) - 1 &&
+		strends(name, BPF_ARENA_NULLABLE_SUFFIX));
+}
+
+/* Modifies state->ret_type_id and state->parms[i].type_id for arena kfuncs */
 static int btf__add_bpf_arena_type_tags(struct btf *btf, struct btf_encoder_func_state *state)
 {
 	uint32_t flags = state->elf->kfunc_flags;
 	int ret_type_id;
-	int err;
+	int err, i;
 
 	if (!btf__add_type_attr) {
 		fprintf(stderr, "btf__add_type_attr is not available, is libbpf < 1.6?\n");
@@ -830,14 +852,10 @@ static int btf__add_bpf_arena_type_tags(struct btf *btf, struct btf_encoder_func
 		state->ret_type_id = ret_type_id;
 	}
 
-	if (KF_ARENA_ARG1 & flags) {
-		err = btf__tag_bpf_arena_arg(btf, state, 0);
-		if (err < 0)
-			return err;
-	}
-
-	if (KF_ARENA_ARG2 & flags) {
-		err = btf__tag_bpf_arena_arg(btf, state, 1);
+	for (i = 0; i < state->nr_parms; i++) {
+		if (!btf__is_bpf_arena_arg(btf, state, i))
+			continue;
+		err = btf__tag_bpf_arena_arg(btf, state, i);
 		if (err < 0)
 			return err;
 	}
diff --git a/dutil.h b/dutil.h
index 603556fa0308..d55d01abc842 100644
--- a/dutil.h
+++ b/dutil.h
@@ -335,6 +335,19 @@ static inline bool strstarts(const char *str, const char *prefix)
 	return strncmp(str, prefix, strlen(prefix)) == 0;
 }
 
+/**
+ * strends - does @str end with @suffix?
+ * @str: string to examine
+ * @suffix: suffix to look for.
+ */
+static inline bool strends(const char *str, const char *suffix)
+{
+	size_t str_len = strlen(str);
+	size_t suffix_len = strlen(suffix);
+
+	return suffix_len <= str_len && strcmp(str + str_len - suffix_len, suffix) == 0;
+}
+
 void *zalloc(const size_t size);
 
 Elf_Scn *elf_section_by_name(Elf *elf, GElf_Shdr *shp, const char *name, size_t *index);
-- 
2.53.0