libbpf: Ensure btf_dump__new() and btf_dump_opts are future-proof

Change btf_dump__new() and corresponding struct btf_dump_ops structure
to be extensible by using OPTS "framework" ([0]). Given we don't change
the names, we use a similar approach as with bpf_prog_load(), but this
time we ended up with two APIs with the same name and same number of
arguments, so overloading based on number of arguments with
___libbpf_override() doesn't work.

Instead, use "overloading" based on types. In this particular case,
print callback has to be specified, so we detect which argument is
a callback. If it's 4th (last) argument, old implementation of API is
used by user code. If not, it must be 2nd, and thus new implementation
is selected. The rest is handled by the same symbol versioning approach.

btf_ext argument is dropped as it was never used and isn't necessary
either. If in the future we'll need btf_ext, that will be added into
OPTS-based struct btf_dump_opts.

struct btf_dump_opts is reused for both old API and new APIs. ctx field
is marked deprecated in v0.7+ and it's put at the same memory location
as OPTS's sz field. Any user of new-style btf_dump__new() will have to
set sz field and doesn't/shouldn't use ctx, as ctx is now passed along
the callback as mandatory input argument, following the other APIs in
libbpf that accept callbacks consistently.

Again, this is quite ugly in implementation, but is done in the name of
backwards compatibility and uniform and extensible future APIs (at the
same time, sigh). And it will be gone in libbpf 1.0.

  [0] Closes: https://github.com/libbpf/libbpf/issues/283

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211111053624.190580-5-andrii@kernel.org

1 files changed, 22 insertions, 9 deletions

diff --git a/tools/lib/bpf/btf_dump.c b/tools/lib/bpf/btf_dump.c
index 17db62b5002e..b8cd7e4f557a 100644
--- a/tools/lib/bpf/btf_dump.c
+++ b/tools/lib/bpf/btf_dump.c
@@ -77,9 +77,8 @@ struct btf_dump_data {
 
 struct btf_dump {
 	const struct btf *btf;
-	const struct btf_ext *btf_ext;
 	btf_dump_printf_fn_t printf_fn;
-	struct btf_dump_opts opts;
+	void *cb_ctx;
 	int ptr_sz;
 	bool strip_mods;
 	bool skip_anon_defs;
@@ -138,29 +137,32 @@ static void btf_dump_printf(const struct btf_dump *d, const char *fmt, ...)
 	va_list args;
 
 	va_start(args, fmt);
-	d->printf_fn(d->opts.ctx, fmt, args);
+	d->printf_fn(d->cb_ctx, fmt, args);
 	va_end(args);
 }
 
 static int btf_dump_mark_referenced(struct btf_dump *d);
 static int btf_dump_resize(struct btf_dump *d);
 
-struct btf_dump *btf_dump__new(const struct btf *btf,
-			       const struct btf_ext *btf_ext,
-			       const struct btf_dump_opts *opts,
-			       btf_dump_printf_fn_t printf_fn)
+DEFAULT_VERSION(btf_dump__new_v0_6_0, btf_dump__new, LIBBPF_0.6.0)
+struct btf_dump *btf_dump__new_v0_6_0(const struct btf *btf,
+				      btf_dump_printf_fn_t printf_fn,
+				      void *ctx,
+				      const struct btf_dump_opts *opts)
 {
 	struct btf_dump *d;
 	int err;
 
+	if (!printf_fn)
+		return libbpf_err_ptr(-EINVAL);
+
 	d = calloc(1, sizeof(struct btf_dump));
 	if (!d)
 		return libbpf_err_ptr(-ENOMEM);
 
 	d->btf = btf;
-	d->btf_ext = btf_ext;
 	d->printf_fn = printf_fn;
-	d->opts.ctx = opts ? opts->ctx : NULL;
+	d->cb_ctx = ctx;
 	d->ptr_sz = btf__pointer_size(btf) ? : sizeof(void *);
 
 	d->type_names = hashmap__new(str_hash_fn, str_equal_fn, NULL);
@@ -186,6 +188,17 @@ err:
 	return libbpf_err_ptr(err);
 }
 
+COMPAT_VERSION(btf_dump__new_deprecated, btf_dump__new, LIBBPF_0.0.4)
+struct btf_dump *btf_dump__new_deprecated(const struct btf *btf,
+					  const struct btf_ext *btf_ext,
+					  const struct btf_dump_opts *opts,
+					  btf_dump_printf_fn_t printf_fn)
+{
+	if (!printf_fn)
+		return libbpf_err_ptr(-EINVAL);
+	return btf_dump__new_v0_6_0(btf, printf_fn, opts ? opts->ctx : NULL, opts);
+}
+
 static int btf_dump_resize(struct btf_dump *d)
 {
 	int err, last_id = btf__type_cnt(d->btf) - 1;