[PATCH v2 bpf-next 1/2] libbpf: support stripping modifiers for btf_dump

[Andrii Nakryiko <andriin@xxxxxx>]

One important use case when emitting const/volatile/restrict is undesirable is
BPF skeleton generation of DATASEC layout. These are further memory-mapped and
can be written/read from user-space directly.

For important case of .rodata variables, bpftool strips away first-level
modifiers, to make their use on user-space side simple and not requiring extra
type casts to override compiler complaining about writing to const variables.

This logic works mostly fine, but breaks in some more complicated cases. E.g.:

    const volatile int params[10];

Because in BTF it's a chain of ARRAY -> CONST -> VOLATILE -> INT, bpftool
stops at ARRAY and doesn't strip CONST and VOLATILE. In skeleton this variable
will be emitted as is. So when used from user-space, compiler will complain
about writing to const array. This is problematic, as also mentioned in [0].

To solve this for arrays and other non-trivial cases (e.g., inner
const/volatile fields inside the struct), teach btf_dump to strip away any
modifier, when requested. This is done as an extra option on
btf_dump__emit_type_decl() API.

Reported-by: Anton Protopopov <a.s.protopopov@xxxxxxxxx>
Signed-off-by: Andrii Nakryiko <andriin@xxxxxx>
---
 tools/lib/bpf/btf.h      |  2 ++
 tools/lib/bpf/btf_dump.c | 10 ++++++++--
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h
index 70c1b7ec2bd0..be98dd75b791 100644
--- a/tools/lib/bpf/btf.h
+++ b/tools/lib/bpf/btf.h
@@ -143,6 +143,8 @@ struct btf_dump_emit_type_decl_opts {
 	 * necessary indentation already
 	 */
 	int indent_level;
+	/* strip all the const/volatile/restrict mods */
+	bool strip_mods;
 };
 #define btf_dump_emit_type_decl_opts__last_field indent_level
 
diff --git a/tools/lib/bpf/btf_dump.c b/tools/lib/bpf/btf_dump.c
index bbb430317260..e1c344504cae 100644
--- a/tools/lib/bpf/btf_dump.c
+++ b/tools/lib/bpf/btf_dump.c
@@ -60,6 +60,7 @@ struct btf_dump {
 	const struct btf_ext *btf_ext;
 	btf_dump_printf_fn_t printf_fn;
 	struct btf_dump_opts opts;
+	bool strip_mods;
 
 	/* per-type auxiliary state */
 	struct btf_dump_type_aux_state *type_states;
@@ -1032,7 +1033,9 @@ int btf_dump__emit_type_decl(struct btf_dump *d, __u32 id,
 
 	fname = OPTS_GET(opts, field_name, "");
 	lvl = OPTS_GET(opts, indent_level, 0);
+	d->strip_mods = OPTS_GET(opts, strip_mods, false);
 	btf_dump_emit_type_decl(d, id, fname, lvl);
+	d->strip_mods = false;
 	return 0;
 }
 
@@ -1045,6 +1048,10 @@ static void btf_dump_emit_type_decl(struct btf_dump *d, __u32 id,
 
 	stack_start = d->decl_stack_cnt;
 	for (;;) {
+		t = btf__type_by_id(d->btf, id);
+		if (d->strip_mods && btf_is_mod(t))
+			goto skip_mod;
+
 		err = btf_dump_push_decl_stack_id(d, id);
 		if (err < 0) {
 			/*
@@ -1056,12 +1063,11 @@ static void btf_dump_emit_type_decl(struct btf_dump *d, __u32 id,
 			d->decl_stack_cnt = stack_start;
 			return;
 		}
-
+skip_mod:
 		/* VOID */
 		if (id == 0)
 			break;
 
-		t = btf__type_by_id(d->btf, id);
 		switch (btf_kind(t)) {
 		case BTF_KIND_PTR:
 		case BTF_KIND_VOLATILE:
-- 
2.24.1