From: Andrii Nakryiko <andriin@fb.com>
Date: Mon, 13 Jul 2020 16:24:08 -0700
Subject: libbpf: Support stripping modifiers for btf_dump
Patch-mainline: v5.9-rc1
Git-commit: 7c819e701382d4969ca4837b8cbe157895f5d0bf
References: bsc#1177028
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@gmail.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200713232409.3062144-2-andriin@fb.com
Acked-by: Gary Lin <glin@suse.com>
---
tools/lib/bpf/btf.h | 2 ++
tools/lib/bpf/btf_dump.c | 10 ++++++++--
2 files changed, 10 insertions(+), 2 deletions(-)
--- a/tools/lib/bpf/btf.h
+++ b/tools/lib/bpf/btf.h
@@ -145,6 +145,8 @@ 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
--- 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;
@@ -1021,7 +1022,9 @@ int btf_dump__emit_type_decl(struct btf_
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;
}
@@ -1034,6 +1037,10 @@ static void btf_dump_emit_type_decl(stru
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) {
/*
@@ -1045,12 +1052,11 @@ static void btf_dump_emit_type_decl(stru
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: