[PATCH AUTOSEL 5.7 371/388] libbpf: Handle GCC noreturn-turned-volatile quirk

Sasha Levin
2020-06-18T01:19:15Z

From: Andrii Nakryiko <andriin@fb.com>

[ Upstream commit 32022fd97ed34f6812802bf1288db27c313576f4 ]

Handle a GCC quirk of emitting extra volatile modifier in DWARF (and
subsequently preserved in BTF by pahole) for function pointers marked as
__attribute__((noreturn)). This was the way to mark such functions before GCC
2.5 added noreturn attribute. Drop such func_proto modifiers, similarly to how
it's done for array (also to handle GCC quirk/bug).

Such volatile attribute is emitted by GCC only, so existing selftests can't
express such test. Simple repro is like this (compiled with GCC + BTF
generated by pahole):

struct my_struct {
void __attribute__((noreturn)) (*fn)(int);
};
struct my_struct a;

Without this fix, output will be:

struct my_struct {
voidvolatile (*fn)(int);
};

With the fix:

struct my_struct {
void (*fn)(int);
};

Fixes: 351131b51c7a ("libbpf: add btf_dump API for BTF-to-C conversion")
Reported-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Link: https://lore.kernel.org/bpf/20200610052335.2862559-1-andriin@fb.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
tools/lib/bpf/btf_dump.c | 33 ++++++++++++++++++++++++---------
1 file changed, 24 insertions(+), 9 deletions(-)

diff --git a/tools/lib/bpf/btf_dump.c b/tools/lib/bpf/btf_dump.c
index 0c28ee82834b..653dbbe2e366 100644
--- a/tools/lib/bpf/btf_dump.c
+++ b/tools/lib/bpf/btf_dump.c
@@ -1137,6 +1137,20 @@ static void btf_dump_emit_mods(struct btf_dump *d, struct id_stack *decl_stack)
}
}

+static void btf_dump_drop_mods(struct btf_dump *d, struct id_stack *decl_stack)
+{
+ const struct btf_type *t;
+ __u32 id;
+
+ while (decl_stack->cnt) {
+ id = decl_stack->ids[decl_stack->cnt - 1];
+ t = btf__type_by_id(d->btf, id);
+ if (!btf_is_mod(t))
+ return;
+ decl_stack->cnt--;
+ }
+}
+
static void btf_dump_emit_name(const struct btf_dump *d,
const char *name, bool last_was_ptr)
{
@@ -1235,14 +1249,7 @@ static void btf_dump_emit_type_chain(struct btf_dump *d,
* a const/volatile modifier for array, so we are
* going to silently skip them here.
*/
- while (decls->cnt) {
- next_id = decls->ids[decls->cnt - 1];
- next_t = btf__type_by_id(d->btf, next_id);
- if (btf_is_mod(next_t))
- decls->cnt--;
- else
- break;
- }
+ btf_dump_drop_mods(d, decls);

if (decls->cnt == 0) {
btf_dump_emit_name(d, fname, last_was_ptr);
@@ -1270,7 +1277,15 @@ static void btf_dump_emit_type_chain(struct btf_dump *d,
__u16 vlen = btf_vlen(t);
int i;

- btf_dump_emit_mods(d, decls);
+ /*
+ * GCC emits extra volatile qualifier for
+ * __attribute__((noreturn)) function pointers. Clang
+ * doesn't do it. It's a GCC quirk for backwards
+ * compatibility with code written for GCC <2.5. So,
+ * similarly to extra qualifiers for array, just drop
+ * them, instead of handling them.
+ */
+ btf_dump_drop_mods(d, decls);
if (decls->cnt) {
btf_dump_printf(d, " (");
btf_dump_emit_type_chain(d, decls, fname, lvl);
--
2.25.1