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Commit f3f9ee42 authored by Andrii Nakryiko's avatar Andrii Nakryiko Committed by Greg Kroah-Hartman
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libbpf: Handle GCC noreturn-turned-volatile quirk 



[ 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: 351131b5 ("libbpf: add btf_dump API for BTF-to-C conversion")
Reported-by: default avatarJean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: default avatarAndrii Nakryiko <andriin@fb.com>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Tested-by: default avatarJean-Philippe Brucker <jean-philippe@linaro.org>
Link: https://lore.kernel.org/bpf/20200610052335.2862559-1-andriin@fb.com


Signed-off-by: default avatarSasha Levin <sashal@kernel.org>
parent ab482b58

tools/lib/bpf/btf_dump.c

+24 −9
Original line number Diff line number Diff line
@@ -1141,6 +1141,20 @@ 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)

{
@@ -1239,14 +1253,7 @@ 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);
@@ -1274,7 +1281,15 @@ 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);