libbpf: Handle GCC noreturn-turned-volatile quirk (32022fd9) · Commits · 戴 / test

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: ("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

tools/lib/bpf/btf_dump.c

+24 −9

Original line number	Diff line number	Diff line
		@@ -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);

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