[PATCH 3/5] objtool: Support Clang non-section symbols in ORC generation

Josh Poimboeuf
2020-04-01T18:23:51Z

When compiling the kernel with AS=clang, objtool produces a lot of
warnings:

warning: objtool: missing symbol for section .text
warning: objtool: missing symbol for section .init.text
warning: objtool: missing symbol for section .ref.text

It then fails to generate the ORC table.

The problem is that objtool assumes text section symbols always exist.
But the Clang assembler is aggressive about removing them.

When generating relocations for the ORC table, objtool always tries to
reference instructions by their section symbol offset. If the section
symbol doesn't exist, it bails.

Do a fallback: when a section symbol isn't available, reference a
function symbol instead.

Link: https://github.com/ClangBuiltLinux/linux/issues/669
Cc: Nick Desaulniers <ndesaulniers@google.com>
Reported-by: Dmitry Golovin <dima@golovin.in>
Tested-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
tools/objtool/orc_gen.c | 33 ++++++++++++++++++++++++++-------
1 file changed, 26 insertions(+), 7 deletions(-)

diff --git a/tools/objtool/orc_gen.c b/tools/objtool/orc_gen.c
index 41e4a2754da4..4c0dabd28000 100644
--- a/tools/objtool/orc_gen.c
+++ b/tools/objtool/orc_gen.c
@@ -88,11 +88,6 @@ static int create_orc_entry(struct elf *elf, struct section *u_sec, struct secti
struct orc_entry *orc;
struct rela *rela;

- if (!insn_sec->sym) {
- WARN("missing symbol for section %s", insn_sec->name);
- return -1;
- }
-
/* populate ORC data */
orc = (struct orc_entry *)u_sec->data->d_buf + idx;
memcpy(orc, o, sizeof(*orc));
@@ -105,8 +100,32 @@ static int create_orc_entry(struct elf *elf, struct section *u_sec, struct secti
}
memset(rela, 0, sizeof(*rela));

- rela->sym = insn_sec->sym;
- rela->addend = insn_off;
+ if (insn_sec->sym) {
+ rela->sym = insn_sec->sym;
+ rela->addend = insn_off;
+ } else {
+ /*
+ * The Clang assembler doesn't produce section symbols, so we
+ * have to reference the function symbol instead:
+ */
+ rela->sym = find_symbol_containing(insn_sec, insn_off);
+ if (!rela->sym) {
+ /*
+ * Hack alert. This happens when we need to reference
+ * the NOP pad insn immediately after the function.
+ */
+ rela->sym = find_symbol_containing(insn_sec,
+ insn_off - 1);
+ }
+ if (!rela->sym) {
+ WARN("missing symbol for insn at offset 0x%lx\n",
+ insn_off);
+ return -1;
+ }
+
+ rela->addend = insn_off - rela->sym->offset;
+ }
+
rela->type = R_X86_64_PC32;
rela->offset = idx * sizeof(int);
rela->sec = ip_relasec;
--
2.21.1