objtool/klp: Add .klp.symid for sympos disambiguation

Livepatch identifies a duplicate-named symbol by its position (sympos)
among same-named kallsyms entries, which for vmlinux are counted in
ascending address order in the final linked kernel.  That order can't be
reliably derived from vmlinux.o: the final link reorders sub-sections
(.text.unlikely*, .data..*, etc).

Bridge the gap with a new .klp.symid section which can be used to
correlate symbols between vmlinux.o and vmlinux so that klp-diff can
reliably determine the sympos.

The table can't survive --gc-sections: keeping it alive would keep every
duplicate-named symbol's section alive, so the reference kernel would
stop matching the one which ships.  klp-build rejects
CONFIG_LD_DEAD_CODE_DATA_ELIMINATION instead.  Nothing is lost today:
x86_64 is the only HAVE_KLP_BUILD arch and doesn't select
HAVE_LD_DEAD_CODE_DATA_ELIMINATION, arm64 and s390 have never selected
it either, and on powerpc, it's still EXPERIMENTAL and disabled by every
distro kernel.

This is the build-time half of reliable vmlinux sympos computation;
"objtool klp diff" will consume the table in a subsequent commit.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: live-patching@vger.kernel.org
Link: https://patch.msgid.link/64d50f077b569f47883c015cdb7079edb068efe8.1785727106.git.jpoimboe@kernel.org

2 files changed, 16 insertions, 0 deletions

diff --git a/tools/objtool/include/objtool/builtin.h b/tools/objtool/include/objtool/builtin.h
index e844e9c82b7b..349690bb1c50 100644
--- a/tools/objtool/include/objtool/builtin.h
+++ b/tools/objtool/include/objtool/builtin.h
@@ -16,6 +16,7 @@ struct opts {
 	bool hack_noinstr;
 	bool hack_skylake;
 	bool ibt;
+	bool klp_symids;
 	bool mcount;
 	bool noabs;
 	bool noinstr;
diff --git a/tools/objtool/include/objtool/klp.h b/tools/objtool/include/objtool/klp.h
index aab6db42052d..4d3c3bd462aa 100644
--- a/tools/objtool/include/objtool/klp.h
+++ b/tools/objtool/include/objtool/klp.h
@@ -31,6 +31,21 @@ struct klp_reloc {
 	u32 type;
 };
 
+/*
+ * .klp.symid is used to correlate symbols between vmlinux.o and vmlinux, for
+ * calculating sympos to disambiguate duplicately-named symbols.
+ */
+#define KLP_SYMID_SEC	".klp.symid"
+
+struct klp_symid {
+	u64 id;
+	u64 addr;
+};
+
+struct objtool_file;
+
+int klp_create_symid_sections(struct objtool_file *file);
+
 int cmd_klp_checksum(int argc, const char **argv);
 int cmd_klp_diff(int argc, const char **argv);
 int cmd_klp_post_link(int argc, const char **argv);