objtool/klp: Fix symbol resolution for duplicate data symbols

find_sympos() calculates a sympos used by livepatch to disambiguate
duplicately-named symbols.  For function symbols, there's a hack which
counts .text.unlikely symbols before other .text symbols, matching the
linker script's section ordering.

Not only is the hack fragile, data symbols can have the same problem.
So for example, adding a reference to pwq_cache in
ep_unregister_pollwait() can trigger a corrupt sympos and a relocation
to the wrong pwq_cache symbol in the livepatch module, resulting in a
crash or undefined behavior.

Remove the existing hack in favor of a fully deterministic solution,
using the new .klp.symid table to derive the symbol-to-id mapping from
the original vmlinux.o and the id-to-address mapping from the
corresponding vmlinux, which can then be used to determine the exact
sympos associated with the original vmlinux.

Modules don't need any special treatment: the .ko has the same
section/symbol ordering as the original whole-archive symbol table.

Fixes: dd590d4d57eb ("objtool/klp: Introduce klp diff subcommand for diffing object files")
Reported-by: Ben Procknow <bprockno@redhat.com>
Reported-by: Joe Lawrence <joe.lawrence@redhat.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: live-patching@vger.kernel.org
Link: https://lore.kernel.org/20260710153042.3156788-1-joe.lawrence@redhat.com
Link: https://lore.kernel.org/20260724221730.3126529-1-joe.lawrence@redhat.com
Link: https://patch.msgid.link/919785e3bf2245db02ff6391e735d9cb139170b1.1785727106.git.jpoimboe@kernel.org

1 files changed, 5 insertions, 61 deletions

diff --git a/tools/objtool/klp-diff.c b/tools/objtool/klp-diff.c
index 75ba0e060a34..c5284d275207 100644
--- a/tools/objtool/klp-diff.c
+++ b/tools/objtool/klp-diff.c
@@ -898,65 +898,6 @@ static int correlate_symbols(struct elfs *e)
 	return 0;
 }
 
-/* "sympos" is used by livepatch to disambiguate duplicate symbol names */
-static unsigned long find_sympos(struct elf *elf, struct symbol *sym)
-{
-	bool vmlinux = str_ends_with(objname, "vmlinux.o");
-	unsigned long sympos = 0, nr_matches = 0;
-	bool has_dup = false;
-	struct symbol *s;
-
-	if (sym->bind != STB_LOCAL)
-		return 0;
-
-	if (vmlinux && is_func_sym(sym)) {
-		/*
-		 * HACK: Unfortunately, symbol ordering can differ between
-		 * vmlinux.o and vmlinux due to the linker script emitting
-		 * .text.unlikely* before .text*.  Count .text.unlikely* first.
-		 *
-		 * TODO: Disambiguate symbols more reliably (checksums?)
-		 */
-		for_each_sym(elf, s) {
-			if (strstarts(s->sec->name, ".text.unlikely") &&
-			    !strcmp(s->name, sym->name)) {
-				nr_matches++;
-				if (s == sym)
-					sympos = nr_matches;
-				else
-					has_dup = true;
-			}
-		}
-		for_each_sym(elf, s) {
-			if (!strstarts(s->sec->name, ".text.unlikely") &&
-			    !strcmp(s->name, sym->name)) {
-				nr_matches++;
-				if (s == sym)
-					sympos = nr_matches;
-				else
-					has_dup = true;
-			}
-		}
-	} else {
-		for_each_sym(elf, s) {
-			if (!strcmp(s->name, sym->name)) {
-				nr_matches++;
-				if (s == sym)
-					sympos = nr_matches;
-				else
-					has_dup = true;
-			}
-		}
-	}
-
-	if (!sympos) {
-		ERROR("can't find sympos for %s", sym->name);
-		return ULONG_MAX;
-	}
-
-	return has_dup ? sympos : 0;
-}
-
 static int clone_sym_relocs(struct elfs *e, struct symbol *patched_sym);
 
 static struct symbol *__clone_symbol(struct elf *elf, struct symbol *patched_sym,
@@ -1418,7 +1359,7 @@ static int clone_reloc_klp(struct elfs *e, struct reloc *patched_reloc,
 			return -1;
 
 		sym_orig_name = patched_sym->twin->name;
-		sympos = find_sympos(e->orig, patched_sym->twin);
+		sympos = klp_find_sympos(e->orig, patched_sym->twin);
 		if (sympos == ULONG_MAX)
 			return -1;
 	}
@@ -2036,7 +1977,7 @@ static int create_klp_sections(struct elfs *e)
 
 		/* klp_func_ext.sympos */
 		BUILD_BUG_ON(sizeof(sympos) != sizeof_field(struct klp_func_ext, sympos));
-		sympos = find_sympos(e->orig, sym->clone->twin);
+		sympos = klp_find_sympos(e->orig, sym->clone->twin);
 		if (sympos == ULONG_MAX)
 			return -1;
 		memcpy(func_data + offsetof(struct klp_func_ext, sympos), &sympos,
@@ -2190,6 +2131,9 @@ int cmd_klp_diff(int argc, const char **argv)
 	if (!e.orig || !e.patched)
 		return -1;
 
+	if (klp_sympos_init(e.orig))
+		return -1;
+
 	if (read_exports())
 		return -1;