objtool/klp: Fix symbol correlation for orphaned local symbols

When compiling with CONFIG_LTO_CLANG_THIN, vmlinux.o has
__irf_[start|end] before the first FILE entry:

$ readelf -sW vmlinux.o
Symbol table '.symtab' contains 597706 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000000000 0 NOTYPE LOCAL DEFAULT 18 __irf_start
2: 0000000000000200 0 NOTYPE LOCAL DEFAULT 18 __irf_end
3: 0000000000000000 0 SECTION LOCAL DEFAULT 17 .text
4: 0000000000000000 0 SECTION LOCAL DEFAULT 18 .init.ramfs

This causes klp-build warnings like:

vmlinux.o: warning: objtool: no correlation: __irf_start
vmlinux.o: warning: objtool: no correlation: __irf_end

The problem is that Clang LTO is stripping the initramfs_data.o FILE
symbol, causing those two symbols to be orphaned and not noticed by
klp-diff's correlation logic. Add a loop to correlate any symbols found
before the first FILE symbol.

Fixes: dd590d4d57eb ("objtool/klp: Introduce klp diff subcommand for diffing object files")
Reported-by: Song Liu <[e-mail]>
Acked-by: Song Liu <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Josh Poimboeuf <[e-mail]>

tools/objtool/klp-diff.c

···
 	struct symbol *file1_sym, *file2_sym;
 	struct symbol *sym1, *sym2;
 
-	/* Correlate locals */
-	for (file1_sym = first_file_symbol(e->orig),
-	     file2_sym = first_file_symbol(e->patched); ;
-	     file1_sym = next_file_symbol(e->orig, file1_sym),
-	     file2_sym = next_file_symbol(e->patched, file2_sym)) {
+	file1_sym = first_file_symbol(e->orig);
+	file2_sym = first_file_symbol(e->patched);
+
+	/*
+	 * Correlate any locals before the first FILE symbol.  This has been
+	 * seen when LTO inexplicably strips the initramfs_data.o FILE symbol
+	 * due to the file only containing data and no code.
+	 */
+	for_each_sym(e->orig, sym1) {
+		if (sym1 == file1_sym || !is_local_sym(sym1))
+			break;
+
+		if (dont_correlate(sym1))
+			continue;
+
+		for_each_sym(e->patched, sym2) {
+			if (sym2 == file2_sym || !is_local_sym(sym2))
+				break;
+
+			if (sym2->twin || dont_correlate(sym2))
+				continue;
+
+			if (strcmp(sym1->demangled_name, sym2->demangled_name))
+				continue;
+
+			sym1->twin = sym2;
+			sym2->twin = sym1;
+			break;
+		}
+	}
+
+	/* Correlate locals after the first FILE symbol */
+	for (; ; file1_sym = next_file_symbol(e->orig, file1_sym),
+		 file2_sym = next_file_symbol(e->patched, file2_sym)) {
 
 		if (!file1_sym && file2_sym) {
 			ERROR("FILE symbol mismatch: NULL != %s", file2_sym->name);