[PATCH v2] objtool/klp: Fix checksums for constant pool references

[Josh Poimboeuf <jpoimboe@kernel.org>]

Adding a line of code to __link_shadow_page() with a literal string
causes a false positive changed function with GCC:

  arch/x86/kvm/kvm.ko.o: changed function: kvm_tdp_mmu_map_private_pfn

While the patch only touched __link_shadow_page(), the string addition
triggered a rename of .LC64 -> .LC65 in kvm_tdp_mmu_map_private_pfn()
even though the underlying referenced constant data didn't change.

So for .LC* symbols, the suffix is arbitrary but the data isn't.  Add
the underlying data to the checksum calculation rather than the symbol
name.

Clang also uses .LC* symbols, but also uses anonymous data.  Both
compilers put this data in .rodata.cst<num> sections.

Fixes: 0d83da43b1e1 ("objtool/klp: Add --checksum option to generate per-function checksums")
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
 tools/objtool/klp-checksum.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/tools/objtool/klp-checksum.c b/tools/objtool/klp-checksum.c
index b8e47f28997e9..ebe25f9c5260a 100644
--- a/tools/objtool/klp-checksum.c
+++ b/tools/objtool/klp-checksum.c
@@ -54,6 +54,19 @@ static int checksum_debug_init(struct objtool_file *file)
 	return 0;
 }
 
+/*
+ * Detect a reference to anonymous constant pool data which the compiler places
+ * in .rodata.cst<num> and which either has an .LC<num> symbol associated with
+ * it or (with Clang) no symbol at all.  These are typically initializers for
+ * local function stack data, so they're considered part of the function rather
+ * than data per se.
+ */
+static bool is_anonymous_const_data(struct symbol *sym)
+{
+	return strstarts(sym->sec->name, ".rodata.cst") &&
+	       (is_sec_sym(sym) || strstarts(sym->name, ".LC"));
+}
+
 static void checksum_update_insn(struct objtool_file *file, struct symbol *func,
 				 struct instruction *insn)
 {
@@ -129,6 +142,14 @@ static void checksum_update_insn(struct objtool_file *file, struct symbol *func,
 		goto alts;
 	}
 
+	if (is_anonymous_const_data(sym)) {
+		void *cst;
+
+		cst = sym->sec->data->d_buf + sym->offset + offset;
+		__checksum_update_insn(func, insn, cst, sym->sec->sh.sh_entsize);
+		goto alts;
+	}
+
 	if (is_sec_sym(sym)) {
 		sym = find_symbol_containing(reloc->sym->sec, offset);
 		if (!sym)
-- 
2.55.0