objtool/klp: Fix cross-module klp relocation section naming

A klp relocation section is .klp.rela.<objname>.<secname>, where objname
is the object being patched.

klp-build wrongly derives objname from where the referenced symbol
lives, not where it's referenced.  For a cross-module reference like
patched can_isotp code calling can.ko's can_rx_unregister(), that gives
.klp.rela.can..text rather than .klp.rela.can_isotp..text.  Unless the
patch happens to patch can.ko as well, the relocation never gets applied
and the call goes off into the weeds.

Name the intermediate section __klp_relocs.<objname> so post-link can
read the patched object's name from there.

Fixes: dd590d4d57 ("objtool/klp: Introduce klp diff subcommand for diffing object files")
Reported-by: Joe Lawrence <joe.lawrence@redhat.com>
Link: https://lore.kernel.org/20260720145658.1103243-2-joe.lawrence@redhat.com
Acked-by: Song Liu <song@kernel.org>
Acked-by: Joe Lawrence <joe.lawrence@redhat.com>
Link: https://patch.msgid.link/ee93a08f3e55e76ffa67d04e283917ddaa893f09.1786138493.git.jpoimboe@kernel.org
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>

tools/objtool/include/objtool/klp.h

@@ -14,11 +14,15 @@
 #define KLP_FUNCS_SEC	".init.klp_funcs"
 
 /*
- * __klp_relocs is an intermediate section which are created by klp diff and
- * converted into KLP symbols/relas by "objtool klp post-link".  This is needed
- * to work around the linker, which doesn't preserve SHN_LIVEPATCH or
+ * __klp_relocs.<objname> are intermediate sections which are created by klp
+ * diff and converted into KLP symbols/relas by "objtool klp post-link".  This
+ * is needed to work around the linker, which doesn't preserve SHN_LIVEPATCH or
  * SHF_RELA_LIVEPATCH, nor does it support having two RELA sections for a
  * single PROGBITS section.
+ *
+ * "objname" is the name of the object being patched ("vmlinux" or a module
+ * name).  post-link uses it to name the resulting
+ * .klp.rela.objname.section_name sections.
  */
 #define KLP_RELOCS_SEC	"__klp_relocs"
 #define KLP_STRINGS_SEC	".rodata.klp.str1.1"

tools/objtool/klp-diff.c

@@ -1381,8 +1381,8 @@ static int clone_reloc_klp(struct elfs *e, struct reloc *patched_reloc,
 	}
 
 	/*
-	 * Create the __klp_relocs entry.  This will be converted to an actual
-	 * KLP rela by "objtool klp post-link".
+	 * Create the __klp_relocs.<objname> entry.  This will be converted to
+	 * an actual KLP rela by "objtool klp post-link".
 	 *
 	 * This intermediate step is necessary to prevent corruption by the
 	 * linker, which doesn't know how to properly handle two rela sections
@@ -1390,7 +1390,18 @@ static int clone_reloc_klp(struct elfs *e, struct reloc *patched_reloc,
 	 */
 
 	if (!klp_relocs) {
-		klp_relocs = elf_create_section(e->out, KLP_RELOCS_SEC, 0,
+		const char *objname = find_modname(e);
+		char sec_name[SEC_NAME_LEN];
+
+		if (!objname)
+			return -1;
+
+		/* section format: __klp_relocs.objname */
+		if (snprintf_check(sec_name, SEC_NAME_LEN,
+				   KLP_RELOCS_SEC ".%s", objname))
+			return -1;
+
+		klp_relocs = elf_create_section(e->out, sec_name, 0,
 						0, SHT_PROGBITS, 8, SHF_ALLOC);
 		if (!klp_relocs)
 			return -1;

tools/objtool/klp-post-link.c

@@ -19,19 +19,11 @@
 #include <objtool/util.h>
 #include <linux/livepatch_external.h>
 
-static int fix_klp_relocs(struct elf *elf)
+static int fix_klp_reloc_sec(struct elf *elf, struct section *symtab,
+			     struct section *klp_relocs)
 {
-	struct section *symtab, *klp_relocs;
-
-	klp_relocs = find_section_by_name(elf, KLP_RELOCS_SEC);
-	if (!klp_relocs)
-		return 0;
-
-	symtab = find_section_by_name(elf, ".symtab");
-	if (!symtab) {
-		ERROR("missing .symtab");
-		return -1;
-	}
+	/* section format: __klp_relocs.sec_objname */
+	const char *sec_objname = klp_relocs->name + strlen(KLP_RELOCS_SEC ".");
 
 	for (int i = 0; i < sec_size(klp_relocs) / sizeof(struct klp_reloc); i++) {
 		struct klp_reloc *klp_reloc;
@@ -39,7 +31,6 @@ static int fix_klp_relocs(struct elf *elf)
 		struct section *sec, *tmp, *klp_rsec;
 		unsigned long offset;
 		struct reloc *reloc;
-		char sym_modname[64];
 		char rsec_name[SEC_NAME_LEN];
 		u64 addend;
 		struct symbol *sym, *klp_sym;
@@ -55,7 +46,7 @@ static int fix_klp_relocs(struct elf *elf)
 		reloc = find_reloc_by_dest(elf, klp_relocs,
 					   klp_reloc_off + offsetof(struct klp_reloc, offset));
 		if (!reloc) {
-			ERROR("malformed " KLP_RELOCS_SEC " section");
+			ERROR("malformed %s section", klp_relocs->name);
 			return -1;
 		}
 
@@ -66,17 +57,13 @@ static int fix_klp_relocs(struct elf *elf)
 		reloc = find_reloc_by_dest(elf, klp_relocs,
 					   klp_reloc_off + offsetof(struct klp_reloc, sym));
 		if (!reloc) {
-			ERROR("malformed " KLP_RELOCS_SEC " section");
+			ERROR("malformed %s section", klp_relocs->name);
 			return -1;
 		}
 
 		klp_sym = reloc->sym;
 		addend = reloc_addend(reloc);
 
-		/* symbol format: .klp.sym.modname.sym_name,sympos */
-		if (sscanf(klp_sym->name + strlen(KLP_SYM_PREFIX), "%55[^.]", sym_modname) != 1)
-			ERROR("can't find modname in klp symbol '%s'", klp_sym->name);
-
 		/*
 		 * Create the KLP rela:
 		 */
@@ -84,7 +71,7 @@ static int fix_klp_relocs(struct elf *elf)
 		/* section format: .klp.rela.sec_objname.section_name */
 		if (snprintf_check(rsec_name, SEC_NAME_LEN,
 				   KLP_RELOC_SEC_PREFIX "%s.%s",
-				   sym_modname, sec->name))
+				   sec_objname, sec->name))
 			return -1;
 
 		klp_rsec = find_section_by_name(elf, rsec_name);
@@ -134,10 +121,32 @@ static int fix_klp_relocs(struct elf *elf)
 	return 0;
 }
 
+static int fix_klp_relocs(struct elf *elf)
+{
+	struct section *symtab, *sec;
+
+	symtab = find_section_by_name(elf, ".symtab");
+	if (!symtab) {
+		ERROR("missing .symtab");
+		return -1;
+	}
+
+	for_each_sec(elf, sec) {
+		if (strncmp(sec->name, KLP_RELOCS_SEC ".",
+			    strlen(KLP_RELOCS_SEC ".")))
+			continue;
+
+		if (fix_klp_reloc_sec(elf, symtab, sec))
+			return -1;
+	}
+
+	return 0;
+}
+
 /*
  * This runs on the livepatch module after all other linking has been done.  It
- * converts the intermediate __klp_relocs section into proper KLP relocs to be
- * processed by livepatch.  This needs to run last to avoid linker wreckage.
+ * converts the intermediate __klp_relocs.* sections into proper KLP relocs to
+ * be processed by livepatch.  This needs to run last to avoid linker wreckage.
  * Linkers don't tend to handle the "two rela sections for a single base
  * section" case very well, nor do they appreciate SHN_LIVEPATCH.
  */