[PATCH 4/4] objtool/klp: strip klp-induced module dependencies from livepatch modules

[Joe Lawrence <joe.lawrence@redhat.com>]

The klp-diff pipeline creates weak UNDEF placeholder symbols for
cross-module references that will become klp-relocations.  When modpost
runs on the linked livepatch .ko, it sees these UNDEF symbols, resolves
them via Module.symvers, and adds the providing modules to the depends=
field in .modinfo.

These dependencies are unnecessary: klp-relocations are resolved by the
livepatch infrastructure at patch-enable time, not by the module loader.
Adding hard module dependencies defeats late-module patching, since the
kernel will refuse to load the livepatch module unless all depended
modules are already present.

Fix this by having klp-post-link strip any depends= entry that is only
the result of klp-symbol (.klp.sym.<mod>.*).  The check is conservative:
if any remaining SHN_UNDEF symbol matches a known export name from that
module, the dependency is kept (indicating a legitimate non-KLP
reference).

Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
---
 tools/objtool/klp-post-link.c | 175 ++++++++++++++++++++++++++++++++++
 1 file changed, 175 insertions(+)

diff --git a/tools/objtool/klp-post-link.c b/tools/objtool/klp-post-link.c
index 141b0a46ca52..8bdec0b38f65 100644
--- a/tools/objtool/klp-post-link.c
+++ b/tools/objtool/klp-post-link.c
@@ -19,6 +19,178 @@
 #include <objtool/util.h>
 #include <linux/livepatch_external.h>
 
+static bool modnames_match(const char *a, const char *b, size_t len)
+{
+	for (size_t i = 0; i < len; i++) {
+		char ca = a[i] == '-' ? '_' : a[i];
+		char cb = b[i] == '-' ? '_' : b[i];
+		if (ca != cb)
+			return false;
+	}
+	return true;
+}
+
+/*
+ * Extract the export name length from a KLP symbol name suffix: "<name>,<pos>"
+ */
+static size_t klp_sym_name_len(const char *name_start)
+{
+	const char *comma = strrchr(name_start, ',');
+	return comma ? (size_t)(comma - name_start) : strlen(name_start);
+}
+
+/*
+ * Check if the dependency on 'mod' is purely from KLP relocations.
+ *
+ * Returns true only if:
+ *  (a) .klp.sym.<mod>.* symbols exist (the dep is KLP-related), AND
+ *  (b) no remaining SHN_UNDEF symbol shares a name with any of those exports
+ *      (which would indicate a non-KLP reference to the same module)
+ */
+static bool is_klp_only_dep(struct elf *elf, const char *mod)
+{
+	const char *exports[1024];
+	size_t export_lens[1024];
+	int nr_exports = 0;
+	struct symbol *sym;
+	size_t prefix_len = strlen(KLP_SYM_PREFIX);
+
+	/* Collect export names from .klp.sym.<mod>.* symbols */
+	for_each_sym(elf, sym) {
+		const char *name = sym->name;
+		const char *dot;
+
+		if (!strstarts(name, KLP_SYM_PREFIX))
+			continue;
+
+		dot = strchr(name + prefix_len, '.');
+		if (!dot)
+			continue;
+
+		if (strlen(mod) != (size_t)(dot - name - prefix_len) ||
+		    !modnames_match(name + prefix_len, mod, dot - name - prefix_len))
+			continue;
+
+		if (nr_exports >= 1024) {
+			WARN("too many KLP exports for module %s, skipping dependency stripping", mod);
+			return false;
+		}
+
+		exports[nr_exports] = dot + 1;
+		export_lens[nr_exports] = klp_sym_name_len(dot + 1);
+		nr_exports++;
+	}
+
+	if (!nr_exports)
+		return false;
+
+	/*
+	 * Verify no remaining UNDEF symbol matches an export name from this
+	 * module.  After fix_klp_relocs(), KLP placeholder symbols have been
+	 * converted to SHN_LIVEPATCH.  Any leftover UNDEF with a matching name
+	 * indicates a legitimate non-KLP dependency.
+	 */
+	for_each_sym(elf, sym) {
+		const char *name = sym->name;
+
+		if (sym->sym.st_shndx != SHN_UNDEF)
+			continue;
+		if (!name || !*name)
+			continue;
+
+		for (int i = 0; i < nr_exports; i++) {
+			if (strlen(name) == export_lens[i] &&
+			    !strncmp(name, exports[i], export_lens[i]))
+				return false;
+		}
+	}
+
+	return true;
+}
+
+/*
+ * Remove modules from .modinfo depends= that are only referenced via KLP
+ * relocations.  These dependencies are an artifact of the placeholder symbols
+ * created by klp-diff for the linker/objtool and should not constrain module
+ * load ordering (which would break late-module patching).
+ */
+static int fix_modinfo_depends(struct elf *elf)
+{
+	struct section *sec;
+	char *data, *data_end, *depends = NULL;
+	char *new_depends, *p;
+	char *tok, *save;
+	char *dep_val, *dep_copy;
+	size_t old_entry_len, new_entry_len;
+
+	sec = find_section_by_name(elf, ".modinfo");
+	if (!sec || !sec->data || !sec->data->d_buf)
+		return 0;
+
+	data = sec->data->d_buf;
+	data_end = data + sec->data->d_size;
+
+	for (char *s = data; s < data_end; s += strlen(s) + 1) {
+		if (strstarts(s, "depends=")) {
+			depends = s;
+			break;
+		}
+	}
+
+	if (!depends)
+		return 0;
+
+	dep_val = depends + strlen("depends=");
+	if (!*dep_val)
+		return 0;
+
+	new_depends = strdup(dep_val);
+	if (!new_depends) {
+		ERROR_GLIBC("strdup");
+		return -1;
+	}
+
+	dep_copy = strdup(dep_val);
+	if (!dep_copy) {
+		ERROR_GLIBC("strdup");
+		free(new_depends);
+		return -1;
+	}
+
+	p = new_depends;
+	*p = '\0';
+
+	tok = strtok_r(dep_copy, ",", &save);
+	while (tok) {
+		if (!is_klp_only_dep(elf, tok)) {
+			if (p != new_depends)
+				*p++ = ',';
+			strcpy(p, tok);
+			p += strlen(tok);
+		}
+		tok = strtok_r(NULL, ",", &save);
+	}
+
+	old_entry_len = strlen(depends) + 1;
+	new_entry_len = strlen("depends=") + strlen(new_depends) + 1;
+
+	if (strcmp(dep_val, new_depends) && new_entry_len <= old_entry_len) {
+		snprintf(depends, old_entry_len, "depends=%s", new_depends);
+		memset(depends + new_entry_len, '\0',
+		       old_entry_len - new_entry_len);
+		mark_sec_changed(elf, sec, true);
+	} else if (new_entry_len > old_entry_len) {
+		ERROR("new depends= string exceeds original size (shouldn't happen)");
+		free(dep_copy);
+		free(new_depends);
+		return -1;
+	}
+
+	free(dep_copy);
+	free(new_depends);
+	return 0;
+}
+
 static int fix_klp_relocs(struct elf *elf)
 {
 	struct section *symtab, *klp_relocs;
@@ -163,6 +335,9 @@ int cmd_klp_post_link(int argc, const char **argv)
 	if (fix_klp_relocs(elf))
 		return -1;
 
+	if (fix_modinfo_depends(elf))
+		return -1;
+
 	if (elf_write(elf))
 		return -1;
 
-- 
2.54.0