create-diff-object: fix Clang UBSAN diff failure by ignoring unnamed data

This fixes a diff failure when generating livepatches for objects
compiled with Clang and UBSAN (Undefined Behavior Sanitizer).

When UBSAN is enabled, Clang heavily utilizes `.data..L__unnamed_XX`
sections to store type descriptors and source location metadata.
kpatch-build currently fails to handle these sections, resulting in
errors like:

  fsnotify.o: changed section .data..L__unnamed_1 not selected for inclusion
  ERROR: fsnotify.o: 1 unsupported section change(s)
  create-diff-object: unreconcilable difference

The numeric suffix (e.g., `_1`) in these section names is an unstable
internal counter. If the patch modifies the code, these indices often
shift, causing `create-diff-object` to incorrectly correlate unrelated
UBSAN metadata between the original and patched objects. Since kpatch
cannot support changes to existing data sections, the build aborts.

Fix this by treating `.data..L__unnamed_` sections as unstable and
uncorrelatable.

This patch introduces `is_clang_unnamed_data()` to:
1. Explicitly skip these sections in `kpatch_correlate_sections()`.
2. Prevent `kpatch_correlate_static_local_variables()` from forcing
   correlation based on symbol usage.

This ensures that UBSAN metadata sections are always marked as 'NEW'
and allocated safely in the patch module, enabling support for
livepatching UBSAN-instrumented kernels.

Signed-off-by: Florent Revest <[email protected]>

Author: FlorentRevest

kpatch-build/create-diff-object.c

@@ -396,6 +396,18 @@ static bool is_string_literal_section(struct section *sec)
 	return !strncmp(sec->name, ".rodata.", 8) && strstr(sec->name, ".str");
 }
 
+/*
+ * Clang generates .data..L__unnamed_XX sections for anonymous constants.
+ * The numeric suffix is unstable (it can change if code is added/removed).
+ * Therefore, we must never correlate these by name; the patched object
+ * must always allocate a fresh copy (Status: NEW).
+ */
+static bool is_clang_unnamed_data(const char *name)
+{
+	return !strncmp(name, ".data..L__unnamed_", 18) ||
+		!strncmp(name, ".data.__unnamed_", 16);
+}
+
 /*
  * This function detects whether the given symbol is a "special" static local
  * variable (for lack of a better term).
@@ -425,6 +437,10 @@ static bool is_special_static(struct symbol *sym)
 	if (is_dynamic_debug_symbol(sym))
 		return true;
 
+	/* Do not try to correlate statics inside unstable Clang sections */
+	if (sym->sec && is_clang_unnamed_data(sym->sec->name))
+		return true;
+
 	if (sym->type == STT_SECTION) {
 		/* make sure section is bundled */
 		if (!sym->sec->sym)
@@ -1142,6 +1158,11 @@ static void kpatch_correlate_sections(struct list_head *seclist_orig,
 			if (is_ubsan_sec(sec_orig->name))
 				continue;
 
+			/* Skip correlation for unstable Clang anonymous sections */
+			if (is_clang_unnamed_data(sec_orig->name) ||
+			    is_clang_unnamed_data(sec_patched->name))
+				continue;
+
 			if (is_special_static(is_rela_section(sec_orig) ?
 					      sec_orig->base->secsym :
 					      sec_orig->secsym))
@@ -1785,6 +1806,7 @@ static void kpatch_replace_sections_syms(struct kpatch_elf *kelf)
 			}
 
 			if (!found && !is_string_literal_section(rela->sym->sec) &&
+			    !is_clang_unnamed_data(rela->sym->name) &&
 			    strncmp(rela->sym->name, ".rodata", 7)) {
 				ERROR("%s+0x%x: can't find replacement symbol for %s+%ld reference",
 				      relasec->base->name, rela->offset, rela->sym->name, rela->addend);