objtool/klp: Don't correlate .rodata.cst* constant pool objects

Clang aggregates UBSAN type descriptors into shared anonymous
.data..L__unnamed_* sections.  This data is used by UBSAN trap handlers.

When a changed function has an UBSAN bounds check, klp-diff clones the
entire UBSAN data section associated with the TU.  Relocations within
the cloned section that reference named rodata objects in .rodata.cst*
(like 'exponent', 'pirq_ali_set.irqmap') become KLP relocations because
those objects now get correlated.

That results in a .klp.rela.vmlinux..data section which can easily have
thousands of KLP relocs, most of which are completely superfluous, used
by functions which aren't cloned to the patch module.

The .rodata.cst* sections are SHF_MERGE constant pool sections
containing small fixed-size data (lookup tables, bitmasks) that is only
read by value.  Pointer identity is never relevant for these objects, so
correlating them is unnecessary.

Exclude .rodata.cst* objects from correlation so they get cloned as
local data instead of generating KLP relocations.

It might be possible to someday treat UBSAN data sections as special
sections, and only extract the few needed entries.  But this works for
now.

Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>

diff --git a/tools/objtool/klp-diff.c b/tools/objtool/klp-diff.c
index bf37c652188bd7586c91152bf4762d69f9a0c305..ca87bcb9afa34f27fc2fe5be8eff1506bc536724 100644 (file)
--- a/tools/objtool/klp-diff.c
+++ b/tools/objtool/klp-diff.c
@@ -372,6 +372,21 @@ static bool is_initcall_sym(struct symbol *sym)
               strstarts(sym->name, "__initstub__");
 }
 
+/*
+ * Some .rodata is anonymous and can't be correlated due to there being no
+ * symbol names.
+ *
+ * The .rodata.cst* sections aren't technically anonymous, they're SHF_MERGE
+ * constant pool sections containing small fixed-size data (lookup tables,
+ * bitmasks) which are only read by value, so pointer equivalence isn't needed.
+ * They are typically referenced by UBSAN data sections.
+ */
+static bool is_anonymous_rodata(struct symbol *sym)
+{
+       return is_rodata_sec(sym->sec) &&
+              (!is_object_sym(sym) || strstarts(sym->sec->name, ".rodata.cst"));
+}
+
 /*
  * These symbols should never be correlated, so their local patched versions
  * are used instead of linking to the originals.
@@ -386,7 +401,7 @@ static bool dont_correlate(struct symbol *sym)
               is_uncorrelated_static_local(sym) ||
               is_local_label(sym) ||
               is_string_sec(sym->sec) ||
-              (is_rodata_sec(sym->sec) && !is_object_sym(sym)) ||
+              is_anonymous_rodata(sym) ||
               is_initcall_sym(sym) ||
               is_addressable_sym(sym) ||
               is_special_section(sym->sec) ||