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. Signed-off-by: Josh Poimboeuf <[email protected]> --- tools/objtool/klp-diff.c | 17 ++++++++++++++++- 1 file changed, 16 insertions(+), 1 deletion(-) diff --git a/tools/objtool/klp-diff.c b/tools/objtool/klp-diff.c index bf37c652188b..ca87bcb9afa3 100644 --- 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) || -- 2.53.0
