From 636f230ce21e2730072c58280f1fa0bed4a00eed Mon Sep 17 00:00:00 2001 From: Josh Poimboeuf Date: Fri, 7 Aug 2026 14:37:48 -0700 Subject: [PATCH] objtool/klp: Ignore replacement offset of empty x86 alternatives An x86 alternative with an empty replacement, e.g. the second entry of ALTERNATIVE_2("orig", "repl", ft1, "", ft2) has a replacementlen of zero. Its replacement offset still gets a relocation, but the label it points at is the end of the previous replacement, which is also the beginning of the *next* alternative's replacement. The value is meaningless; get_alt_entry() already ignores it for that reason. klp diff doesn't ignore it. When such an alternative belongs to a changed function, cloning its relocations drags in the unrelated neighboring replacement, along with everything that replacement references. On an x86 clang/lto build an empty alternative in meminfo_proc_show() pulled in the replacement of an alternative in proc_kcore_init(), silently emitting a klp relocation against init text which has long since been freed by the time the patch is applied. Add arch_alt_ignore_new_reloc() and skip such relocations when cloning. This has to be arch specific: on arm64 a zero-length replacement instead identifies an alternative callback, whose replacement offset points at the callback function and must be preserved. Fixes: dd590d4d57eb ("objtool/klp: Introduce klp diff subcommand for diffing object files") Acked-by: Song Liu Acked-by: Joe Lawrence Link: https://patch.msgid.link/7a885b70974795c3417f3358869e62aafd4ef783.1786138493.git.jpoimboe@kernel.org Signed-off-by: Josh Poimboeuf --- tools/objtool/arch/x86/special.c | 27 +++++++++++++++++++++++++ tools/objtool/include/objtool/special.h | 7 +++++++ tools/objtool/klp-diff.c | 6 +++++- 3 files changed, 39 insertions(+), 1 deletion(-) diff --git a/tools/objtool/arch/x86/special.c b/tools/objtool/arch/x86/special.c index e817a3fff449..1e84c81bfcd8 100644 --- a/tools/objtool/arch/x86/special.c +++ b/tools/objtool/arch/x86/special.c @@ -1,6 +1,7 @@ // SPDX-License-Identifier: GPL-2.0-or-later #include +#include #include #include #include @@ -9,6 +10,32 @@ /* cpu feature name array generated from cpufeatures.h */ #include "cpu-feature-names.c" +/* + * An alternative with an empty replacement, e.g. the second entry of + * + * ALTERNATIVE_2("orig", "repl", ft1, "", ft2) + * + * still gets a relocation for its replacement offset. But the label it points + * at is the end of the previous entry's replacement, which is also the + * beginning of the *next* entry's replacement. The value is meaningless: it's + * only ever used with a length of zero. + */ +bool arch_alt_ignore_new_reloc(struct section *sec, unsigned long offset) +{ + unsigned long entry_off; + + if (strcmp(sec->name, ".altinstructions")) + return false; + + entry_off = offset - (offset % ALT_ENTRY_SIZE); + + if (offset - entry_off != ALT_NEW_OFFSET) + return false; + + return !*(unsigned char *)(sec->data->d_buf + entry_off + + ALT_NEW_LEN_OFFSET); +} + void arch_handle_alternative(struct special_alt *alt) { static struct special_alt *group, *prev; diff --git a/tools/objtool/include/objtool/special.h b/tools/objtool/include/objtool/special.h index 121c3761899c..620dbf6cb0e5 100644 --- a/tools/objtool/include/objtool/special.h +++ b/tools/objtool/include/objtool/special.h @@ -32,6 +32,13 @@ int special_get_alts(struct elf *elf, struct list_head *alts); void arch_handle_alternative(struct special_alt *alt); +/* + * Should the reloc at @offset -- the "new" (replacement) field of a special + * section group entry -- be ignored? The meaning of a zero-length replacement + * is arch specific, so the arch decides. + */ +bool arch_alt_ignore_new_reloc(struct section *sec, unsigned long offset); + bool arch_support_alt_relocation(struct special_alt *special_alt, struct instruction *insn, struct reloc *reloc); diff --git a/tools/objtool/klp-diff.c b/tools/objtool/klp-diff.c index b4220e12a8e2..df2fe014ec7e 100644 --- a/tools/objtool/klp-diff.c +++ b/tools/objtool/klp-diff.c @@ -12,7 +12,7 @@ #include #include #include -#include +#include #include #include @@ -1538,6 +1538,10 @@ static int clone_sym_relocs(struct elfs *e, struct symbol *patched_sym) !strcmp(patched_reloc->sym->sec->name, ".altinstr_aux")) continue; + if (arch_alt_ignore_new_reloc(patched_sym->sec, + reloc_offset(patched_reloc))) + continue; + ret = convert_reloc_sym(e->patched, patched_reloc); if (ret < 0) { ERROR_FUNC(patched_rsec->base, reloc_offset(patched_reloc),