From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 9A35C38886A; Wed, 5 Aug 2026 14:30:03 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785940212; cv=none; b=nZA/6VBujN5GEsnOXOTgxsSZTNzPqmfttgn096JqrSdrI8ccJ9agkbr5qc5Eef+HvmmJgNEIUMP3SNFOzod8UAqTvDYXrzrJn28shYbnW9f5MnvsHRTjDzwxYBG+DwPb/ATdQQARIie6aswAHBE3sIMxKtU/ki98RnIoAK05ZbI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785940212; c=relaxed/simple; bh=EYNuaF6doaxxQJyOIofIXN8oHkPcU316iaavUCAc9KQ=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=e7Urwp8Ouf9JwUIgF5LL+ifoUA7R/mvTdGzDiyowZII3mjJjRenVe5z2JfVNBJTZ6M3QKGELLOEp5ZIypIQfL7uvABrKK9shNrG4yAhRD+bKV3xDAAuHq20I80IIGwBRs1vcspG7Ejyc7QkkN6V/g5VX4zGHKDhsLhZMA0WbvKw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=er/NksyN; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="er/NksyN" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B8B861F00ACA; Wed, 5 Aug 2026 14:29:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785940200; bh=8C1aQofDV7HsvKHeAFCooZp2csmiOoW6Cpz9YcY4RoM=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=er/NksyN5Ta4Z2qymms4Xly4o57Eu5urtHfVV1lHdZP22WqeD0ijqOrMJwc6/q9WA WIEnvNx5oaqh+TdUUtWG8x7dpv7lUAqnF05TK85IPUwCoDibItrnGIiggv2c6EBcVE zUVGGGFKHSzuaFHIrE0MbiaLz0iyzyVsTO1O93D4ebRpy3aOCZB2/juD0J9AjiTKRV J1JXqDZM9HuTop9w+m235cOnG6e/qXREsQXSJU1RORMfLffHfJR80q+4bZr5FnyNVC oAEE6WC5KPBmiFCBjvISgcZV7SQs2DwPodd7awMABOwBLRRSJeH4XxrLhdzc/V1Iyx RoiAlkQgZCOVg== From: Josh Poimboeuf To: x86@kernel.org Cc: linux-kernel@vger.kernel.org, live-patching@vger.kernel.org, Peter Zijlstra , Joe Lawrence , Miroslav Benes , Petr Mladek , Song Liu Subject: [PATCH v2 3/7] objtool/klp: Ignore replacement offset of empty x86 alternatives Date: Wed, 5 Aug 2026 07:29:40 -0700 Message-ID: <9f9c0c444e078869f2bceac7ecc72fe7310e188f.1785939903.git.jpoimboe@kernel.org> X-Mailer: git-send-email 2.54.0 In-Reply-To: References: Precedence: bulk X-Mailing-List: live-patching@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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") 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 e817a3fff4491..1e84c81bfcd81 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 121c3761899c1..620dbf6cb0e58 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 11e8f3ddbb0e6..07cc8e2703260 100644 --- a/tools/objtool/klp-diff.c +++ b/tools/objtool/klp-diff.c @@ -12,7 +12,7 @@ #include #include #include -#include +#include #include #include @@ -1537,6 +1537,10 @@ 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), -- 2.54.0