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 6933D36D9E9; Mon, 3 Aug 2026 03:24:55 +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=1785727497; cv=none; b=O6jehoyT4uw/OoxRa86cAW7fijv7+pBi1eshqM4vO12Kn+L6YiLhjHExRELXbFAdaXubkh/b1lkHztHNIbJE5d+K2qHo1LgmtIb60j46DzVw10dcCqVguqW8x2LryZrVfgEQPnaDPP8mvlvlw4u9LPP5wz7cCLyi6jBJCpi+LVU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785727497; c=relaxed/simple; bh=+i6d5aIZoP6EZsiI43w/o9faePszHVo7G+PuVfaNO7A=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=oUPBFs/UTRjeca3ZZ37yhOo6+xYpVyyF0xP3oYzxwCs23i1TuZSW1+Nuch+RHJaALZ1iV3mZeZTdZy3BkvqbqlmP8sFZlVVsUF4aq3qNVURAwSjc56zAfA5KDd0QVHfR5y29cFOmdubloOKIBvcb4ZnuvODuX+ORzlrOzWIEFAQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=YZCrFjjw; 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="YZCrFjjw" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D41571F000E9; Mon, 3 Aug 2026 03:24:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785727495; bh=4oZotfaHgciDLlJjYrspLV8HQAzomvenh71Q/xI58mY=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=YZCrFjjwaCyM+erXNydkp1U8xJjKUXuxcfxQ1NIa7AgD47NlVWeNuLYAxrlRUfb3T ig5P1UyzElF2x1ybRFm5KHCfkPltdM1Mv3HhCSUoxgh/vNLk5jfYFQkxNhmVgy1770 Ck3ZUiMRcafJvFc5Lxg5RK7BKdg9nPhOjEjsqHkg2Ma4i3v/X/+8mdAXLA3nxhR5h0 VHoMyOUWdRDRrZDfb1ApjnCN7HFmo1vfsyE7MacvI9O0PYl1D1oOJBvMr0Deb2s1Gm zLBtRa0LkIjPNAs8DqNzxLWQs3hre5NeK7TVpCcVNBLH0v0PcdaRJaEn4qr5i+pSjE QqJ5zPtNrSLMw== 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 , Luis Chamberlain , Petr Pavlu , Daniel Gomez , Sami Tolvanen , linux-modules@vger.kernel.org Subject: [PATCH 10/14] objtool/klp: Ignore replacement offset of empty x86 alternatives Date: Sun, 2 Aug 2026 20:24:32 -0700 Message-ID: X-Mailer: git-send-email 2.54.0 In-Reply-To: References: Precedence: bulk X-Mailing-List: linux-kernel@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 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 257e7f924928..cf6fc88bc979 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), -- 2.54.0