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From: Joe Lawrence <joe.lawrence@redhat.com>
To: x86@kernel.org, Josh Poimboeuf <jpoimboe@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org, Miroslav Benes <mbenes@suse.cz>
Subject: [PATCH v2 1/2] objtool: make .altinstructions section entry size consistent
Date: Fri, 20 Aug 2021 15:44:52 -0400	[thread overview]
Message-ID: <20210820194453.395548-2-joe.lawrence@redhat.com> (raw)
In-Reply-To: <20210820194453.395548-1-joe.lawrence@redhat.com>

commit e31694e0a7a7 ("objtool: Don't make .altinstructions writable")
aligned objtool-created and kernel-created .altinstructions section
flags, but there remains a minor discrepency in their use of a section
entry size: objtool sets one while the kernel build does not.

While sh_entsize of sizeof(struct alt_instr) seems intuitive, this small
deviation can cause failures with external tooling (kpatch-build).

Fix this by creating new .altinstructions sections with sh_entsize of 0
and then later updating sec->sh_size as alternatives are added to the
section.  An added benefit is avoiding the data descriptor and buffer
created by elf_create_section(), but previously unused by
elf_add_alternative().

Fixes: 9bc0bb50727c ("objtool/x86: Rewrite retpoline thunk calls")
Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
---
 tools/objtool/arch/x86/decode.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
index bc821056aba9..0893436cc09f 100644
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -684,7 +684,7 @@ static int elf_add_alternative(struct elf *elf,
 	sec = find_section_by_name(elf, ".altinstructions");
 	if (!sec) {
 		sec = elf_create_section(elf, ".altinstructions",
-					 SHF_ALLOC, size, 0);
+					 SHF_ALLOC, 0, 0);
 
 		if (!sec) {
 			WARN_ELF("elf_create_section");
-- 
2.26.3


  reply	other threads:[~2021-08-20 19:45 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-20 19:44 [PATCH v2 0/2] objtool: altinstructions fix and cleanup Joe Lawrence
2021-08-20 19:44 ` Joe Lawrence [this message]
2021-08-20 19:44 ` [PATCH v2 2/2] objtool: remove redundant len value from struct section Joe Lawrence
2021-08-22  1:17   ` Joe Lawrence

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