linux-kernel - [PATCH] objtool: x86: .altinstructions doesn't need section entry size

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Date:   Mon, 16 Aug 2021 21:49:58 -0400
From:   Joe Lawrence <joe.lawrence@...hat.com>
To:     x86@...nel.org, Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Miroslav Benes <mbenes@...e.cz>
Subject: [PATCH] objtool: x86: .altinstructions doesn't need section entry size

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 like kpatch-build.

Fix this by creating new .altinstructions sections with sh_entsize of 0
and then later updating sec->len 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@...hat.com>
---

Hi Josh, this is a follow up for
https://github.com/dynup/kpatch/issues/1194 where I'll add some more
comments on the kpatch-side of this.  We could probably work around it
over there, but this objtool tweak looks small enough to maintain closer
kernel-built .altinstructions section properties.

 tools/objtool/arch/x86/decode.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
index bc821056aba9..e7087aa473f8 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");
@@ -692,6 +692,8 @@ static int elf_add_alternative(struct elf *elf,
 		}
 	}

+	sec->len += size;
+
 	s = elf_getscn(elf->elf, sec->idx);
 	if (!s) {
 		WARN_ELF("elf_getscn");
-- 
2.26.3