linux-kernel - [PATCH 5.10 044/148] objtool: Make .altinstructions section entry size consistent

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Date:   Sat, 23 Jul 2022 11:54:16 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Joe Lawrence <joe.lawrence@...hat.com>,
        Miroslav Benes <mbenes@...e.cz>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andy Lavr <andy.lavr@...il.com>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        Ben Hutchings <ben@...adent.org.uk>
Subject: [PATCH 5.10 044/148] objtool: Make .altinstructions section entry size consistent

From: Joe Lawrence <joe.lawrence@...hat.com>

commit dc02368164bd0ec603e3f5b3dd8252744a667b8a upstream.

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@...hat.com>
Reviewed-by: Miroslav Benes <mbenes@...e.cz>
Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
Link: https://lore.kernel.org/r/20210822225037.54620-2-joe.lawrence@redhat.com
Cc: Andy Lavr <andy.lavr@...il.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: x86@...nel.org
Cc: linux-kernel@...r.kernel.org
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 tools/objtool/arch/x86/decode.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -611,7 +611,7 @@ static int elf_add_alternative(struct el
 	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");