linux-kernel - [RFC PATCH 16/17] objtool: arm64: Ignore replacement section for alternative callback

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Date:   Wed, 20 Jan 2021 18:37:59 +0100
From:   Julien Thierry <jthierry@...hat.com>
To:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Cc:     catalin.marinas@....com, will@...nel.org, ardb@...nel.org,
        masahiroy@...nel.org, keescook@...omium.org,
        michal.lkml@...kovi.net, jpoimboe@...hat.com, peterz@...radead.org,
        mark.rutland@....com, broonie@...nel.org,
        linux-efi@...r.kernel.org, linux-hardening@...r.kernel.org,
        Julien Thierry <jthierry@...hat.com>
Subject: [RFC PATCH 16/17] objtool: arm64: Ignore replacement section for alternative callback

ARM64_CB_PATCH doesn't have static replacement instructions. Skip
trying to validate the alternative section.

Signed-off-by: Julien Thierry <jthierry@...hat.com>
---
 tools/objtool/arch/arm64/special.c | 12 ++++++++++++
 tools/objtool/check.c              |  3 +++
 2 files changed, 15 insertions(+)

diff --git a/tools/objtool/arch/arm64/special.c b/tools/objtool/arch/arm64/special.c
index d47e5590ed60..aff8577e71e9 100644
--- a/tools/objtool/arch/arm64/special.c
+++ b/tools/objtool/arch/arm64/special.c
@@ -24,6 +24,18 @@ struct switch_table_info {
 
 void arch_handle_alternative(unsigned short feature, struct special_alt *alt)
 {
+	if (alt->orig_len && !alt->new_len) {
+		/*
+		 * ARM64_CB_PATCH has no alternative instruction.
+		 * a callback is called at alternative replacement time
+		 * to dynamically change the original instructions.
+		 *
+		 * ARM64_CB_PATCH is the last ARM64 feature, it's value changes
+		 * every time a new feature is added. So the orig/alt region
+		 * length are used to detect those alternatives
+		 */
+		alt->skip_alt = true;
+	}
 }
 
 bool arch_support_alt_relocation(struct special_alt *special_alt,
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index d902697a388e..8840af09f843 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1182,6 +1182,9 @@ static int add_special_section_alts(struct objtool_file *file)
 				continue;
 			}
 
+			if (special_alt->skip_alt && !special_alt->new_len)
+				continue;
+
 			ret = handle_group_alt(file, special_alt, orig_insn,
 					       &new_insn);
 			if (ret)
-- 
2.25.4