linux-kernel - [tip: objtool/core] jump_label, x86: Emit short JMP

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Date:   Wed, 12 May 2021 13:19:49 -0000
From:   "tip-bot2 for Peter Zijlstra" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: objtool/core] jump_label, x86: Emit short JMP

The following commit has been merged into the objtool/core branch of tip:

Commit-ID:     e7bf1ba97afdde75b0ef43e4bdb718bf843613f1
Gitweb:        https://git.kernel.org/tip/e7bf1ba97afdde75b0ef43e4bdb718bf843613f1
Author:        Peter Zijlstra <peterz@...radead.org>
AuthorDate:    Thu, 06 May 2021 21:34:01 +02:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Wed, 12 May 2021 14:54:55 +02:00

jump_label, x86: Emit short JMP

Now that we can patch short JMP/NOP, allow the compiler/assembler to
emit short JMP instructions.

There is no way to have the assembler emit short NOPs based on the
potential displacement, so leave those long for now.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Link: https://lore.kernel.org/r/20210506194157.967034497@infradead.org
---
 arch/x86/include/asm/jump_label.h | 3 +--
 arch/x86/kernel/jump_label.c      | 8 +++++++-
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h
index d85802a..ef819e3 100644
--- a/arch/x86/include/asm/jump_label.h
+++ b/arch/x86/include/asm/jump_label.h
@@ -35,8 +35,7 @@ l_yes:
 static __always_inline bool arch_static_branch_jump(struct static_key * const key, const bool branch)
 {
 	asm_volatile_goto("1:"
-		".byte 0xe9 \n\t"
-		".long %l[l_yes] - (. + 4) \n\t"
+		"jmp %l[l_yes]\n\t"
 		JUMP_TABLE_ENTRY
 		: :  "i" (key), "i" (branch) : : l_yes);
 
diff --git a/arch/x86/kernel/jump_label.c b/arch/x86/kernel/jump_label.c
index 190d810..a762dc1 100644
--- a/arch/x86/kernel/jump_label.c
+++ b/arch/x86/kernel/jump_label.c
@@ -15,12 +15,18 @@
 #include <asm/kprobes.h>
 #include <asm/alternative.h>
 #include <asm/text-patching.h>
+#include <asm/insn.h>
 
 #define JUMP_LABEL_NOP_SIZE	JMP32_INSN_SIZE
 
 int arch_jump_entry_size(struct jump_entry *entry)
 {
-	return JMP32_INSN_SIZE;
+	struct insn insn = {};
+
+	insn_decode_kernel(&insn, (void *)jump_entry_code(entry));
+	BUG_ON(insn.length != 2 && insn.length != 5);
+
+	return insn.length;
 }
 
 struct jump_label_patch {