linux-kernel - [PATCH v1 05/12] x86, alternatives: Use optimized NOPs for padding

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Date:	Tue,  3 Feb 2015 19:16:23 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	X86 ML <x86@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH v1 05/12] x86, alternatives: Use optimized NOPs for padding

From: Borislav Petkov <bp@...e.de>

Alternatives allow now for empty old instruction. In this case we go
and pad the space with NOPs at assembly time. However, there are the
optimal, longer NOPs which should be used. Do that at patching time.

Signed-off-by: Borislav Petkov <bp@...e.de>
---
 arch/x86/kernel/alternative.c | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 715af37bf008..dd0cdb6b179c 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -323,6 +323,21 @@ done:
 		n_dspl, (unsigned long)orig_insn + n_dspl + repl_len);
 }
 
+static void __init_or_module optimize_nops(u8 *instr, u8 max_len)
+{
+	int i = 0;
+
+	while (instr[i] == 0x90 && i < max_len)
+		i++;
+
+	if (!i)
+		return;
+
+	add_nops(instr, i);
+
+	DUMP_BYTES(instr, i, "%p: optimized NOPs: ", instr);
+}
+
 /*
  * Replace instructions with better alternatives for this CPU type. This runs
  * before SMP is initialized to avoid SMP problems with self modifying code.
@@ -354,8 +369,11 @@ void __init_or_module apply_alternatives(struct alt_instr *start,
 		replacement = (u8 *)&a->repl_offset + a->repl_offset;
 		BUG_ON(a->instrlen > sizeof(insnbuf));
 		BUG_ON(a->cpuid >= (NCAPINTS + NBUGINTS) * 32);
-		if (!boot_cpu_has(a->cpuid))
+		if (!boot_cpu_has(a->cpuid)) {
+			if (instr[0] == 0x90)
+				optimize_nops(instr, a->instrlen);
 			continue;
+		}
 
 		DPRINTK("feat: %d*32+%d, old: (%p, len: %d), repl: (%p, len: %d)",
 			a->cpuid >> 5,
-- 
2.2.0.33.gc18b867

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