linux-kernel - Re: [PATCH v3 2/4] x86/alternative: Support relocations in alternatives

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Date:   Fri, 17 Feb 2023 23:21:21 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, mhiramat@...nel.org,
        kirill.shutemov@...ux.intel.com, Andrew.Cooper3@...rix.com,
        jpoimboe@...hat.com
Subject: Re: [PATCH v3 2/4] x86/alternative: Support relocations in
 alternatives

On Wed, Feb 08, 2023 at 06:10:52PM +0100, Peter Zijlstra wrote:
> +			if (insn.opcode.bytes[0] == JMP32_INSN_OPCODE) {
> +				s32 imm = insn.immediate.value;
> +				imm += src - dest;
> +				imm += JMP32_INSN_SIZE - JMP8_INSN_SIZE;
> +				if ((imm >> 31) == (imm >> 7)) {
> +					buf[i+0] = JMP8_INSN_OPCODE;
> +					buf[i+1] = (s8)imm;
> +					for (int j = 2; j < insn.length; j++)
> +						buf[i+j] = INT3_INSN_OPCODE;

Let's get rid of that third nested loop:

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index e14bc15bf646..28eb1d0bc4a0 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -308,8 +308,8 @@ apply_relocation(u8 *buf, size_t len, u8 *dest, u8 *src, size_t src_len)
 				if ((imm >> 31) == (imm >> 7)) {
 					buf[i+0] = JMP8_INSN_OPCODE;
 					buf[i+1] = (s8)imm;
-					for (int j = 2; j < insn.length; j++)
-						buf[i+j] = INT3_INSN_OPCODE;
+
+					memset(&buf[i+2], INT3_INSN_OPCODE, insn.length - 2);
 				}
 			}
 			break;

-- 
Regards/Gruss,
    Boris.

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