RE: [RFC PATCH v2 20/21] x86: Add support for CONFIG_CFI_CLANG
From: David Laight <hidden>
Date: 2022-05-17 07:56:43
Also in:
linux-hardening, lkml, llvm
From: Sami Tolvanen
Sent: 16 May 2022 23:03 On Mon, May 16, 2022 at 2:44 PM Peter Zijlstra [off-list ref] wrote:quoted
On Mon, May 16, 2022 at 09:32:55PM +0000, David Laight wrote:quoted
quoted
The compiler always generates this specific instruction sequence.Yes, but there are several ways to encode 'cmpl imm,-6(reg)'.Yes, but we don't care. This *always* uses the 32bit immediate form. Even if the immediate is small.Yes, that part is not a problem, but it's a valid point that LLVM might not always use r8-r15 here, so I will have to check for the REX prefix before blindly attempting to decode the instruction.
Are you allowing for the REX prefix at all? The encoding of:
quoted
quoted
+ * cmpl <id>, -6(%reg) ; 7 bytes
is <opcode><mod/TTT/rm><off8><imm32> which is 7 bytes without the REX. If reg is r11 there is an extra REX byte - for 8 in total. Without the REX byte the decode will be using %bx. So the testing should all have failed. Which means that something else is wrong as well. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales) _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel