linux-kernel - Re: [tip: objtool/core] objtool/x86: objtool can confuse memory and stack access

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Message-ID: <20240708164846.GFZowYbmQpBu2Y4GeL@fat_crate.local>
Date: Mon, 8 Jul 2024 18:48:46 +0200
From: Borislav Petkov <bp@...en8.de>
To: linux-kernel@...r.kernel.org
Cc: linux-tip-commits@...r.kernel.org, kernel test robot <lkp@...el.com>,
	Alexandre Chartre <alexandre.chartre@...cle.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>, x86@...nel.org
Subject: Re: [tip: objtool/core] objtool/x86: objtool can confuse memory and
 stack access

On Mon, Jul 08, 2024 at 11:49:24AM -0000, tip-bot2 for Alexandre Chartre wrote:
>  4c 8b 24 25 e0 ff ff    mov    0xffffffffffffffe0,%r12

Right, this is missing a "ff" which is the 4th byte of a disp32.

I.e., ModRM=0, SIB=5 simply means that what follows is a disp32 field:

 REX:                   0x4c { 4 [w]: 1 [r]: 1 [x]: 0 [b]: 0 }
Opcode:                 0x8b
ModRM:                  0x24  [mod:0b][.R:1b,reg:1100b][.B:0b,r/m:100b]
                        register-indirect mode, offset 0
SIB:                    0x25 [.B:0b,base:101b][.X:0b,idx:100b][scale: 0]

 MOV Gv,Ev; MOV reg{16,32,64} reg/mem{16,32,64}
               0:       4c 8b 24 25 e0 ff ff    mov 0xffffffffffffffe0,%r12
               7:       ff
-- 
Regards/Gruss,
    Boris.

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