linux-kernel - Re: [PATCH v2 1/2] selftests/sgx: Use rip relative addressing for encl

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Date:   Wed, 30 Mar 2022 22:22:43 +0300
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Reinette Chatre <reinette.chatre@...el.com>
Cc:     Shuah Khan <skhan@...uxfoundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Shuah Khan <shuah@...nel.org>,
        "open list:INTEL SGX" <linux-sgx@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] selftests/sgx: Use rip relative addressing for
 encl_stack

On Wed, 2022-03-30 at 22:03 +0300, Jarkko Sakkinen wrote:
> On Wed, 2022-03-30 at 10:40 -0700, Reinette Chatre wrote:
> > Could you please elaborate how the compiler will fix it up?
> 
> Sure.
> 
> Here's the disassembly of the RBX version:
> 
> [0x000021a9]> pi 1
> lea rax, [rbx + loc.encl_stack]
> 
> Here's the same with s/RBX/RIP/:
> 
> [0x000021a9]> pi 5
> lea rax, loc.encl_stack
> Compiler will substitute correct offset relative to the RIP,
> well, because it can and it makes sense.
> 
> It is treated differently than other registers, e.g. when
> LEA is assembled.
> 
> BR, Jarkko


To demonstrate this I did a couple of simple sessions with Rizin
(fork/continuation/something of Radare2):

jarkko@...pilovahvero ~/Downloads (main)> rizin test_encl.rbx.elf
 -- Temporally drop the verbosity prefixing the commands with ':'
[0x00002000]> aaa
[x] Analyze all flags starting with sym. and entry0 (aa)
[x] Analyze function calls (aac)
[x] Analyze len bytes of instructions for references (aar)
[x] Check for classes
[x] Type matching analysis for all functions (aaft)
[x] Propagate noreturn information
[x] Use -AA or aaaa to perform additional experimental analysis.
[0x00002000]> /ad lea
0x0000206e   # 7: lea rax, [rip + 0xf8b]
0x0000206f   # 6: lea eax, [rip + 0xf8b]
0x0000207e   # 1: leave
0x000020a1   # 7: lea rcx, [rip + 0xf58]
0x000020a2   # 6: lea ecx, [rip + 0xf58]
0x000020b4   # 1: leave
0x000020ee   # 1: leave
0x00002128   # 1: leave
0x00002145   # 7: lea rax, [rip - 0x102]
0x00002146   # 6: lea eax, [rip - 0x102]
0x00002150   # 7: lea rax, [rip - 0xd7]
0x00002151   # 6: lea eax, [rip - 0xd7]
0x0000215b   # 7: lea rax, [rip - 0xac]
0x0000215c   # 6: lea eax, [rip - 0xac]
0x00002166   # 7: lea rax, [rip - 0x7d]
0x00002167   # 6: lea eax, [rip - 0x7d]
0x00002171   # 7: lea rax, [rip - 0x4e]
0x00002172   # 6: lea eax, [rip - 0x4e]
0x000021a7   # 1: leave
0x000021a9   # 7: lea rax, [rbx + loc.encl_stack]
0x000021aa   # 6: lea eax, [rbx + loc.encl_stack]
[0x00002000]> s 0x21a9
[0x000021a9]> pi 1
lea rax, [rbx + loc.encl_stack]
[0x000021a9]> 

jarkko@...pilovahvero ~/Downloads (main)> rizin test_encl.elf
 -- Use V! to enter into the visual panels mode (dwm style)
[0x00002000]> aaa
[x] Analyze all flags starting with sym. and entry0 (aa)
[x] Analyze function calls (aac)
[x] Analyze len bytes of instructions for references (aar)
[x] Check for classes
[x] Type matching analysis for all functions (aaft)
[x] Propagate noreturn information
[x] Use -AA or aaaa to perform additional experimental analysis.
[0x00002000]> /ad lea
0x0000206e   # 7: lea rax, [rip + 0xf8b]
0x0000206f   # 6: lea eax, [rip + 0xf8b]
0x0000207e   # 1: leave
0x000020a1   # 7: lea rcx, [rip + 0xf58]
0x000020a2   # 6: lea ecx, [rip + 0xf58]
0x000020b4   # 1: leave
0x000020ee   # 1: leave
0x00002128   # 1: leave
0x00002145   # 7: lea rax, [rip - 0x102]
0x00002146   # 6: lea eax, [rip - 0x102]
0x00002150   # 7: lea rax, [rip - 0xd7]
0x00002151   # 6: lea eax, [rip - 0xd7]
0x0000215b   # 7: lea rax, [rip - 0xac]
0x0000215c   # 6: lea eax, [rip - 0xac]
0x00002166   # 7: lea rax, [rip - 0x7d]
0x00002167   # 6: lea eax, [rip - 0x7d]
0x00002171   # 7: lea rax, [rip - 0x4e]
0x00002172   # 6: lea eax, [rip - 0x4e]
0x000021a7   # 1: leave
0x000021a9   # 7: lea rax, [rip + 0x5e50]
0x000021aa   # 6: lea eax, [rip + 0x5e50]
[0x00002000]> s 0x21a9
[0x000021a9]> pi 1
lea rax, loc.encl_stack
[0x000021a9]> 

BR, Jarkko