Re: [PATCH v1 01/27] x86/crypto: Adapt assembly for PIE support

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 20 October 2017 at 09:24, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Thomas Garnier <thgarnie@google.com> wrote:
>
>> Change the assembly code to use only relative references of symbols for the
>> kernel to be PIE compatible.
>>
>> Position Independent Executable (PIE) support will allow to extended the
>> KASLR randomization range below the -2G memory limit.
>
>> diff --git a/arch/x86/crypto/aes-x86_64-asm_64.S b/arch/x86/crypto/aes-x86_64-asm_64.S
>> index 8739cf7795de..86fa068e5e81 100644
>> --- a/arch/x86/crypto/aes-x86_64-asm_64.S
>> +++ b/arch/x86/crypto/aes-x86_64-asm_64.S
>> @@ -48,8 +48,12 @@
>>  #define R10  %r10
>>  #define R11  %r11
>>
>> +/* Hold global for PIE suport */
>> +#define RBASE        %r12
>> +
>>  #define prologue(FUNC,KEY,B128,B192,r1,r2,r5,r6,r7,r8,r9,r10,r11) \
>>       ENTRY(FUNC);                    \
>> +     pushq   RBASE;                  \
>>       movq    r1,r2;                  \
>>       leaq    KEY+48(r8),r9;          \
>>       movq    r10,r11;                \
>> @@ -74,54 +78,63 @@
>>       movl    r6 ## E,4(r9);          \
>>       movl    r7 ## E,8(r9);          \
>>       movl    r8 ## E,12(r9);         \
>> +     popq    RBASE;                  \
>>       ret;                            \
>>       ENDPROC(FUNC);
>>
>> +#define round_mov(tab_off, reg_i, reg_o) \
>> +     leaq    tab_off(%rip), RBASE; \
>> +     movl    (RBASE,reg_i,4), reg_o;
>> +
>> +#define round_xor(tab_off, reg_i, reg_o) \
>> +     leaq    tab_off(%rip), RBASE; \
>> +     xorl    (RBASE,reg_i,4), reg_o;
>> +
>>  #define round(TAB,OFFSET,r1,r2,r3,r4,r5,r6,r7,r8,ra,rb,rc,rd) \
>>       movzbl  r2 ## H,r5 ## E;        \
>>       movzbl  r2 ## L,r6 ## E;        \
>> -     movl    TAB+1024(,r5,4),r5 ## E;\
>> +     round_mov(TAB+1024, r5, r5 ## E)\
>>       movw    r4 ## X,r2 ## X;        \
>> -     movl    TAB(,r6,4),r6 ## E;     \
>> +     round_mov(TAB, r6, r6 ## E)     \
>>       roll    $16,r2 ## E;            \
>>       shrl    $16,r4 ## E;            \
>>       movzbl  r4 ## L,r7 ## E;        \
>>       movzbl  r4 ## H,r4 ## E;        \
>>       xorl    OFFSET(r8),ra ## E;     \
>>       xorl    OFFSET+4(r8),rb ## E;   \
>> -     xorl    TAB+3072(,r4,4),r5 ## E;\
>> -     xorl    TAB+2048(,r7,4),r6 ## E;\
>> +     round_xor(TAB+3072, r4, r5 ## E)\
>> +     round_xor(TAB+2048, r7, r6 ## E)\
>>       movzbl  r1 ## L,r7 ## E;        \
>>       movzbl  r1 ## H,r4 ## E;        \
>> -     movl    TAB+1024(,r4,4),r4 ## E;\
>> +     round_mov(TAB+1024, r4, r4 ## E)\
>>       movw    r3 ## X,r1 ## X;        \
>>       roll    $16,r1 ## E;            \
>>       shrl    $16,r3 ## E;            \
>> -     xorl    TAB(,r7,4),r5 ## E;     \
>> +     round_xor(TAB, r7, r5 ## E)     \
>>       movzbl  r3 ## L,r7 ## E;        \
>>       movzbl  r3 ## H,r3 ## E;        \
>> -     xorl    TAB+3072(,r3,4),r4 ## E;\
>> -     xorl    TAB+2048(,r7,4),r5 ## E;\
>> +     round_xor(TAB+3072, r3, r4 ## E)\
>> +     round_xor(TAB+2048, r7, r5 ## E)\
>>       movzbl  r1 ## L,r7 ## E;        \
>>       movzbl  r1 ## H,r3 ## E;        \
>>       shrl    $16,r1 ## E;            \
>> -     xorl    TAB+3072(,r3,4),r6 ## E;\
>> -     movl    TAB+2048(,r7,4),r3 ## E;\
>> +     round_xor(TAB+3072, r3, r6 ## E)\
>> +     round_mov(TAB+2048, r7, r3 ## E)\
>>       movzbl  r1 ## L,r7 ## E;        \
>>       movzbl  r1 ## H,r1 ## E;        \
>> -     xorl    TAB+1024(,r1,4),r6 ## E;\
>> -     xorl    TAB(,r7,4),r3 ## E;     \
>> +     round_xor(TAB+1024, r1, r6 ## E)\
>> +     round_xor(TAB, r7, r3 ## E)     \
>>       movzbl  r2 ## H,r1 ## E;        \
>>       movzbl  r2 ## L,r7 ## E;        \
>>       shrl    $16,r2 ## E;            \
>> -     xorl    TAB+3072(,r1,4),r3 ## E;\
>> -     xorl    TAB+2048(,r7,4),r4 ## E;\
>> +     round_xor(TAB+3072, r1, r3 ## E)\
>> +     round_xor(TAB+2048, r7, r4 ## E)\
>>       movzbl  r2 ## H,r1 ## E;        \
>>       movzbl  r2 ## L,r2 ## E;        \
>>       xorl    OFFSET+8(r8),rc ## E;   \
>>       xorl    OFFSET+12(r8),rd ## E;  \
>> -     xorl    TAB+1024(,r1,4),r3 ## E;\
>> -     xorl    TAB(,r2,4),r4 ## E;
>> +     round_xor(TAB+1024, r1, r3 ## E)\
>> +     round_xor(TAB, r2, r4 ## E)
>
> This appears to be adding unconditional overhead to a function that was moved to
> assembly to improve its performance.
>

I did some benchmarking on this code a while ago and, interestingly,
it was slower than the generic C implementation (on a Pentium E2200),
so we may want to consider whether we still need this driver in the
first place.