Query: Regarding mtrack-speculation support in gcc latest version

Gaurav Kohli gkohli@codeaurora.org
Tue Jan 7 11:32:00 GMT 2020


Hi Richard,

Thanks a lot, it is working now.
So for user space also, we have to work like Linux kernel only use 
array_index_nospec in required places.

Is there any plan to automate this thing from compiler only?

Regards
Gaurav

On 1/6/2020 9:23 PM, Richard Earnshaw (lists) wrote:
> On 03/01/2020 13:08, Gaurav Kohli wrote:
>> Hi,
>>
>> I am trying latest gcc 9.1 version and executing below code with 
>> option -mtrack-speculation -march=armv8.5-a
>>
>> #include<stdio.h>
>> #include<stdlib.h>
>> #define limit 10
>> int arr[limit] = {0};
>> void main() {
>>
>> int inp;
>> printf("enter inp\n");
>> scanf("%d\n",&inp);
>> if(inp < limit)
>>          printf("val=%d\n",arr[inp]);
>> else
>>          printf("crossing limit\n");
>> return;
>>
>> }
>>
>>
>> But in disassembly only csel is generating, no csdb is coming:
>>
>>   40069c:       9a9fd1ef        csel    x15, x15, xzr, le
>>    4006a0:       b9401fe1        ldr     w1, [sp, #28]
>>    4006a4:       b0000080        adrp    x0, 411000 
>> <__libc_start_main@GLIBC_2.17>
>>    4006a8:       91012000        add     x0, x0, #0x48
>>    4006ac:       93407c21        sxtw    x1, w1
>>    4006b0:       b8617800        ldr     w0, [x0, x1, lsl #2]
>>    4006b4:       2a0003e1        mov     w1, w0
>>    4006b8:       90000000        adrp    x0, 400000 <_init-0x4a0>
>>    4006bc:       911f8000        add     x0, x0, #0x7e0
>>    4006c0:       910003ee        mov     x14, sp
>>    4006c4:       8a0f01ce        and     x14, x14, x15
>>    4006c8:       910001df        mov     sp, x14
>>    4006cc:       97ffff99        bl      400530 <printf@plt>
>>    4006d0:       f10003ff        cmp     sp, #0x0
>>    4006d4:       da9f03ef        csetm   x15, ne  // ne = any
>>    4006d8:       1400000b        b       400704 <main+0xb8>
>>    4006dc:       9a9fc1ef        csel    x15, x15, xzr, gt
>>
>>
>> Can someone please point, when cdsb will come?
>>
>> Regards
>> Gaurav
>>
> 
> As the manual says, -mtrack-speculation must be used in conjunction with 
> __builtin_speculation_safe_copy to annotate memory accesses that might 
> lead to potentially unsafe speculation.  See the manual for more details.

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