Query: Regarding mtrack-speculation support in gcc latest version

[Richard Earnshaw (lists)]

On 07/01/2020 11:32, Gaurav Kohli wrote:
> 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.
> 

You can use the GCC builtin I mentioned with or without 
-mtrack-speculation.  Depending on your code, the number of locations 
you need to annotate and the frequency with which sensitive data might 
be accessed, -mtrack-speculation might, or might not, improve the 
overall performance (you trade code to track the conditions against a 
lighter weight speculation barrier when you need to inhibit speculation.

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

Not at present.

R.

> 
> 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)
>>> Â Â Â Â Â Â Â Â  printf("val=%d\n",arr[inp]);
>>> else
>>> Â Â Â Â Â Â Â Â  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.
>