Re: [PATCH 06/14] bpf/tests: Add more BPF_LSH/RSH/ARSH tests for ALU64

[Yonghong Song <yhs@fb.com>]

On 7/29/21 5:34 AM, Johan Almbladh wrote:
> On Thu, Jul 29, 2021 at 1:30 AM Yonghong Song <yhs@fb.com> wrote:
>>> @@ -4139,6 +4139,106 @@ static struct bpf_test tests[] = {
>>>                { },
>>>                { { 0, 0x80000000 } },
>>>        },
>>> +     {
>>> +             "ALU64_LSH_X: Shift < 32, low word",
>>> +             .u.insns_int = {
>>> +                     BPF_LD_IMM64(R0, 0x0123456789abcdefLL),
>>> +                     BPF_ALU32_IMM(BPF_MOV, R1, 12),
>>> +                     BPF_ALU64_REG(BPF_LSH, R0, R1),
>>> +                     BPF_EXIT_INSN(),
>>> +             },
>>> +             INTERNAL,
>>> +             { },
>>> +             { { 0, 0xbcdef000 } }
>>
>> In bpf_test struct, the result is defined as __u32
>>           struct {
>>                   int data_size;
>>                   __u32 result;
>>           } test[MAX_SUBTESTS];
>>
>> But the above result 0xbcdef000 does not really capture the bpf program
>> return value, which should be 0x3456789abcdef000.
>> Can we change "result" type to __u64 so the result truly captures the
>> program return value?
> 
> This was also my though at first, but I don't think that is possible.
> As I understand it, the eBPF functions have the prototype int
> func(struct *ctx). While the context pointer will have a different
> size on 32-bit and 64-bit architectures, the return value will always
> be 32 bits on most, or all, platforms.

Thanks for explanation. Yes, all BPF_PROG_RUN variables have bpf program
return type u32, so you are right, we cannot really check prog return
value against a 64bit R0.

> 
>> We have several other similar cases for the rest of this patch.
> 
> I have used two ways to check the full 64-bit result in such cases.
> 
> 1) Load the expected result as a 64-bit value in a register. Then jump
> conditionally if the result matches this value or not. The jump
> destinations each set a distinct value in R0, which is finally
> examined as the result.
> 
> 2) Run the test twice. The first one returns the low 32-bits of R0.
> The second adds a 32-bit right shift to return the high 32 bits.
> 
> When I first wrote the tests I tried to use as few complex
> instructions not under test as possible, in order to test each
> instruction in isolation. Since the 32-bit right shift is a much
> simpler operation than conditional jumps, at least in the 32-bit MIPS
> JIT, I chose method (2) for most of the tests. Existing tests seem to
> use method (1), so in some cases I used that instead when adding more
> tests of the same operation. The motivation for the simple one-by-one
> tests is mainly convenience and better diagnostics during JIT
> development. Both methods (1) and (2) are equally valid of course.

it is totally okay to use (2). Your tests are fine in that regard.

> 
> By the way, thanks a lot for the review, Yonghong!

You are welcome!

> 
> Johan
>