The bpf_csum_diff() helper has been fixed to return a 16-bit value for
all archs, so now we don't need to mask the result.

This commit is basically reverting the below:

commit 6185266c5a85 ("selftests/bpf: Mask bpf_csum_diff() return value
to 16 bits in test_verifier")

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
---
 tools/testing/selftests/bpf/progs/verifier_array_access.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/tools/testing/selftests/bpf/progs/verifier_array_access.c b/tools/testing/selftests/bpf/progs/verifier_array_access.c
index 95d7ecc12963b..4195aa824ba55 100644
--- a/tools/testing/selftests/bpf/progs/verifier_array_access.c
+++ b/tools/testing/selftests/bpf/progs/verifier_array_access.c
@@ -368,8 +368,7 @@ __naked void a_read_only_array_2_1(void)
 	r4 = 0;						\
 	r5 = 0;						\
 	call %[bpf_csum_diff];				\
-l0_%=:	r0 &= 0xffff;					\
-	exit;						\
+l0_%=:	exit;						\
 "	:
 	: __imm(bpf_csum_diff),
 	  __imm(bpf_map_lookup_elem),
-- 
2.40.1

Puranjay Mohan <puranjay@kernel.org> writes:

> The bpf_csum_diff() helper has been fixed to return a 16-bit value for
> all archs, so now we don't need to mask the result.
>
> This commit is basically reverting the below:
>
> commit 6185266c5a85 ("selftests/bpf: Mask bpf_csum_diff() return value
> to 16 bits in test_verifier")
>
> Signed-off-by: Puranjay Mohan <puranjay@kernel.org>

Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>

On 10/21/24 2:21 PM, Puranjay Mohan wrote:
> The bpf_csum_diff() helper has been fixed to return a 16-bit value for
> all archs, so now we don't need to mask the result.
> 
> This commit is basically reverting the below:
> 
> commit 6185266c5a85 ("selftests/bpf: Mask bpf_csum_diff() return value
> to 16 bits in test_verifier")
> 
> Signed-off-by: Puranjay Mohan <puranjay@kernel.org>

Acked-by: Daniel Borkmann <daniel@iogearbox.net>

On 10/21/24 14:21, Puranjay Mohan wrote:
> The bpf_csum_diff() helper has been fixed to return a 16-bit value for
> all archs, so now we don't need to mask the result.
>
> ...
> --- a/tools/testing/selftests/bpf/progs/verifier_array_access.c
> +++ b/tools/testing/selftests/bpf/progs/verifier_array_access.c
> @@ -368,8 +368,7 @@ __naked void a_read_only_array_2_1(void)
>   	r4 = 0;						\
>   	r5 = 0;						\
>   	call %[bpf_csum_diff];				\
> -l0_%=:	r0 &= 0xffff;					\
> -	exit;						\
> +l0_%=:	exit;						\

Instead of dropping the masking, would it make sense to
check here if (r0 >> 16) == 0 ?

Helge

Helge Deller <deller@gmx.de> writes:

> On 10/21/24 14:21, Puranjay Mohan wrote:
>> The bpf_csum_diff() helper has been fixed to return a 16-bit value for
>> all archs, so now we don't need to mask the result.
>>
>> ...
>> --- a/tools/testing/selftests/bpf/progs/verifier_array_access.c
>> +++ b/tools/testing/selftests/bpf/progs/verifier_array_access.c
>> @@ -368,8 +368,7 @@ __naked void a_read_only_array_2_1(void)
>>   	r4 = 0;						\
>>   	r5 = 0;						\
>>   	call %[bpf_csum_diff];				\
>> -l0_%=:	r0 &= 0xffff;					\
>> -	exit;						\
>> +l0_%=:	exit;						\
>
> Instead of dropping the masking, would it make sense to
> check here if (r0 >> 16) == 0 ?

We define the expected value in R0 to be 65507(0xffe3) in the line at the top:
__success __retval(65507)

So, we should just not do anything to R0 and it should contain this value
after returning from bpf_csum_diff()

This masking hack was added in:

6185266c5a853 ("selftests/bpf: Mask bpf_csum_diff() return value to 16 bits in test_verifier")

because without the fix in patch 2 bpf_csum_diff() would return the
following for this test:

x86                    :    -29 : 0xffffffe3
generic (arm64, riscv) :  65507 : 0x0000ffe3


Thanks,
Puranjay

On 10/21/24 15:14, Puranjay Mohan wrote:
> Helge Deller <deller@gmx.de> writes:
>
>> On 10/21/24 14:21, Puranjay Mohan wrote:
>>> The bpf_csum_diff() helper has been fixed to return a 16-bit value for
>>> all archs, so now we don't need to mask the result.
>>>
>>> ...
>>> --- a/tools/testing/selftests/bpf/progs/verifier_array_access.c
>>> +++ b/tools/testing/selftests/bpf/progs/verifier_array_access.c
>>> @@ -368,8 +368,7 @@ __naked void a_read_only_array_2_1(void)
>>>    	r4 = 0;						\
>>>    	r5 = 0;						\
>>>    	call %[bpf_csum_diff];				\
>>> -l0_%=:	r0 &= 0xffff;					\
>>> -	exit;						\
>>> +l0_%=:	exit;						\
>>
>> Instead of dropping the masking, would it make sense to
>> check here if (r0 >> 16) == 0 ?
>
> We define the expected value in R0 to be 65507(0xffe3) in the line at the top:
> __success __retval(65507)
>
> So, we should just not do anything to R0 and it should contain this value
> after returning from bpf_csum_diff()
>
> This masking hack was added in:
>
> 6185266c5a853 ("selftests/bpf: Mask bpf_csum_diff() return value to 16 bits in test_verifier")
>
> because without the fix in patch 2 bpf_csum_diff() would return the
> following for this test:
>
> x86                    :    -29 : 0xffffffe3
> generic (arm64, riscv) :  65507 : 0x0000ffe3

You're right.
Thanks for explaining.

Helge