Re: [PATCH bpf-next 3/5] selftests/bpf: don't mask result of bpf_csum_diff() in test_verifier

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On 10/21/24 15:14, Puranjay Mohan wrote:
Helge Deller <deller@xxxxxx> 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





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