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SubjectRe: [PATCH v6 bpf-next 4/6] selftests/bpf: add bpf_snprintf_btf helper tests
On Wed, Sep 23, 2020 at 06:46:26PM +0100, Alan Maguire wrote:
> +static int __strncmp(const void *m1, const void *m2, size_t len)
> +{
> + const unsigned char *s1 = m1;
> + const unsigned char *s2 = m2;
> + int i, delta = 0;
> +
> +#pragma clang loop unroll(full)

Shouldn't be needed?
The verifier supports bounded loops.

> + for (i = 0; i < len; i++) {
> + delta = s1[i] - s2[i];
> + if (delta || s1[i] == 0 || s2[i] == 0)
> + break;
> + }
> + return delta;
> +}
> +
> +/* Use __builtin_btf_type_id to test snprintf_btf by type id instead of name */
> +#if __has_builtin(__builtin_btf_type_id)
> +#define TEST_BTF_BY_ID(_str, _typestr, _ptr, _hflags) \
> + do { \
> + int _expected_ret = ret; \
> + _ptr.type = 0; \
> + _ptr.type_id = __builtin_btf_type_id(_typestr, 0); \

The test is passing for me, but I don't understand why :)
__builtin_btf_type_id(, 0); means btf_id of the bpf program.
While bpf_snprintf_btf() is treating it as btf_id of vmlinux_btf.
So it really should have been __builtin_btf_type_id(,1);

The following diff works:
diff --git a/tools/testing/selftests/bpf/progs/netif_receive_skb.c b/tools/testing/selftests/bpf/progs/netif_receive_skb.c
index b4f96f1f6830..bffa786e3b03 100644
--- a/tools/testing/selftests/bpf/progs/netif_receive_skb.c
+++ b/tools/testing/selftests/bpf/progs/netif_receive_skb.c
@@ -45,7 +45,7 @@ static int __strncmp(const void *m1, const void *m2, size_t len)
do { \
int _expected_ret = ret; \
_ptr.type = 0; \
- _ptr.type_id = __builtin_btf_type_id(_typestr, 0); \
+ _ptr.type_id = __builtin_btf_type_id(_typestr, 1); \
ret = bpf_snprintf_btf(_str, STRSIZE, &_ptr, \
sizeof(_ptr), _hflags); \
if (ret != _expected_ret) { \
@@ -88,7 +88,7 @@ static int __strncmp(const void *m1, const void *m2, size_t len)
ret = -EBADMSG; \
break; \
} \
- TEST_BTF_BY_ID(_str, #_type, _ptr, _hflags); \
+ TEST_BTF_BY_ID(_str, _ptr, _ptr, _hflags); \

But still makes me suspicious of the test. I haven't debugged further.

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