bpf: selftests: Add verifier tests for CO-RE bitfield writes

Add some tests that exercise BPF_CORE_WRITE_BITFIELD() macro. Since some
non-trivial bit fiddling is going on, make sure various edge cases (such
as adjacent bitfields and bitfields at the edge of structs) are
exercised.

Acked-by: Andrii Nakryiko <[e-mail]>
Signed-off-by: Daniel Xu <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Martin KaFai Lau <[e-mail]>

tools/testing/selftests/bpf/prog_tests/verifier.c

···
 #include "verifier_and.skel.h"
 #include "verifier_array_access.skel.h"
 #include "verifier_basic_stack.skel.h"
+#include "verifier_bitfield_write.skel.h"
 #include "verifier_bounds.skel.h"
 #include "verifier_bounds_deduction.skel.h"
 #include "verifier_bounds_deduction_non_const.skel.h"
···
 
 void test_verifier_and(void)                  { RUN(verifier_and); }
 void test_verifier_basic_stack(void)          { RUN(verifier_basic_stack); }
+void test_verifier_bitfield_write(void)       { RUN(verifier_bitfield_write); }
 void test_verifier_bounds(void)               { RUN(verifier_bounds); }
 void test_verifier_bounds_deduction(void)     { RUN(verifier_bounds_deduction); }
 void test_verifier_bounds_deduction_non_const(void)     { RUN(verifier_bounds_deduction_non_const); }

tools/testing/selftests/bpf/progs/verifier_bitfield_write.c

···
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/bpf.h>
+#include <stdint.h>
+
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_core_read.h>
+
+#include "bpf_misc.h"
+
+struct core_reloc_bitfields {
+	/* unsigned bitfields */
+	uint8_t		ub1: 1;
+	uint8_t		ub2: 2;
+	uint32_t	ub7: 7;
+	/* signed bitfields */
+	int8_t		sb4: 4;
+	int32_t		sb20: 20;
+	/* non-bitfields */
+	uint32_t	u32;
+	int32_t		s32;
+} __attribute__((preserve_access_index));
+
+SEC("tc")
+__description("single CO-RE bitfield roundtrip")
+__btf_path("btf__core_reloc_bitfields.bpf.o")
+__success
+__retval(3)
+int single_field_roundtrip(struct __sk_buff *ctx)
+{
+	struct core_reloc_bitfields bitfields;
+
+	__builtin_memset(&bitfields, 0, sizeof(bitfields));
+	BPF_CORE_WRITE_BITFIELD(&bitfields, ub2, 3);
+	return BPF_CORE_READ_BITFIELD(&bitfields, ub2);
+}
+
+SEC("tc")
+__description("multiple CO-RE bitfield roundtrip")
+__btf_path("btf__core_reloc_bitfields.bpf.o")
+__success
+__retval(0x3FD)
+int multiple_field_roundtrip(struct __sk_buff *ctx)
+{
+	struct core_reloc_bitfields bitfields;
+	uint8_t ub2;
+	int8_t sb4;
+
+	__builtin_memset(&bitfields, 0, sizeof(bitfields));
+	BPF_CORE_WRITE_BITFIELD(&bitfields, ub2, 1);
+	BPF_CORE_WRITE_BITFIELD(&bitfields, sb4, -1);
+
+	ub2 = BPF_CORE_READ_BITFIELD(&bitfields, ub2);
+	sb4 = BPF_CORE_READ_BITFIELD(&bitfields, sb4);
+
+	return (((uint8_t)sb4) << 2) | ub2;
+}
+
+SEC("tc")
+__description("adjacent CO-RE bitfield roundtrip")
+__btf_path("btf__core_reloc_bitfields.bpf.o")
+__success
+__retval(7)
+int adjacent_field_roundtrip(struct __sk_buff *ctx)
+{
+	struct core_reloc_bitfields bitfields;
+	uint8_t ub1, ub2;
+
+	__builtin_memset(&bitfields, 0, sizeof(bitfields));
+	BPF_CORE_WRITE_BITFIELD(&bitfields, ub1, 1);
+	BPF_CORE_WRITE_BITFIELD(&bitfields, ub2, 3);
+
+	ub1 = BPF_CORE_READ_BITFIELD(&bitfields, ub1);
+	ub2 = BPF_CORE_READ_BITFIELD(&bitfields, ub2);
+
+	return (ub2 << 1) | ub1;
+}
+
+SEC("tc")
+__description("multibyte CO-RE bitfield roundtrip")
+__btf_path("btf__core_reloc_bitfields.bpf.o")
+__success
+__retval(0x21)
+int multibyte_field_roundtrip(struct __sk_buff *ctx)
+{
+	struct core_reloc_bitfields bitfields;
+	uint32_t ub7;
+	uint8_t ub1;
+
+	__builtin_memset(&bitfields, 0, sizeof(bitfields));
+	BPF_CORE_WRITE_BITFIELD(&bitfields, ub1, 1);
+	BPF_CORE_WRITE_BITFIELD(&bitfields, ub7, 16);
+
+	ub1 = BPF_CORE_READ_BITFIELD(&bitfields, ub1);
+	ub7 = BPF_CORE_READ_BITFIELD(&bitfields, ub7);
+
+	return (ub7 << 1) | ub1;
+}
+
+char _license[] SEC("license") = "GPL";