selftests/bpf: Test access from RO map from xdp_store_bytes

This new test simply checks that helper bpf_xdp_store_bytes can
successfully read from a read-only map.

Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>
Link: https://lore.kernel.org/r/4fdb934a713b2d7cf133288c77f6cfefe9856440.1769875479.git.paul.chaignon@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: pchaigno

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

@@ -5,6 +5,14 @@
 #include <bpf/bpf_helpers.h>
 #include "bpf_misc.h"
 
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 1);
+	__type(key, int);
+	__type(value, __u64);
+	__uint(map_flags, BPF_F_RDONLY_PROG);
+} map_array_ro SEC(".maps");
+
 SEC("xdp")
 __description("XDP, using ifindex from netdev")
 __success __retval(1)
@@ -21,4 +29,31 @@ l0_%=:	exit;						\
 	: __clobber_all);
 }
 
+SEC("xdp")
+__description("XDP, using xdp_store_bytes from RO map")
+__success __retval(0)
+__naked void xdp_store_bytes_from_ro_map(void)
+{
+	asm volatile ("					\
+	r6 = r1;					\
+	r1 = 0;                                         \
+	*(u64*)(r10 - 8) = r1;                          \
+	r2 = r10;                                       \
+	r2 += -8;                                       \
+	r1 = %[map_array_ro] ll;                        \
+	call %[bpf_map_lookup_elem];                    \
+	if r0 == 0 goto l0_%=;                          \
+	r1 = r6;					\
+	r2 = 0;						\
+	r3 = r0;					\
+	r4 = 8;						\
+	call %[bpf_xdp_store_bytes];			\
+l0_%=:	exit;						\
+"	:
+	: __imm(bpf_map_lookup_elem),
+	  __imm(bpf_xdp_store_bytes),
+	  __imm_addr(map_array_ro)
+	: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";