bpf, selftests: Add test for BPF_STX BPF_B storing R10

This patch adds a test to test_verifier that writes the lower 8 bits of
R10 (aka FP) using BPF_B to an array map and reads the result back. The
expected behavior is that the result should be the same as first copying
R10 to R9, and then storing / loading the lower 8 bits of R9.

This test catches a bug that was present in the x86-64 JIT that caused
an incorrect encoding for BPF_STX BPF_B when the source operand is R10.
Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Luke Nelson <luke.r.nels@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200418232655.23870-2-luke.r.nels@gmail.com

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

@@ -315,3 +315,43 @@
 	},
 	.result = ACCEPT,
 },
+{
+	"store PTR_TO_STACK in R10 to array map using BPF_B",
+	.insns = {
+	/* Load pointer to map. */
+	BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
+	BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
+	BPF_ST_MEM(BPF_DW, BPF_REG_2, 0, 0),
+	BPF_LD_MAP_FD(BPF_REG_1, 0),
+	BPF_EMIT_CALL(BPF_FUNC_map_lookup_elem),
+	BPF_JMP_IMM(BPF_JNE, BPF_REG_0, 0, 2),
+	BPF_MOV64_IMM(BPF_REG_0, 2),
+	BPF_EXIT_INSN(),
+	BPF_MOV64_REG(BPF_REG_1, BPF_REG_0),
+	/* Copy R10 to R9. */
+	BPF_MOV64_REG(BPF_REG_9, BPF_REG_10),
+	/* Pollute other registers with unaligned values. */
+	BPF_MOV64_IMM(BPF_REG_2, -1),
+	BPF_MOV64_IMM(BPF_REG_3, -1),
+	BPF_MOV64_IMM(BPF_REG_4, -1),
+	BPF_MOV64_IMM(BPF_REG_5, -1),
+	BPF_MOV64_IMM(BPF_REG_6, -1),
+	BPF_MOV64_IMM(BPF_REG_7, -1),
+	BPF_MOV64_IMM(BPF_REG_8, -1),
+	/* Store both R9 and R10 with BPF_B and read back. */
+	BPF_STX_MEM(BPF_B, BPF_REG_1, BPF_REG_10, 0),
+	BPF_LDX_MEM(BPF_B, BPF_REG_2, BPF_REG_1, 0),
+	BPF_STX_MEM(BPF_B, BPF_REG_1, BPF_REG_9, 0),
+	BPF_LDX_MEM(BPF_B, BPF_REG_3, BPF_REG_1, 0),
+	/* Should read back as same value. */
+	BPF_JMP_REG(BPF_JEQ, BPF_REG_2, BPF_REG_3, 2),
+	BPF_MOV64_IMM(BPF_REG_0, 1),
+	BPF_EXIT_INSN(),
+	BPF_MOV64_IMM(BPF_REG_0, 42),
+	BPF_EXIT_INSN(),
+	},
+	.fixup_map_array_48b = { 3 },
+	.result = ACCEPT,
+	.retval = 42,
+	.prog_type = BPF_PROG_TYPE_SCHED_CLS,
+},