selftests/bpf: Test variable offset stack access

Test different scenarios of indirect variable-offset stack access: out of
bound access (>0), min_off below initialized part of the stack,
max_off+size above initialized part of the stack, initialized stack.

Example of output:
  ...
  #856/p indirect variable-offset stack access, out of bound OK
  #857/p indirect variable-offset stack access, max_off+size > max_initialized OK
  #858/p indirect variable-offset stack access, min_off < min_initialized OK
  #859/p indirect variable-offset stack access, ok OK
  ...
Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -40,7 +40,7 @@
 	.prog_type = BPF_PROG_TYPE_LWT_IN,
 },
 {
-	"indirect variable-offset stack access",
+	"indirect variable-offset stack access, out of bound",
 	.insns = {
 	/* Fill the top 8 bytes of the stack */
 	BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
@@ -60,7 +60,82 @@
 	BPF_EXIT_INSN(),
 	},
 	.fixup_map_hash_8b = { 5 },
-	.errstr = "variable stack read R2",
+	.errstr = "invalid stack type R2 var_off",
 	.result = REJECT,
 	.prog_type = BPF_PROG_TYPE_LWT_IN,
 },
+{
+	"indirect variable-offset stack access, max_off+size > max_initialized",
+	.insns = {
+	/* Fill only the second from top 8 bytes of the stack. */
+	BPF_ST_MEM(BPF_DW, BPF_REG_10, -16, 0),
+	/* Get an unknown value. */
+	BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1, 0),
+	/* Make it small and 4-byte aligned. */
+	BPF_ALU64_IMM(BPF_AND, BPF_REG_2, 4),
+	BPF_ALU64_IMM(BPF_SUB, BPF_REG_2, 16),
+	/* Add it to fp.  We now have either fp-12 or fp-16, but we don't know
+	 * which. fp-12 size 8 is partially uninitialized stack.
+	 */
+	BPF_ALU64_REG(BPF_ADD, BPF_REG_2, BPF_REG_10),
+	/* Dereference it indirectly. */
+	BPF_LD_MAP_FD(BPF_REG_1, 0),
+	BPF_EMIT_CALL(BPF_FUNC_map_lookup_elem),
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_EXIT_INSN(),
+	},
+	.fixup_map_hash_8b = { 5 },
+	.errstr = "invalid indirect read from stack var_off",
+	.result = REJECT,
+	.prog_type = BPF_PROG_TYPE_LWT_IN,
+},
+{
+	"indirect variable-offset stack access, min_off < min_initialized",
+	.insns = {
+	/* Fill only the top 8 bytes of the stack. */
+	BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
+	/* Get an unknown value */
+	BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1, 0),
+	/* Make it small and 4-byte aligned. */
+	BPF_ALU64_IMM(BPF_AND, BPF_REG_2, 4),
+	BPF_ALU64_IMM(BPF_SUB, BPF_REG_2, 16),
+	/* Add it to fp.  We now have either fp-12 or fp-16, but we don't know
+	 * which. fp-16 size 8 is partially uninitialized stack.
+	 */
+	BPF_ALU64_REG(BPF_ADD, BPF_REG_2, BPF_REG_10),
+	/* Dereference it indirectly. */
+	BPF_LD_MAP_FD(BPF_REG_1, 0),
+	BPF_EMIT_CALL(BPF_FUNC_map_lookup_elem),
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_EXIT_INSN(),
+	},
+	.fixup_map_hash_8b = { 5 },
+	.errstr = "invalid indirect read from stack var_off",
+	.result = REJECT,
+	.prog_type = BPF_PROG_TYPE_LWT_IN,
+},
+{
+	"indirect variable-offset stack access, ok",
+	.insns = {
+	/* Fill the top 16 bytes of the stack. */
+	BPF_ST_MEM(BPF_DW, BPF_REG_10, -16, 0),
+	BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
+	/* Get an unknown value. */
+	BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1, 0),
+	/* Make it small and 4-byte aligned. */
+	BPF_ALU64_IMM(BPF_AND, BPF_REG_2, 4),
+	BPF_ALU64_IMM(BPF_SUB, BPF_REG_2, 16),
+	/* Add it to fp.  We now have either fp-12 or fp-16, we don't know
+	 * which, but either way it points to initialized stack.
+	 */
+	BPF_ALU64_REG(BPF_ADD, BPF_REG_2, BPF_REG_10),
+	/* Dereference it indirectly. */
+	BPF_LD_MAP_FD(BPF_REG_1, 0),
+	BPF_EMIT_CALL(BPF_FUNC_map_lookup_elem),
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_EXIT_INSN(),
+	},
+	.fixup_map_hash_8b = { 6 },
+	.result = ACCEPT,
+	.prog_type = BPF_PROG_TYPE_LWT_IN,
+},