bpf/tests: Add tail call limit test with external function call

This patch adds a tail call limit test where the program also emits
a BPF_CALL to an external function prior to the tail call. Mainly
testing that JITed programs preserve its internal register state, for
example tail call count, across such external calls.

Signed-off-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210914091842.4186267-15-johan.almbladh@anyfinetworks.com

lib/test_bpf.c

@@ -12207,6 +12207,30 @@ struct tail_call_test {
 		     offset, TAIL_CALL_MARKER),	       \
 	BPF_JMP_IMM(BPF_TAIL_CALL, 0, 0, 0)
 
+/*
+ * A test function to be called from a BPF program, clobbering a lot of
+ * CPU registers in the process. A JITed BPF program calling this function
+ * must save and restore any caller-saved registers it uses for internal
+ * state, for example the current tail call count.
+ */
+BPF_CALL_1(bpf_test_func, u64, arg)
+{
+	char buf[64];
+	long a = 0;
+	long b = 1;
+	long c = 2;
+	long d = 3;
+	long e = 4;
+	long f = 5;
+	long g = 6;
+	long h = 7;
+
+	return snprintf(buf, sizeof(buf),
+			"%ld %lu %lx %ld %lu %lx %ld %lu %x",
+			a, b, c, d, e, f, g, h, (int)arg);
+}
+#define BPF_FUNC_test_func __BPF_FUNC_MAX_ID
+
 /*
  * Tail call tests. Each test case may call any other test in the table,
  * including itself, specified as a relative index offset from the calling
@@ -12266,6 +12290,28 @@ static struct tail_call_test tail_call_tests[] = {
 		.flags = FLAG_NEED_STATE | FLAG_RESULT_IN_STATE,
 		.result = (MAX_TAIL_CALL_CNT + 1 + 1) * MAX_TESTRUNS,
 	},
+	{
+		"Tail call count preserved across function calls",
+		.insns = {
+			BPF_LDX_MEM(BPF_W, R2, R1, 0),
+			BPF_ALU64_IMM(BPF_ADD, R2, 1),
+			BPF_STX_MEM(BPF_W, R1, R2, 0),
+			BPF_STX_MEM(BPF_DW, R10, R1, -8),
+			BPF_CALL_REL(BPF_FUNC_get_numa_node_id),
+			BPF_CALL_REL(BPF_FUNC_ktime_get_ns),
+			BPF_CALL_REL(BPF_FUNC_ktime_get_boot_ns),
+			BPF_CALL_REL(BPF_FUNC_ktime_get_coarse_ns),
+			BPF_CALL_REL(BPF_FUNC_jiffies64),
+			BPF_CALL_REL(BPF_FUNC_test_func),
+			BPF_LDX_MEM(BPF_DW, R1, R10, -8),
+			BPF_ALU32_REG(BPF_MOV, R0, R1),
+			TAIL_CALL(0),
+			BPF_EXIT_INSN(),
+		},
+		.stack_depth = 8,
+		.flags = FLAG_NEED_STATE | FLAG_RESULT_IN_STATE,
+		.result = (MAX_TAIL_CALL_CNT + 1 + 1) * MAX_TESTRUNS,
+	},
 	{
 		"Tail call error path, NULL target",
 		.insns = {
@@ -12344,24 +12390,58 @@ static __init int prepare_tail_call_tests(struct bpf_array **pprogs)
 		/* Relocate runtime tail call offsets and addresses */
 		for (i = 0; i < len; i++) {
 			struct bpf_insn *insn = &fp->insnsi[i];
-
-			if (insn->imm != TAIL_CALL_MARKER)
-				continue;
+			long addr = 0;
 
 			switch (insn->code) {
 			case BPF_LD | BPF_DW | BPF_IMM:
+				if (insn->imm != TAIL_CALL_MARKER)
+					break;
 				insn[0].imm = (u32)(long)progs;
 				insn[1].imm = ((u64)(long)progs) >> 32;
 				break;
 
 			case BPF_ALU | BPF_MOV | BPF_K:
+				if (insn->imm != TAIL_CALL_MARKER)
+					break;
 				if (insn->off == TAIL_CALL_NULL)
 					insn->imm = ntests;
 				else if (insn->off == TAIL_CALL_INVALID)
 					insn->imm = ntests + 1;
 				else
 					insn->imm = which + insn->off;
 				insn->off = 0;
+				break;
+
+			case BPF_JMP | BPF_CALL:
+				if (insn->src_reg != BPF_PSEUDO_CALL)
+					break;
+				switch (insn->imm) {
+				case BPF_FUNC_get_numa_node_id:
+					addr = (long)&numa_node_id;
+					break;
+				case BPF_FUNC_ktime_get_ns:
+					addr = (long)&ktime_get_ns;
+					break;
+				case BPF_FUNC_ktime_get_boot_ns:
+					addr = (long)&ktime_get_boot_fast_ns;
+					break;
+				case BPF_FUNC_ktime_get_coarse_ns:
+					addr = (long)&ktime_get_coarse_ns;
+					break;
+				case BPF_FUNC_jiffies64:
+					addr = (long)&get_jiffies_64;
+					break;
+				case BPF_FUNC_test_func:
+					addr = (long)&bpf_test_func;
+					break;
+				default:
+					err = -EFAULT;
+					goto out_err;
+				}
+				*insn = BPF_EMIT_CALL(BPF_CAST_CALL(addr));
+				if ((long)__bpf_call_base + insn->imm != addr)
+					*insn = BPF_JMP_A(0); /* Skip: NOP */
+				break;
 			}
 		}