[PATCH bpf-next 4/4] selftests/bpf: validate jit behaviour for tail calls

Eduard Zingerman
2024-08-09T01:05:54Z

A program calling sub-program which does a tail call.
The idea is to verify instructions generated by jit for tail calls:
- in program and sub-program prologues;
- for subprogram call instruction;
- for tail call itself.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
---
.../selftests/bpf/prog_tests/verifier.c | 2 +
.../bpf/progs/verifier_tailcall_jit.c | 103 ++++++++++++++++++
2 files changed, 105 insertions(+)
create mode 100644 tools/testing/selftests/bpf/progs/verifier_tailcall_jit.c

diff --git a/tools/testing/selftests/bpf/prog_tests/verifier.c b/tools/testing/selftests/bpf/prog_tests/verifier.c
index f8f546eba488..cf3662dbd24f 100644
--- a/tools/testing/selftests/bpf/prog_tests/verifier.c
+++ b/tools/testing/selftests/bpf/prog_tests/verifier.c
@@ -75,6 +75,7 @@
#include "verifier_stack_ptr.skel.h"
#include "verifier_subprog_precision.skel.h"
#include "verifier_subreg.skel.h"
+#include "verifier_tailcall_jit.skel.h"
#include "verifier_typedef.skel.h"
#include "verifier_uninit.skel.h"
#include "verifier_unpriv.skel.h"
@@ -198,6 +199,7 @@ void test_verifier_spin_lock(void) { RUN(verifier_spin_lock); }
void test_verifier_stack_ptr(void) { RUN(verifier_stack_ptr); }
void test_verifier_subprog_precision(void) { RUN(verifier_subprog_precision); }
void test_verifier_subreg(void) { RUN(verifier_subreg); }
+void test_verifier_tailcall_jit(void) { RUN(verifier_tailcall_jit); }
void test_verifier_typedef(void) { RUN(verifier_typedef); }
void test_verifier_uninit(void) { RUN(verifier_uninit); }
void test_verifier_unpriv(void) { RUN(verifier_unpriv); }
diff --git a/tools/testing/selftests/bpf/progs/verifier_tailcall_jit.c b/tools/testing/selftests/bpf/progs/verifier_tailcall_jit.c
new file mode 100644
index 000000000000..1a09c76d7be0
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/verifier_tailcall_jit.c
@@ -0,0 +1,103 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+#include "bpf_misc.h"
+
+int main(void);
+
+struct {
+ __uint(type, BPF_MAP_TYPE_PROG_ARRAY);
+ __uint(max_entries, 1);
+ __uint(key_size, sizeof(__u32));
+ __array(values, void (void));
+} jmp_table SEC(".maps") = {
+ .values = {
+ [0] = (void *) &main,
+ },
+};
+
+__noinline __auxiliary
+static __naked int sub(void)
+{
+ asm volatile (
+ "r2 = %[jmp_table] ll;"
+ "r3 = 0;"
+ "call 12;"
+ "exit;"
+ :
+ : __imm_addr(jmp_table)
+ : __clobber_all);
+}
+
+__success
+/* program entry for main(), regular function prologue */
+__jit_x86(" endbr64")
+__jit_x86(" nopl (%rax,%rax)")
+__jit_x86(" xorq %rax, %rax")
+__jit_x86(" pushq %rbp")
+__jit_x86(" movq %rsp, %rbp")
+/* tail call prologue for program:
+ * - establish memory location for tail call counter at &rbp[-8];
+ * - spill tail_call_cnt_ptr at &rbp[-16];
+ * - expect tail call counter to be passed in rax;
+ * - for entry program rax is a raw counter, value < 33;
+ * - for tail called program rax is tail_call_cnt_ptr (value > 33).
+ */
+__jit_x86(" endbr64")
+__jit_x86(" cmpq $0x21, %rax")
+__jit_x86(" ja L0")
+__jit_x86(" pushq %rax")
+__jit_x86(" movq %rsp, %rax")
+__jit_x86(" jmp L1")
+__jit_x86("L0: pushq %rax") /* rbp[-8] = rax */
+__jit_x86("L1: pushq %rax") /* rbp[-16] = rax */
+/* on subprogram call restore rax to be tail_call_cnt_ptr from rbp[-16]
+ * (cause original rax might be clobbered by this point)
+ */
+__jit_x86(" movq -0x10(%rbp), %rax")
+__jit_x86(" callq 0x[0-9a-f]\\+") /* call to sub() */
+__jit_x86(" xorl %eax, %eax")
+__jit_x86(" leave")
+__jit_x86(" retq")
+/* subprogram entry for sub(), regular function prologue */
+__jit_x86(" endbr64")
+__jit_x86(" nopl (%rax,%rax)")
+__jit_x86(" nopl (%rax)")
+__jit_x86(" pushq %rbp")
+__jit_x86(" movq %rsp, %rbp")
+/* tail call prologue for subprogram address of tail call counter
+ * stored at rbp[-16].
+ */
+__jit_x86(" endbr64")
+__jit_x86(" pushq %rax") /* rbp[-8] = rax */
+__jit_x86(" pushq %rax") /* rbp[-16] = rax */
+__jit_x86(" movabsq $-0x[0-9a-f]\\+, %rsi") /* r2 = &jmp_table */
+__jit_x86(" xorl %edx, %edx") /* r3 = 0 */
+/* bpf_tail_call implementation:
+ * - load tail_call_cnt_ptr from rbp[-16];
+ * - if *tail_call_cnt_ptr < 33, increment it and jump to target;
+ * - otherwise do nothing.
+ */
+__jit_x86(" movq -0x10(%rbp), %rax")
+__jit_x86(" cmpq $0x21, (%rax)")
+__jit_x86(" jae L0")
+__jit_x86(" nopl (%rax,%rax)")
+__jit_x86(" addq $0x1, (%rax)") /* *tail_call_cnt_ptr += 1 */
+__jit_x86(" popq %rax")
+__jit_x86(" popq %rax")
+__jit_x86(" jmp 0x[0-9a-f]\\+") /* jump to tail call tgt */
+__jit_x86("L0: leave")
+__jit_x86(" retq")
+SEC("tc")
+__naked int main(void)
+{
+ asm volatile (
+ "call %[sub];"
+ "r0 = 0;"
+ "exit;"
+ :
+ : __imm(sub)
+ : __clobber_all);
+}
+
+char __license[] SEC("license") = "GPL";
--
2.45.2