bpf: Add verifier support for timed may_goto

Implement support in the verifier for replacing may_goto implementation
from a counter-based approach to one which samples time on the local CPU
to have a bigger loop bound.

We implement it by maintaining 16-bytes per-stack frame, and using 8
bytes for maintaining the count for amortizing time sampling, and 8
bytes for the starting timestamp. To minimize overhead, we need to avoid
spilling and filling of registers around this sequence, so we push this
cost into the time sampling function 'arch_bpf_timed_may_goto'. This is
a JIT-specific wrapper around bpf_check_timed_may_goto which returns us
the count to store into the stack through BPF_REG_AX. All caller-saved
registers (r0-r5) are guaranteed to remain untouched.

The loop can be broken by returning count as 0, otherwise we dispatch
into the function when the count drops to 0, and the runtime chooses to
refresh it (by returning count as BPF_MAX_TIMED_LOOPS) or returning 0
and aborting the loop on next iteration.

Since the check for 0 is done right after loading the count from the
stack, all subsequent cond_break sequences should immediately break as
well, of the same loop or subsequent loops in the program.

We pass in the stack_depth of the count (and thus the timestamp, by
adding 8 to it) to the arch_bpf_timed_may_goto call so that it can be
passed in to bpf_check_timed_may_goto as an argument after r1 is saved,
by adding the offset to r10/fp. This adjustment will be arch specific,
and the next patch will introduce support for x86.

Note that depending on loop complexity, time spent in the loop can be
more than the current limit (250 ms), but imposing an upper bound on
program runtime is an orthogonal problem which will be addressed when
program cancellations are supported.

The current time afforded by cond_break may not be enough for cases
where BPF programs want to implement locking algorithms inline, and use
cond_break as a promise to the verifier that they will eventually
terminate.

Below are some benchmarking numbers on the time taken per-iteration for
an empty loop that counts the number of iterations until cond_break
fires. For comparison, we compare it against bpf_for/bpf_repeat which is
another way to achieve the same number of spins (BPF_MAX_LOOPS). The
hardware used for benchmarking was a Sapphire Rapids Intel server with
performance governor enabled, mitigations were enabled.

+-----------------------------+--------------+--------------+------------------+
| Loop type | Iterations | Time (ms) | Time/iter (ns) |
+-----------------------------|--------------+--------------+------------------+
| may_goto | 8388608 | 3 | 0.36 |
| timed_may_goto (count=65535)| 589674932 | 250 | 0.42 |
| bpf_for | 8388608 | 10 | 1.19 |
+-----------------------------+--------------+--------------+------------------+

This gives a good approximation at low overhead while staying close to
the current implementation.

Signed-off-by: Kumar Kartikeya Dwivedi <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

include/linux/bpf.h

···
  */
 enum {
 	BPF_MAX_LOOPS = 8 * 1024 * 1024,
+	BPF_MAX_TIMED_LOOPS = 0xffff,
 };
 
 #define BPF_F_ACCESS_MASK	(BPF_F_RDONLY |		\

include/linux/filter.h

···
 	struct u64_stats_sync syncp;
 } __aligned(2 * sizeof(u64));
 
+struct bpf_timed_may_goto {
+	u64 count;
+	u64 timestamp;
+};
+
 struct sk_filter {
 	refcount_t	refcnt;
 	struct rcu_head	rcu;
···
 bool bpf_jit_supports_arena(void);
 bool bpf_jit_supports_insn(struct bpf_insn *insn, bool in_arena);
 bool bpf_jit_supports_private_stack(void);
+bool bpf_jit_supports_timed_may_goto(void);
 u64 bpf_arch_uaddress_limit(void);
 void arch_bpf_stack_walk(bool (*consume_fn)(void *cookie, u64 ip, u64 sp, u64 bp), void *cookie);
+u64 arch_bpf_timed_may_goto(void);
+u64 bpf_check_timed_may_goto(struct bpf_timed_may_goto *);
 bool bpf_helper_changes_pkt_data(enum bpf_func_id func_id);
 
 static inline bool bpf_dump_raw_ok(const struct cred *cred)

kernel/bpf/core.c

···
 {
 }
 
+bool __weak bpf_jit_supports_timed_may_goto(void)
+{
+	return false;
+}
+
+u64 __weak arch_bpf_timed_may_goto(void)
+{
+	return 0;
+}
+
+u64 bpf_check_timed_may_goto(struct bpf_timed_may_goto *p)
+{
+	u64 time = ktime_get_mono_fast_ns();
+
+	/* Populate the timestamp for this stack frame, and refresh count. */
+	if (!p->timestamp) {
+		p->timestamp = time;
+		return BPF_MAX_TIMED_LOOPS;
+	}
+	/* Check if we've exhausted our time slice, and zero count. */
+	if (time - p->timestamp >= (NSEC_PER_SEC / 4))
+		return 0;
+	/* Refresh the count for the stack frame. */
+	return BPF_MAX_TIMED_LOOPS;
+}
+
 /* for configs without MMU or 32-bit */
 __weak const struct bpf_map_ops arena_map_ops;
 __weak u64 bpf_arena_get_user_vm_start(struct bpf_arena *arena)

kernel/bpf/verifier.c

···
 			goto next_insn;
 		}
 
-		if (is_may_goto_insn(insn)) {
+		if (is_may_goto_insn(insn) && bpf_jit_supports_timed_may_goto()) {
+			int stack_off_cnt = -stack_depth - 16;
+
+			/*
+			 * Two 8 byte slots, depth-16 stores the count, and
+			 * depth-8 stores the start timestamp of the loop.
+			 *
+			 * The starting value of count is BPF_MAX_TIMED_LOOPS
+			 * (0xffff).  Every iteration loads it and subs it by 1,
+			 * until the value becomes 0 in AX (thus, 1 in stack),
+			 * after which we call arch_bpf_timed_may_goto, which
+			 * either sets AX to 0xffff to keep looping, or to 0
+			 * upon timeout. AX is then stored into the stack. In
+			 * the next iteration, we either see 0 and break out, or
+			 * continue iterating until the next time value is 0
+			 * after subtraction, rinse and repeat.
+			 */
+			stack_depth_extra = 16;
+			insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_AX, BPF_REG_10, stack_off_cnt);
+			if (insn->off >= 0)
+				insn_buf[1] = BPF_JMP_IMM(BPF_JEQ, BPF_REG_AX, 0, insn->off + 5);
+			else
+				insn_buf[1] = BPF_JMP_IMM(BPF_JEQ, BPF_REG_AX, 0, insn->off - 1);
+			insn_buf[2] = BPF_ALU64_IMM(BPF_SUB, BPF_REG_AX, 1);
+			insn_buf[3] = BPF_JMP_IMM(BPF_JNE, BPF_REG_AX, 0, 2);
+			/*
+			 * AX is used as an argument to pass in stack_off_cnt
+			 * (to add to r10/fp), and also as the return value of
+			 * the call to arch_bpf_timed_may_goto.
+			 */
+			insn_buf[4] = BPF_MOV64_IMM(BPF_REG_AX, stack_off_cnt);
+			insn_buf[5] = BPF_EMIT_CALL(arch_bpf_timed_may_goto);
+			insn_buf[6] = BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_AX, stack_off_cnt);
+			cnt = 7;
+
+			new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
+			if (!new_prog)
+				return -ENOMEM;
+
+			delta += cnt - 1;
+			env->prog = prog = new_prog;
+			insn = new_prog->insnsi + i + delta;
+			goto next_insn;
+		} else if (is_may_goto_insn(insn)) {
 			int stack_off = -stack_depth - 8;
 
 			stack_depth_extra = 8;
···
 
 	env->prog->aux->stack_depth = subprogs[0].stack_depth;
 	for (i = 0; i < env->subprog_cnt; i++) {
+		int delta = bpf_jit_supports_timed_may_goto() ? 2 : 1;
 		int subprog_start = subprogs[i].start;
 		int stack_slots = subprogs[i].stack_extra / 8;
+		int slots = delta, cnt = 0;
 
 		if (!stack_slots)
 			continue;
-		if (stack_slots > 1) {
+		/* We need two slots in case timed may_goto is supported. */
+		if (stack_slots > slots) {
 			verbose(env, "verifier bug: stack_slots supports may_goto only\n");
 			return -EFAULT;
 		}
 
-		/* Add ST insn to subprog prologue to init extra stack */
-		insn_buf[0] = BPF_ST_MEM(BPF_DW, BPF_REG_FP,
-					 -subprogs[i].stack_depth, BPF_MAX_LOOPS);
+		stack_depth = subprogs[i].stack_depth;
+		if (bpf_jit_supports_timed_may_goto()) {
+			insn_buf[cnt++] = BPF_ST_MEM(BPF_DW, BPF_REG_FP, -stack_depth,
+						     BPF_MAX_TIMED_LOOPS);
+			insn_buf[cnt++] = BPF_ST_MEM(BPF_DW, BPF_REG_FP, -stack_depth + 8, 0);
+		} else {
+			/* Add ST insn to subprog prologue to init extra stack */
+			insn_buf[cnt++] = BPF_ST_MEM(BPF_DW, BPF_REG_FP, -stack_depth,
+						     BPF_MAX_LOOPS);
+		}
 		/* Copy first actual insn to preserve it */
-		insn_buf[1] = env->prog->insnsi[subprog_start];
+		insn_buf[cnt++] = env->prog->insnsi[subprog_start];
 
-		new_prog = bpf_patch_insn_data(env, subprog_start, insn_buf, 2);
+		new_prog = bpf_patch_insn_data(env, subprog_start, insn_buf, cnt);
 		if (!new_prog)
 			return -ENOMEM;
 		env->prog = prog = new_prog;
···
 		 * to insn after BPF_ST that inits may_goto count.
 		 * Adjustment will succeed because bpf_patch_insn_data() didn't fail.
 		 */
-		WARN_ON(adjust_jmp_off(env->prog, subprog_start, 1));
+		WARN_ON(adjust_jmp_off(env->prog, subprog_start, delta));
 	}
 
 	/* Since poke tab is now finalized, publish aux to tracker. */