Commit 011832b9 authored by Alexei Starovoitov's avatar Alexei Starovoitov Committed by Andrii Nakryiko
Browse files

bpf: Introduce may_goto instruction 



Introduce may_goto instruction that from the verifier pov is similar to
open coded iterators bpf_for()/bpf_repeat() and bpf_loop() helper, but it
doesn't iterate any objects.
In assembly 'may_goto' is a nop most of the time until bpf runtime has to
terminate the program for whatever reason. In the current implementation
may_goto has a hidden counter, but other mechanisms can be used.
For programs written in C the later patch introduces 'cond_break' macro
that combines 'may_goto' with 'break' statement and has similar semantics:
cond_break is a nop until bpf runtime has to break out of this loop.
It can be used in any normal "for" or "while" loop, like

  for (i = zero; i < cnt; cond_break, i++) {

The verifier recognizes that may_goto is used in the program, reserves
additional 8 bytes of stack, initializes them in subprog prologue, and
replaces may_goto instruction with:
aux_reg = *(u64 *)(fp - 40)
if aux_reg == 0 goto pc+off
aux_reg -= 1
*(u64 *)(fp - 40) = aux_reg

may_goto instruction can be used by LLVM to implement __builtin_memcpy,
__builtin_strcmp.

may_goto is not a full substitute for bpf_for() macro.
bpf_for() doesn't have induction variable that verifiers sees,
so 'i' in bpf_for(i, 0, 100) is seen as imprecise and bounded.

But when the code is written as:
for (i = 0; i < 100; cond_break, i++)
the verifier see 'i' as precise constant zero,
hence cond_break (aka may_goto) doesn't help to converge the loop.
A static or global variable can be used as a workaround:
static int zero = 0;
for (i = zero; i < 100; cond_break, i++) // works!

may_goto works well with arena pointers that don't need to be bounds
checked on access. Load/store from arena returns imprecise unbounded
scalar and loops with may_goto pass the verifier.

Reserve new opcode BPF_JMP | BPF_JCOND for may_goto insn.
JCOND stands for conditional pseudo jump.
Since goto_or_nop insn was proposed, it may use the same opcode.
may_goto vs goto_or_nop can be distinguished by src_reg:
code = BPF_JMP | BPF_JCOND
src_reg = 0 - may_goto
src_reg = 1 - goto_or_nop

Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Signed-off-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Acked-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Acked-by: default avatarEduard Zingerman <eddyz87@gmail.com>
Acked-by: default avatarJohn Fastabend <john.fastabend@gmail.com>
Tested-by: default avatarJohn Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20240306031929.42666-2-alexei.starovoitov@gmail.com
parent 9a9d1d36

include/linux/bpf_verifier.h

+2 −0
Original line number Diff line number Diff line
@@ -449,6 +449,7 @@ struct bpf_verifier_state { 
	u32 jmp_history_cnt;

	u32 dfs_depth;

	u32 callback_unroll_depth;

	u32 may_goto_depth;

};



#define bpf_get_spilled_reg(slot, frame, mask)				\
@@ -619,6 +620,7 @@ struct bpf_subprog_info { 
	u32 start; /* insn idx of function entry point */

	u32 linfo_idx; /* The idx to the main_prog->aux->linfo */

	u16 stack_depth; /* max. stack depth used by this function */

	u16 stack_extra;

	bool has_tail_call: 1;

	bool tail_call_reachable: 1;

	bool has_ld_abs: 1;

include/uapi/linux/bpf.h

+5 −0
Original line number Diff line number Diff line
@@ -42,6 +42,7 @@ 
#define BPF_JSGE	0x70	/* SGE is signed '>=', GE in x86 */

#define BPF_JSLT	0xc0	/* SLT is signed, '<' */

#define BPF_JSLE	0xd0	/* SLE is signed, '<=' */

#define BPF_JCOND	0xe0	/* conditional pseudo jumps: may_goto, goto_or_nop */

#define BPF_CALL	0x80	/* function call */

#define BPF_EXIT	0x90	/* function return */
@@ -50,6 +51,10 @@ 
#define BPF_XCHG	(0xe0 | BPF_FETCH)	/* atomic exchange */

#define BPF_CMPXCHG	(0xf0 | BPF_FETCH)	/* atomic compare-and-write */



enum bpf_cond_pseudo_jmp {

	BPF_MAY_GOTO = 0,

};



/* Register numbers */

enum {

	BPF_REG_0 = 0,

kernel/bpf/core.c

+1 −0
Original line number Diff line number Diff line
@@ -1675,6 +1675,7 @@ bool bpf_opcode_in_insntable(u8 code) 
		[BPF_LD | BPF_IND | BPF_B] = true,

		[BPF_LD | BPF_IND | BPF_H] = true,

		[BPF_LD | BPF_IND | BPF_W] = true,

		[BPF_JMP | BPF_JCOND] = true,

	};

#undef BPF_INSN_3_TBL

#undef BPF_INSN_2_TBL

kernel/bpf/disasm.c

+4 −0
Original line number Diff line number Diff line
@@ -322,6 +322,10 @@ void print_bpf_insn(const struct bpf_insn_cbs *cbs, 
		} else if (insn->code == (BPF_JMP | BPF_JA)) {

			verbose(cbs->private_data, "(%02x) goto pc%+d\n",

				insn->code, insn->off);

		} else if (insn->code == (BPF_JMP | BPF_JCOND) &&

			   insn->src_reg == BPF_MAY_GOTO) {

			verbose(cbs->private_data, "(%02x) may_goto pc%+d\n",

				insn->code, insn->off);

		} else if (insn->code == (BPF_JMP32 | BPF_JA)) {

			verbose(cbs->private_data, "(%02x) gotol pc%+d\n",

				insn->code, insn->imm);

kernel/bpf/verifier.c

+133 −30
Original line number Diff line number Diff line
@@ -533,6 +533,16 @@ static bool is_async_callback_calling_insn(struct bpf_insn *insn) 
	return bpf_helper_call(insn) && is_async_callback_calling_function(insn->imm);

}









static bool is_may_goto_insn(struct bpf_insn *insn)









{









	return insn->code == (BPF_JMP | BPF_JCOND) && insn->src_reg == BPF_MAY_GOTO;









}

















static bool is_may_goto_insn_at(struct bpf_verifier_env *env, int insn_idx)









{









	return is_may_goto_insn(&env->prog->insnsi[insn_idx]);









}

















static bool is_storage_get_function(enum bpf_func_id func_id)









{









	return func_id == BPF_FUNC_sk_storage_get ||








@@ -1429,6 +1439,7 @@ static int copy_verifier_state(struct bpf_verifier_state *dst_state,







	dst_state->dfs_depth = src->dfs_depth;









	dst_state->callback_unroll_depth = src->callback_unroll_depth;









	dst_state->used_as_loop_entry = src->used_as_loop_entry;









	dst_state->may_goto_depth = src->may_goto_depth;









	for (i = 0; i <= src->curframe; i++) {









		dst = dst_state->frame[i];









		if (!dst) {








@@ -14871,11 +14882,36 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,







	int err;

















	/* Only conditional jumps are expected to reach here. */









	if (opcode == BPF_JA || opcode > BPF_JSLE) {









	if (opcode == BPF_JA || opcode > BPF_JCOND) {









		verbose(env, "invalid BPF_JMP/JMP32 opcode %x\n", opcode);









		return -EINVAL;









	}

















	if (opcode == BPF_JCOND) {









		struct bpf_verifier_state *cur_st = env->cur_state, *queued_st, *prev_st;









		int idx = *insn_idx;

















		if (insn->code != (BPF_JMP | BPF_JCOND) ||









		    insn->src_reg != BPF_MAY_GOTO ||









		    insn->dst_reg || insn->imm || insn->off == 0) {









			verbose(env, "invalid may_goto off %d imm %d\n",









				insn->off, insn->imm);









			return -EINVAL;









		}









		prev_st = find_prev_entry(env, cur_st->parent, idx);

















		/* branch out 'fallthrough' insn as a new state to explore */









		queued_st = push_stack(env, idx + 1, idx, false);









		if (!queued_st)









			return -ENOMEM;

















		queued_st->may_goto_depth++;









		if (prev_st)









			widen_imprecise_scalars(env, prev_st, queued_st);









		*insn_idx += insn->off;









		return 0;









	}

















	/* check src2 operand */









	err = check_reg_arg(env, insn->dst_reg, SRC_OP);









	if (err)








@@ -15659,6 +15695,8 @@ static int visit_insn(int t, struct bpf_verifier_env *env)







	default:









		/* conditional jump with two edges */









		mark_prune_point(env, t);









		if (is_may_goto_insn(insn))









			mark_force_checkpoint(env, t);

















		ret = push_insn(t, t + 1, FALLTHROUGH, env);









		if (ret)








@@ -17135,6 +17173,13 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)







				}









				goto skip_inf_loop_check;









			}









			if (is_may_goto_insn_at(env, insn_idx)) {









				if (states_equal(env, &sl->state, cur, true)) {









					update_loop_entry(cur, &sl->state);









					goto hit;









				}









				goto skip_inf_loop_check;









			}









			if (calls_callback(env, insn_idx)) {









				if (states_equal(env, &sl->state, cur, true))









					goto hit;








@@ -17144,6 +17189,7 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)







			if (states_maybe_looping(&sl->state, cur) &&









			    states_equal(env, &sl->state, cur, true) &&









			    !iter_active_depths_differ(&sl->state, cur) &&









			    sl->state.may_goto_depth == cur->may_goto_depth &&









			    sl->state.callback_unroll_depth == cur->callback_unroll_depth) {









				verbose_linfo(env, insn_idx, "; ");









				verbose(env, "infinite loop detected at insn %d\n", insn_idx);








@@ -19408,7 +19454,10 @@ static int do_misc_fixups(struct bpf_verifier_env *env)







	struct bpf_insn insn_buf[16];









	struct bpf_prog *new_prog;









	struct bpf_map *map_ptr;









	int i, ret, cnt, delta = 0;









	int i, ret, cnt, delta = 0, cur_subprog = 0;









	struct bpf_subprog_info *subprogs = env->subprog_info;









	u16 stack_depth = subprogs[cur_subprog].stack_depth;









	u16 stack_depth_extra = 0;

















	if (env->seen_exception && !env->exception_callback_subprog) {









		struct bpf_insn patch[] = {








@@ -19428,7 +19477,7 @@ static int do_misc_fixups(struct bpf_verifier_env *env)







		mark_subprog_exc_cb(env, env->exception_callback_subprog);









	}

















	for (i = 0; i < insn_cnt; i++, insn++) {









	for (i = 0; i < insn_cnt;) {









		/* Make divide-by-zero exceptions impossible. */









		if (insn->code == (BPF_ALU64 | BPF_MOD | BPF_X) ||









		    insn->code == (BPF_ALU64 | BPF_DIV | BPF_X) ||








@@ -19467,7 +19516,7 @@ static int do_misc_fixups(struct bpf_verifier_env *env)







			delta    += cnt - 1;









			env->prog = prog = new_prog;









			insn      = new_prog->insnsi + i + delta;









			continue;









			goto next_insn;









		}

















		/* Implement LD_ABS and LD_IND with a rewrite, if supported by the program type. */








@@ -19487,7 +19536,7 @@ static int do_misc_fixups(struct bpf_verifier_env *env)







			delta    += cnt - 1;









			env->prog = prog = new_prog;









			insn      = new_prog->insnsi + i + delta;









			continue;









			goto next_insn;









		}

















		/* Rewrite pointer arithmetic to mitigate speculation attacks. */








@@ -19502,7 +19551,7 @@ static int do_misc_fixups(struct bpf_verifier_env *env)







			aux = &env->insn_aux_data[i + delta];









			if (!aux->alu_state ||









			    aux->alu_state == BPF_ALU_NON_POINTER)









				continue;









				goto next_insn;

















			isneg = aux->alu_state & BPF_ALU_NEG_VALUE;









			issrc = (aux->alu_state & BPF_ALU_SANITIZE) ==








@@ -19540,19 +19589,39 @@ static int do_misc_fixups(struct bpf_verifier_env *env)







			delta    += cnt - 1;









			env->prog = prog = new_prog;









			insn      = new_prog->insnsi + i + delta;









			continue;









			goto next_insn;









		}

















		if (is_may_goto_insn(insn)) {









			int stack_off = -stack_depth - 8;

















			stack_depth_extra = 8;









			insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_AX, BPF_REG_10, stack_off);









			insn_buf[1] = BPF_JMP_IMM(BPF_JEQ, BPF_REG_AX, 0, insn->off + 2);









			insn_buf[2] = BPF_ALU64_IMM(BPF_SUB, BPF_REG_AX, 1);









			insn_buf[3] = BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_AX, stack_off);









			cnt = 4;

















			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;









		}

















		if (insn->code != (BPF_JMP | BPF_CALL))









			continue;









			goto next_insn;









		if (insn->src_reg == BPF_PSEUDO_CALL)









			continue;









			goto next_insn;









		if (insn->src_reg == BPF_PSEUDO_KFUNC_CALL) {









			ret = fixup_kfunc_call(env, insn, insn_buf, i + delta, &cnt);









			if (ret)









				return ret;









			if (cnt == 0)









				continue;









				goto next_insn;

















			new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);









			if (!new_prog)








@@ -19561,7 +19630,7 @@ static int do_misc_fixups(struct bpf_verifier_env *env)







			delta	 += cnt - 1;









			env->prog = prog = new_prog;









			insn	  = new_prog->insnsi + i + delta;









			continue;









			goto next_insn;









		}

















		if (insn->imm == BPF_FUNC_get_route_realm)








@@ -19609,11 +19678,11 @@ static int do_misc_fixups(struct bpf_verifier_env *env)







				}

















				insn->imm = ret + 1;









				continue;









				goto next_insn;









			}

















			if (!bpf_map_ptr_unpriv(aux))









				continue;









				goto next_insn;

















			/* instead of changing every JIT dealing with tail_call









			 * emit two extra insns:








@@ -19642,7 +19711,7 @@ static int do_misc_fixups(struct bpf_verifier_env *env)







			delta    += cnt - 1;









			env->prog = prog = new_prog;









			insn      = new_prog->insnsi + i + delta;









			continue;









			goto next_insn;









		}

















		if (insn->imm == BPF_FUNC_timer_set_callback) {








@@ -19754,7 +19823,7 @@ static int do_misc_fixups(struct bpf_verifier_env *env)







				delta    += cnt - 1;









				env->prog = prog = new_prog;









				insn      = new_prog->insnsi + i + delta;









				continue;









				goto next_insn;









			}

















			BUILD_BUG_ON(!__same_type(ops->map_lookup_elem,








@@ -19785,31 +19854,31 @@ static int do_misc_fixups(struct bpf_verifier_env *env)







			switch (insn->imm) {









			case BPF_FUNC_map_lookup_elem:









				insn->imm = BPF_CALL_IMM(ops->map_lookup_elem);









				continue;









				goto next_insn;









			case BPF_FUNC_map_update_elem:









				insn->imm = BPF_CALL_IMM(ops->map_update_elem);









				continue;









				goto next_insn;









			case BPF_FUNC_map_delete_elem:









				insn->imm = BPF_CALL_IMM(ops->map_delete_elem);









				continue;









				goto next_insn;









			case BPF_FUNC_map_push_elem:









				insn->imm = BPF_CALL_IMM(ops->map_push_elem);









				continue;









				goto next_insn;









			case BPF_FUNC_map_pop_elem:









				insn->imm = BPF_CALL_IMM(ops->map_pop_elem);









				continue;









				goto next_insn;









			case BPF_FUNC_map_peek_elem:









				insn->imm = BPF_CALL_IMM(ops->map_peek_elem);









				continue;









				goto next_insn;









			case BPF_FUNC_redirect_map:









				insn->imm = BPF_CALL_IMM(ops->map_redirect);









				continue;









				goto next_insn;









			case BPF_FUNC_for_each_map_elem:









				insn->imm = BPF_CALL_IMM(ops->map_for_each_callback);









				continue;









				goto next_insn;









			case BPF_FUNC_map_lookup_percpu_elem:









				insn->imm = BPF_CALL_IMM(ops->map_lookup_percpu_elem);









				continue;









				goto next_insn;









			}

















			goto patch_call_imm;








@@ -19837,7 +19906,7 @@ static int do_misc_fixups(struct bpf_verifier_env *env)







			delta    += cnt - 1;









			env->prog = prog = new_prog;









			insn      = new_prog->insnsi + i + delta;









			continue;









			goto next_insn;









		}

















		/* Implement bpf_get_func_arg inline. */








@@ -19862,7 +19931,7 @@ static int do_misc_fixups(struct bpf_verifier_env *env)







			delta    += cnt - 1;









			env->prog = prog = new_prog;









			insn      = new_prog->insnsi + i + delta;









			continue;









			goto next_insn;









		}

















		/* Implement bpf_get_func_ret inline. */








@@ -19890,7 +19959,7 @@ static int do_misc_fixups(struct bpf_verifier_env *env)







			delta    += cnt - 1;









			env->prog = prog = new_prog;









			insn      = new_prog->insnsi + i + delta;









			continue;









			goto next_insn;









		}

















		/* Implement get_func_arg_cnt inline. */








@@ -19905,7 +19974,7 @@ static int do_misc_fixups(struct bpf_verifier_env *env)















			env->prog = prog = new_prog;









			insn      = new_prog->insnsi + i + delta;









			continue;









			goto next_insn;









		}

















		/* Implement bpf_get_func_ip inline. */








@@ -19920,7 +19989,7 @@ static int do_misc_fixups(struct bpf_verifier_env *env)















			env->prog = prog = new_prog;









			insn      = new_prog->insnsi + i + delta;









			continue;









			goto next_insn;









		}

















		/* Implement bpf_kptr_xchg inline */








@@ -19938,7 +20007,7 @@ static int do_misc_fixups(struct bpf_verifier_env *env)







			delta    += cnt - 1;









			env->prog = prog = new_prog;









			insn      = new_prog->insnsi + i + delta;









			continue;









			goto next_insn;









		}









patch_call_imm:









		fn = env->ops->get_func_proto(insn->imm, env->prog);








@@ -19952,6 +20021,40 @@ static int do_misc_fixups(struct bpf_verifier_env *env)







			return -EFAULT;









		}









		insn->imm = fn->func - __bpf_call_base;









next_insn:









		if (subprogs[cur_subprog + 1].start == i + delta + 1) {









			subprogs[cur_subprog].stack_depth += stack_depth_extra;









			subprogs[cur_subprog].stack_extra = stack_depth_extra;









			cur_subprog++;









			stack_depth = subprogs[cur_subprog].stack_depth;









			stack_depth_extra = 0;









		}









		i++;









		insn++;









	}

















	env->prog->aux->stack_depth = subprogs[0].stack_depth;









	for (i = 0; i < env->subprog_cnt; i++) {









		int subprog_start = subprogs[i].start;









		int stack_slots = subprogs[i].stack_extra / 8;

















		if (!stack_slots)









			continue;









		if (stack_slots > 1) {









			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);









		/* Copy first actual insn to preserve it */









		insn_buf[1] = env->prog->insnsi[subprog_start];

















		new_prog = bpf_patch_insn_data(env, subprog_start, insn_buf, 2);









		if (!new_prog)









			return -ENOMEM;









		env->prog = prog = new_prog;









	}

















	/* Since poke tab is now finalized, publish aux to tracker. */