Subject: [PATCH RFC bpf-next] bpf: per-register parent pointers

From: Alexei Starovoitov <[email-masked]>
Date: Tue, 17 Jul 2018 20:54:47 -0700

By giving each register its own liveness chain, we elide the skip_callee()
logic.  Instead, each register's parent is the state it inherits from;
both check_func_call() and prepare_func_exit() automatically connect
reg states to the correct chain since when they copy the reg state across
(r1-r5 into the callee as args, and r0 out as the return value) they also
copy the parent pointer.

Signed-off-by: Edward Cree <[email-masked]>
Signed-off-by: Alexei Starovoitov <[email-masked]>
---
Ed,

I've started analyzing this old patch and it looks great.
I'd like to apply it, but I see the difference in insn_processed.
Several cilium tests show favorable difference towards new liveness approach.
selftests/bpf/test_xdp_noinline.o also shows the difference.
I'm struggling to see why this patch would make such difference.
Could you please help me analyze why such difference exists?
In particular if you hack test_progs.c to run only test_xdp_noinline
with verifier log_level=1 you will see this delta:
-from 237 to 273: frame2: R0=inv1 R1=pkt(id=0,off=54,r=14,imm=0) R2=fp-95,call_13 R3=fp-94,call_13 R4=pkt(id=0,off=0,r=14,imm=0) R5=pkt_end(id=0,off=0,imm=0) R6=fp-56,call_13 R10=fp0,call_44
-273: (95) exit
-returning from callee:
- frame2: R0=inv1 R1=pkt(id=0,off=54,r=14,imm=0) R2=fp-95,call_13 R3=fp-94,call_13 R4=pkt(id=0,off=0,r=14,imm=0) R5=pkt_end(id=0,off=0,imm=0) R6=fp-56,call_13 R10=fp0,call_44
-to caller at 45:
- frame1: R0=inv1 R6=pkt(id=0,off=0,r=14,imm=0) R7=inv1 R9=pkt_end(id=0,off=0,imm=0) R10=fp0,call_13 fp-8=0 fp-24=0 fp-32=0 fp-40=0 fp-48=0 fp-56=0 fp-64=0 fp-72=0 fp-80=0 fp-112=ctx
-
-from 273 to 45: frame1: R0=inv1 R6=pkt(id=0,off=0,r=14,imm=0) R7=inv1 R9=pkt_end(id=0,off=0,imm=0) R10=fp0,call_13 fp-8=0 fp-24=0 fp-32=0 fp-40=0 fp-48=0 fp-56=0 fp-64=0 fp-72=0 fp-80=0 fp-112=ctx
-45: (05) goto pc+9
-55: safe
+from 237 to 273: safe

-processed 2971 insns (limit 131072), stack depth 0+112+0+0+8+0+4+24+8+16+0+16+8+0+0+0+0+0+32+0+0
-test_xdp_noinline:PASS:ipv4 3292 nsec
-test_xdp_noinline:PASS:ipv6 3105 nsec
+processed 2957 insns (limit 131072), stack depth 0+112+0+0+8+0+4+24+8+16+0+16+8+0+0+0+0+0+32+0+0
+test_xdp_noinline:PASS:ipv4 2922 nsec
+test_xdp_noinline:PASS:ipv6 4296 nsec

Did we miss liveness propagation before or there is a bug in this patch?
Thanks

 include/linux/bpf_verifier.h |   8 +-
 kernel/bpf/verifier.c        | 180 ++++++++---------------------------
 2 files changed, 45 insertions(+), 143 deletions(-)

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 38b04f559ad3..b42b60a83e19 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -41,6 +41,7 @@ enum bpf_reg_liveness {
 };
 
 struct bpf_reg_state {
+	/* Ordering of fields matters.  See states_equal() */
 	enum bpf_reg_type type;
 	union {
 		/* valid when type == PTR_TO_PACKET */
@@ -59,7 +60,6 @@ struct bpf_reg_state {
 	 * came from, when one is tested for != NULL.
 	 */
 	u32 id;
-	/* Ordering of fields matters.  See states_equal() */
 	/* For scalar types (SCALAR_VALUE), this represents our knowledge of
 	 * the actual value.
 	 * For pointer types, this represents the variable part of the offset
@@ -76,15 +76,15 @@ struct bpf_reg_state {
 	s64 smax_value; /* maximum possible (s64)value */
 	u64 umin_value; /* minimum possible (u64)value */
 	u64 umax_value; /* maximum possible (u64)value */
+	/* parentage chain for liveness checking */
+	struct bpf_reg_state *parent;
 	/* Inside the callee two registers can be both PTR_TO_STACK like
 	 * R1=fp-8 and R2=fp-8, but one of them points to this function stack
 	 * while another to the caller's stack. To differentiate them 'frameno'
 	 * is used which is an index in bpf_verifier_state->frame[] array
 	 * pointing to bpf_func_state.
-	 * This field must be second to last, for states_equal() reasons.
 	 */
 	u32 frameno;
-	/* This field must be last, for states_equal() reasons. */
 	enum bpf_reg_liveness live;
 };
 
@@ -107,7 +107,6 @@ struct bpf_stack_state {
  */
 struct bpf_func_state {
 	struct bpf_reg_state regs[MAX_BPF_REG];
-	struct bpf_verifier_state *parent;
 	/* index of call instruction that called into this func */
 	int callsite;
 	/* stack frame number of this function state from pov of
@@ -129,7 +128,6 @@ struct bpf_func_state {
 struct bpf_verifier_state {
 	/* call stack tracking */
 	struct bpf_func_state *frame[MAX_CALL_FRAMES];
-	struct bpf_verifier_state *parent;
 	u32 curframe;
 };
 
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 9e2bf834f13a..5dc379f05b08 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -380,9 +380,9 @@ static int copy_stack_state(struct bpf_func_state *dst,
 /* do_check() starts with zero-sized stack in struct bpf_verifier_state to
  * make it consume minimal amount of memory. check_stack_write() access from
  * the program calls into realloc_func_state() to grow the stack size.
- * Note there is a non-zero 'parent' pointer inside bpf_verifier_state
- * which this function copies over. It points to previous bpf_verifier_state
- * which is never reallocated
+ * Note there is a non-zero parent pointer inside each reg of bpf_verifier_state
+ * which this function copies over. It points to corresponding reg in previous
+ * bpf_verifier_state which is never reallocated
  */
 static int realloc_func_state(struct bpf_func_state *state, int size,
 			      bool copy_old)
@@ -466,7 +466,6 @@ static int copy_verifier_state(struct bpf_verifier_state *dst_state,
 		dst_state->frame[i] = NULL;
 	}
 	dst_state->curframe = src->curframe;
-	dst_state->parent = src->parent;
 	for (i = 0; i <= src->curframe; i++) {
 		dst = dst_state->frame[i];
 		if (!dst) {
@@ -732,6 +731,7 @@ static void init_reg_state(struct bpf_verifier_env *env,
 	for (i = 0; i < MAX_BPF_REG; i++) {
 		mark_reg_not_init(env, regs, i);
 		regs[i].live = REG_LIVE_NONE;
+		regs[i].parent = NULL;
 	}
 
 	/* frame pointer */
@@ -876,74 +876,21 @@ static int check_subprogs(struct bpf_verifier_env *env)
 	return 0;
 }
 
-static
-struct bpf_verifier_state *skip_callee(struct bpf_verifier_env *env,
-				       const struct bpf_verifier_state *state,
-				       struct bpf_verifier_state *parent,
-				       u32 regno)
-{
-	struct bpf_verifier_state *tmp = NULL;
-
-	/* 'parent' could be a state of caller and
-	 * 'state' could be a state of callee. In such case
-	 * parent->curframe < state->curframe
-	 * and it's ok for r1 - r5 registers
-	 *
-	 * 'parent' could be a callee's state after it bpf_exit-ed.
-	 * In such case parent->curframe > state->curframe
-	 * and it's ok for r0 only
-	 */
-	if (parent->curframe == state->curframe ||
-	    (parent->curframe < state->curframe &&
-	     regno >= BPF_REG_1 && regno <= BPF_REG_5) ||
-	    (parent->curframe > state->curframe &&
-	       regno == BPF_REG_0))
-		return parent;
-
-	if (parent->curframe > state->curframe &&
-	    regno >= BPF_REG_6) {
-		/* for callee saved regs we have to skip the whole chain
-		 * of states that belong to callee and mark as LIVE_READ
-		 * the registers before the call
-		 */
-		tmp = parent;
-		while (tmp && tmp->curframe != state->curframe) {
-			tmp = tmp->parent;
-		}
-		if (!tmp)
-			goto bug;
-		parent = tmp;
-	} else {
-		goto bug;
-	}
-	return parent;
-bug:
-	verbose(env, "verifier bug regno %d tmp %p\n", regno, tmp);
-	verbose(env, "regno %d parent frame %d current frame %d\n",
-		regno, parent->curframe, state->curframe);
-	return NULL;
-}
-
+/* Parentage chain of this register (or stack slot) should take care of all
+ * issues like callee-saved registers, stack slot allocation time, etc.
+ */
 static int mark_reg_read(struct bpf_verifier_env *env,
-			 const struct bpf_verifier_state *state,
-			 struct bpf_verifier_state *parent,
-			 u32 regno)
+			 const struct bpf_reg_state *state,
+			 struct bpf_reg_state *parent)
 {
 	bool writes = parent == state->parent; /* Observe write marks */
 
-	if (regno == BPF_REG_FP)
-		/* We don't need to worry about FP liveness because it's read-only */
-		return 0;
-
 	while (parent) {
 		/* if read wasn't screened by an earlier write ... */
-		if (writes && state->frame[state->curframe]->regs[regno].live & REG_LIVE_WRITTEN)
+		if (writes && state->live & REG_LIVE_WRITTEN)
 			break;
-		parent = skip_callee(env, state, parent, regno);
-		if (!parent)
-			return -EFAULT;
 		/* ... then we depend on parent's value */
-		parent->frame[parent->curframe]->regs[regno].live |= REG_LIVE_READ;
+		parent->live |= REG_LIVE_READ;
 		state = parent;
 		parent = state->parent;
 		writes = true;
@@ -969,7 +916,10 @@ static int check_reg_arg(struct bpf_verifier_env *env, u32 regno,
 			verbose(env, "R%d !read_ok\n", regno);
 			return -EACCES;
 		}
-		return mark_reg_read(env, vstate, vstate->parent, regno);
+		/* We don't need to worry about FP liveness because it's read-only */
+		if (regno != BPF_REG_FP)
+			return mark_reg_read(env, &regs[regno],
+					     regs[regno].parent);
 	} else {
 		/* check whether register used as dest operand can be written to */
 		if (regno == BPF_REG_FP) {
@@ -1106,61 +1056,6 @@ static int check_stack_write(struct bpf_verifier_env *env,
 	return 0;
 }
 
-/* registers of every function are unique and mark_reg_read() propagates
- * the liveness in the following cases:
- * - from callee into caller for R1 - R5 that were used as arguments
- * - from caller into callee for R0 that used as result of the call
- * - from caller to the same caller skipping states of the callee for R6 - R9,
- *   since R6 - R9 are callee saved by implicit function prologue and
- *   caller's R6 != callee's R6, so when we propagate liveness up to
- *   parent states we need to skip callee states for R6 - R9.
- *
- * stack slot marking is different, since stacks of caller and callee are
- * accessible in both (since caller can pass a pointer to caller's stack to
- * callee which can pass it to another function), hence mark_stack_slot_read()
- * has to propagate the stack liveness to all parent states at given frame number.
- * Consider code:
- * f1() {
- *   ptr = fp - 8;
- *   *ptr = ctx;
- *   call f2 {
- *      .. = *ptr;
- *   }
- *   .. = *ptr;
- * }
- * First *ptr is reading from f1's stack and mark_stack_slot_read() has
- * to mark liveness at the f1's frame and not f2's frame.
- * Second *ptr is also reading from f1's stack and mark_stack_slot_read() has
- * to propagate liveness to f2 states at f1's frame level and further into
- * f1 states at f1's frame level until write into that stack slot
- */
-static void mark_stack_slot_read(struct bpf_verifier_env *env,
-				 const struct bpf_verifier_state *state,
-				 struct bpf_verifier_state *parent,
-				 int slot, int frameno)
-{
-	bool writes = parent == state->parent; /* Observe write marks */
-
-	while (parent) {
-		if (parent->frame[frameno]->allocated_stack <= slot * BPF_REG_SIZE)
-			/* since LIVE_WRITTEN mark is only done for full 8-byte
-			 * write the read marks are conservative and parent
-			 * state may not even have the stack allocated. In such case
-			 * end the propagation, since the loop reached beginning
-			 * of the function
-			 */
-			break;
-		/* if read wasn't screened by an earlier write ... */
-		if (writes && state->frame[frameno]->stack[slot].spilled_ptr.live & REG_LIVE_WRITTEN)
-			break;
-		/* ... then we depend on parent's value */
-		parent->frame[frameno]->stack[slot].spilled_ptr.live |= REG_LIVE_READ;
-		state = parent;
-		parent = state->parent;
-		writes = true;
-	}
-}
-
 static int check_stack_read(struct bpf_verifier_env *env,
 			    struct bpf_func_state *reg_state /* func where register points to */,
 			    int off, int size, int value_regno)
@@ -1198,8 +1093,8 @@ static int check_stack_read(struct bpf_verifier_env *env,
 			 */
 			state->regs[value_regno].live |= REG_LIVE_WRITTEN;
 		}
-		mark_stack_slot_read(env, vstate, vstate->parent, spi,
-				     reg_state->frameno);
+		mark_reg_read(env, &reg_state->stack[spi].spilled_ptr,
+			      reg_state->stack[spi].spilled_ptr.parent);
 		return 0;
 	} else {
 		int zeros = 0;
@@ -1215,8 +1110,8 @@ static int check_stack_read(struct bpf_verifier_env *env,
 				off, i, size);
 			return -EACCES;
 		}
-		mark_stack_slot_read(env, vstate, vstate->parent, spi,
-				     reg_state->frameno);
+		mark_reg_read(env, &reg_state->stack[spi].spilled_ptr,
+			      reg_state->stack[spi].spilled_ptr.parent);
 		if (value_regno >= 0) {
 			if (zeros == size) {
 				/* any size read into register is zero extended,
@@ -1907,8 +1802,8 @@ static int check_stack_boundary(struct bpf_verifier_env *env, int regno,
 		/* reading any byte out of 8-byte 'spill_slot' will cause
 		 * the whole slot to be marked as 'read'
 		 */
-		mark_stack_slot_read(env, env->cur_state, env->cur_state->parent,
-				     spi, state->frameno);
+		mark_reg_read(env, &state->stack[spi].spilled_ptr,
+			      state->stack[spi].spilled_ptr.parent);
 	}
 	return update_stack_depth(env, state, off);
 }
@@ -2349,11 +2244,13 @@ static int check_func_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 			state->curframe + 1 /* frameno within this callchain */,
 			subprog /* subprog number within this prog */);
 
-	/* copy r1 - r5 args that callee can access */
+	/* copy r1 - r5 args that callee can access.  The copy includes parent
+	 * pointers, which connects us up to the liveness chain
+	 */
 	for (i = BPF_REG_1; i <= BPF_REG_5; i++)
 		callee->regs[i] = caller->regs[i];
 
-	/* after the call regsiters r0 - r5 were scratched */
+	/* after the call registers r0 - r5 were scratched */
 	for (i = 0; i < CALLER_SAVED_REGS; i++) {
 		mark_reg_not_init(env, caller->regs, caller_saved[i]);
 		check_reg_arg(env, caller_saved[i], DST_OP_NO_MARK);
@@ -4337,7 +4234,7 @@ static bool regsafe(struct bpf_reg_state *rold, struct bpf_reg_state *rcur,
 		/* explored state didn't use this */
 		return true;
 
-	equal = memcmp(rold, rcur, offsetof(struct bpf_reg_state, frameno)) == 0;
+	equal = memcmp(rold, rcur, offsetof(struct bpf_reg_state, parent)) == 0;
 
 	if (rold->type == PTR_TO_STACK)
 		/* two stack pointers are equal only if they're pointing to
@@ -4570,7 +4467,7 @@ static bool states_equal(struct bpf_verifier_env *env,
  * equivalent state (jump target or such) we didn't arrive by the straight-line
  * code, so read marks in the state must propagate to the parent regardless
  * of the state's write marks. That's what 'parent == state->parent' comparison
- * in mark_reg_read() and mark_stack_slot_read() is for.
+ * in mark_reg_read() is for.
  */
 static int propagate_liveness(struct bpf_verifier_env *env,
 			      const struct bpf_verifier_state *vstate,
@@ -4591,7 +4488,8 @@ static int propagate_liveness(struct bpf_verifier_env *env,
 		if (vparent->frame[vparent->curframe]->regs[i].live & REG_LIVE_READ)
 			continue;
 		if (vstate->frame[vstate->curframe]->regs[i].live & REG_LIVE_READ) {
-			err = mark_reg_read(env, vstate, vparent, i);
+			err = mark_reg_read(env, &vstate->frame[vstate->curframe]->regs[i],
+					    &vparent->frame[vstate->curframe]->regs[i]);
 			if (err)
 				return err;
 		}
@@ -4606,7 +4504,8 @@ static int propagate_liveness(struct bpf_verifier_env *env,
 			if (parent->stack[i].spilled_ptr.live & REG_LIVE_READ)
 				continue;
 			if (state->stack[i].spilled_ptr.live & REG_LIVE_READ)
-				mark_stack_slot_read(env, vstate, vparent, i, frame);
+				mark_reg_read(env, &state->stack[i].spilled_ptr,
+					      &parent->stack[i].spilled_ptr);
 		}
 	}
 	return err;
@@ -4616,7 +4515,7 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
 {
 	struct bpf_verifier_state_list *new_sl;
 	struct bpf_verifier_state_list *sl;
-	struct bpf_verifier_state *cur = env->cur_state;
+	struct bpf_verifier_state *cur = env->cur_state, *new;
 	int i, j, err;
 
 	sl = env->explored_states[insn_idx];
@@ -4658,16 +4557,18 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
 		return -ENOMEM;
 
 	/* add new state to the head of linked list */
-	err = copy_verifier_state(&new_sl->state, cur);
+	new = &new_sl->state;
+	err = copy_verifier_state(new, cur);
 	if (err) {
-		free_verifier_state(&new_sl->state, false);
+		free_verifier_state(new, false);
 		kfree(new_sl);
 		return err;
 	}
 	new_sl->next = env->explored_states[insn_idx];
 	env->explored_states[insn_idx] = new_sl;
 	/* connect new state to parentage chain */
-	cur->parent = &new_sl->state;
+	for (i = 0; i < BPF_REG_FP; i++)
+		cur_regs(env)[i].parent = &new->frame[new->curframe]->regs[i];
 	/* clear write marks in current state: the writes we did are not writes
 	 * our child did, so they don't screen off its reads from us.
 	 * (There are no read marks in current state, because reads always mark
@@ -4680,9 +4581,13 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
 	/* all stack frames are accessible from callee, clear them all */
 	for (j = 0; j <= cur->curframe; j++) {
 		struct bpf_func_state *frame = cur->frame[j];
+		struct bpf_func_state *newframe = new->frame[j];
 
-		for (i = 0; i < frame->allocated_stack / BPF_REG_SIZE; i++)
+		for (i = 0; i < frame->allocated_stack / BPF_REG_SIZE; i++) {
 			frame->stack[i].spilled_ptr.live = REG_LIVE_NONE;
+			frame->stack[i].spilled_ptr.parent =
+						&newframe->stack[i].spilled_ptr;
+		}
 	}
 	return 0;
 }
@@ -4701,7 +4606,6 @@ static int do_check(struct bpf_verifier_env *env)
 	if (!state)
 		return -ENOMEM;
 	state->curframe = 0;
-	state->parent = NULL;
 	state->frame[0] = kzalloc(sizeof(struct bpf_func_state), GFP_KERNEL);
 	if (!state->frame[0]) {
 		kfree(state);
-- 
2.17.1