bpf: Track equal scalars history on per-instruction level

Use bpf_verifier_state->jmp_history to track which registers were
updated by find_equal_scalars() (renamed to collect_linked_regs())
when conditional jump was verified. Use recorded information in
backtrack_insn() to propagate precision.

E.g. for the following program:

while verifying instructions
1: r1 = r0 |
2: if r1 < 8 goto ... | push r0,r1 as linked registers in jmp_history
3: if r0 > 16 goto ... | push r0,r1 as linked registers in jmp_history
4: r2 = r10 |
5: r2 += r0 v mark_chain_precision(r0)

while doing mark_chain_precision(r0)
5: r2 += r0 | mark r0 precise
4: r2 = r10 |
3: if r0 > 16 goto ... | mark r0,r1 as precise
2: if r1 < 8 goto ... | mark r0,r1 as precise
1: r1 = r0 v

Technically, do this as follows:
- Use 10 bits to identify each register that gains range because of
sync_linked_regs():
- 3 bits for frame number;
- 6 bits for register or stack slot number;
- 1 bit to indicate if register is spilled.
- Use u64 as a vector of 6 such records + 4 bits for vector length.
- Augment struct bpf_jmp_history_entry with a field 'linked_regs'
representing such vector.
- When doing check_cond_jmp_op() remember up to 6 registers that
gain range because of sync_linked_regs() in such a vector.
- Don't propagate range information and reset IDs for registers that
don't fit in 6-value vector.
- Push a pair {instruction index, linked registers vector}
to bpf_verifier_state->jmp_history.
- When doing backtrack_insn() check if any of recorded linked
registers is currently marked precise, if so mark all linked
registers as precise.

This also requires fixes for two test_verifier tests:
- precise: test 1
- precise: test 2

Both tests contain the following instruction sequence:

19: (bf) r2 = r9 ; R2=scalar(id=3) R9=scalar(id=3)
20: (a5) if r2 < 0x8 goto pc+1 ; R2=scalar(id=3,umin=8)
21: (95) exit
22: (07) r2 += 1 ; R2_w=scalar(id=3+1,...)
23: (bf) r1 = r10 ; R1_w=fp0 R10=fp0
24: (07) r1 += -8 ; R1_w=fp-8
25: (b7) r3 = 0 ; R3_w=0
26: (85) call bpf_probe_read_kernel#113

The call to bpf_probe_read_kernel() at (26) forces r2 to be precise.
Previously, this forced all registers with same id to become precise
immediately when mark_chain_precision() is called.
After this change, the precision is propagated to registers sharing
same id only when 'if' instruction is backtracked.
Hence verification log for both tests is changed:
regs=r2,r9 -> regs=r2 for instructions 25..20.

Fixes: 904e6ddf4133 ("bpf: Use scalar ids in mark_chain_precision()")
Reported-by: Hao Sun <[e-mail]>
Suggested-by: Andrii Nakryiko <[e-mail]>
Signed-off-by: Eduard Zingerman <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

Closes: https://lore.kernel.org/bpf/[e-mail]/

include/linux/bpf_verifier.h

···
 	u32 prev_idx : 22;
 	/* special flags, e.g., whether insn is doing register stack spill/load */
 	u32 flags : 10;
+	/* additional registers that need precision tracking when this
+	 * jump is backtracked, vector of six 10-bit records
+	 */
+	u64 linked_regs;
 };
 
 /* Maximum number of register states that can exist at once */

kernel/bpf/verifier.c

···
 	return env->insn_aux_data[insn_idx].jmp_point;
 }
 
+#define LR_FRAMENO_BITS	3
+#define LR_SPI_BITS	6
+#define LR_ENTRY_BITS	(LR_SPI_BITS + LR_FRAMENO_BITS + 1)
+#define LR_SIZE_BITS	4
+#define LR_FRAMENO_MASK	((1ull << LR_FRAMENO_BITS) - 1)
+#define LR_SPI_MASK	((1ull << LR_SPI_BITS)     - 1)
+#define LR_SIZE_MASK	((1ull << LR_SIZE_BITS)    - 1)
+#define LR_SPI_OFF	LR_FRAMENO_BITS
+#define LR_IS_REG_OFF	(LR_SPI_BITS + LR_FRAMENO_BITS)
+#define LINKED_REGS_MAX	6
+
+struct linked_reg {
+	u8 frameno;
+	union {
+		u8 spi;
+		u8 regno;
+	};
+	bool is_reg;
+};
+
+struct linked_regs {
+	int cnt;
+	struct linked_reg entries[LINKED_REGS_MAX];
+};
+
+static struct linked_reg *linked_regs_push(struct linked_regs *s)
+{
+	if (s->cnt < LINKED_REGS_MAX)
+		return &s->entries[s->cnt++];
+
+	return NULL;
+}
+
+/* Use u64 as a vector of 6 10-bit values, use first 4-bits to track
+ * number of elements currently in stack.
+ * Pack one history entry for linked registers as 10 bits in the following format:
+ * - 3-bits frameno
+ * - 6-bits spi_or_reg
+ * - 1-bit  is_reg
+ */
+static u64 linked_regs_pack(struct linked_regs *s)
+{
+	u64 val = 0;
+	int i;
+
+	for (i = 0; i < s->cnt; ++i) {
+		struct linked_reg *e = &s->entries[i];
+		u64 tmp = 0;
+
+		tmp |= e->frameno;
+		tmp |= e->spi << LR_SPI_OFF;
+		tmp |= (e->is_reg ? 1 : 0) << LR_IS_REG_OFF;
+
+		val <<= LR_ENTRY_BITS;
+		val |= tmp;
+	}
+	val <<= LR_SIZE_BITS;
+	val |= s->cnt;
+	return val;
+}
+
+static void linked_regs_unpack(u64 val, struct linked_regs *s)
+{
+	int i;
+
+	s->cnt = val & LR_SIZE_MASK;
+	val >>= LR_SIZE_BITS;
+
+	for (i = 0; i < s->cnt; ++i) {
+		struct linked_reg *e = &s->entries[i];
+
+		e->frameno =  val & LR_FRAMENO_MASK;
+		e->spi     = (val >> LR_SPI_OFF) & LR_SPI_MASK;
+		e->is_reg  = (val >> LR_IS_REG_OFF) & 0x1;
+		val >>= LR_ENTRY_BITS;
+	}
+}
+
 /* for any branch, call, exit record the history of jmps in the given state */
 static int push_jmp_history(struct bpf_verifier_env *env, struct bpf_verifier_state *cur,
-			    int insn_flags)
+			    int insn_flags, u64 linked_regs)
 {
 	u32 cnt = cur->jmp_history_cnt;
 	struct bpf_jmp_history_entry *p;
···
 			  "verifier insn history bug: insn_idx %d cur flags %x new flags %x\n",
 			  env->insn_idx, env->cur_hist_ent->flags, insn_flags);
 		env->cur_hist_ent->flags |= insn_flags;
+		WARN_ONCE(env->cur_hist_ent->linked_regs != 0,
+			  "verifier insn history bug: insn_idx %d linked_regs != 0: %#llx\n",
+			  env->insn_idx, env->cur_hist_ent->linked_regs);
+		env->cur_hist_ent->linked_regs = linked_regs;
 		return 0;
 	}
 
···
 	p->idx = env->insn_idx;
 	p->prev_idx = env->prev_insn_idx;
 	p->flags = insn_flags;
+	p->linked_regs = linked_regs;
 	cur->jmp_history_cnt = cnt;
 	env->cur_hist_ent = p;
 
···
 	return bt->reg_masks[bt->frame] & (1 << reg);
 }
 
+static inline bool bt_is_frame_reg_set(struct backtrack_state *bt, u32 frame, u32 reg)
+{
+	return bt->reg_masks[frame] & (1 << reg);
+}
+
 static inline bool bt_is_frame_slot_set(struct backtrack_state *bt, u32 frame, u32 slot)
 {
 	return bt->stack_masks[frame] & (1ull << slot);
···
 	}
 }
 
+/* If any register R in hist->linked_regs is marked as precise in bt,
+ * do bt_set_frame_{reg,slot}(bt, R) for all registers in hist->linked_regs.
+ */
+static void bt_sync_linked_regs(struct backtrack_state *bt, struct bpf_jmp_history_entry *hist)
+{
+	struct linked_regs linked_regs;
+	bool some_precise = false;
+	int i;
+
+	if (!hist || hist->linked_regs == 0)
+		return;
+
+	linked_regs_unpack(hist->linked_regs, &linked_regs);
+	for (i = 0; i < linked_regs.cnt; ++i) {
+		struct linked_reg *e = &linked_regs.entries[i];
+
+		if ((e->is_reg && bt_is_frame_reg_set(bt, e->frameno, e->regno)) ||
+		    (!e->is_reg && bt_is_frame_slot_set(bt, e->frameno, e->spi))) {
+			some_precise = true;
+			break;
+		}
+	}
+
+	if (!some_precise)
+		return;
+
+	for (i = 0; i < linked_regs.cnt; ++i) {
+		struct linked_reg *e = &linked_regs.entries[i];
+
+		if (e->is_reg)
+			bt_set_frame_reg(bt, e->frameno, e->regno);
+		else
+			bt_set_frame_slot(bt, e->frameno, e->spi);
+	}
+}
+
 static bool calls_callback(struct bpf_verifier_env *env, int insn_idx);
 
 /* For given verifier state backtrack_insn() is called from the last insn to
···
 		verbose(env, "%d: ", idx);
 		print_bpf_insn(&cbs, insn, env->allow_ptr_leaks);
 	}
+
+	/* If there is a history record that some registers gained range at this insn,
+	 * propagate precision marks to those registers, so that bt_is_reg_set()
+	 * accounts for these registers.
+	 */
+	bt_sync_linked_regs(bt, hist);
 
 	if (class == BPF_ALU || class == BPF_ALU64) {
 		if (!bt_is_reg_set(bt, dreg))
···
 			 */
 			bt_set_reg(bt, dreg);
 			bt_set_reg(bt, sreg);
-			 /* else dreg <cond> K
+		} else if (BPF_SRC(insn->code) == BPF_K) {
+			 /* dreg <cond> K
 			  * Only dreg still needs precision before
 			  * this insn, so for the K-based conditional
 			  * there is nothing new to be marked.
···
 			/* to be analyzed */
 			return -ENOTSUPP;
 	}
+	/* Propagate precision marks to linked registers, to account for
+	 * registers marked as precise in this function.
+	 */
+	bt_sync_linked_regs(bt, hist);
 	return 0;
 }
 
···
 
 	if (!src_reg->id && !tnum_is_const(src_reg->var_off))
 		/* Ensure that src_reg has a valid ID that will be copied to
-		 * dst_reg and then will be used by find_equal_scalars() to
+		 * dst_reg and then will be used by sync_linked_regs() to
 		 * propagate min/max range.
 		 */
 		src_reg->id = ++env->id_gen;
···
 	}
 
 	if (insn_flags)
-		return push_jmp_history(env, env->cur_state, insn_flags);
+		return push_jmp_history(env, env->cur_state, insn_flags, 0);
 	return 0;
 }
 
···
 		insn_flags = 0; /* we are not restoring spilled register */
 	}
 	if (insn_flags)
-		return push_jmp_history(env, env->cur_state, insn_flags);
+		return push_jmp_history(env, env->cur_state, insn_flags, 0);
 	return 0;
 }
 
···
 		u64 val = reg_const_value(src_reg, alu32);
 
 		if ((dst_reg->id & BPF_ADD_CONST) ||
-		    /* prevent overflow in find_equal_scalars() later */
+		    /* prevent overflow in sync_linked_regs() later */
 		    val > (u32)S32_MAX) {
 			/*
 			 * If the register already went through rX += val
···
 	} else {
 		/*
 		 * Make sure ID is cleared otherwise dst_reg min/max could be
-		 * incorrectly propagated into other registers by find_equal_scalars()
+		 * incorrectly propagated into other registers by sync_linked_regs()
 		 */
 		dst_reg->id = 0;
 	}
···
 						copy_register_state(dst_reg, src_reg);
 						/* Make sure ID is cleared if src_reg is not in u32
 						 * range otherwise dst_reg min/max could be incorrectly
-						 * propagated into src_reg by find_equal_scalars()
+						 * propagated into src_reg by sync_linked_regs()
 						 */
 						if (!is_src_reg_u32)
 							dst_reg->id = 0;
···
 	return true;
 }
 
-static void find_equal_scalars(struct bpf_verifier_state *vstate,
-			       struct bpf_reg_state *known_reg)
+static void __collect_linked_regs(struct linked_regs *reg_set, struct bpf_reg_state *reg,
+				  u32 id, u32 frameno, u32 spi_or_reg, bool is_reg)
+{
+	struct linked_reg *e;
+
+	if (reg->type != SCALAR_VALUE || (reg->id & ~BPF_ADD_CONST) != id)
+		return;
+
+	e = linked_regs_push(reg_set);
+	if (e) {
+		e->frameno = frameno;
+		e->is_reg = is_reg;
+		e->regno = spi_or_reg;
+	} else {
+		reg->id = 0;
+	}
+}
+
+/* For all R being scalar registers or spilled scalar registers
+ * in verifier state, save R in linked_regs if R->id == id.
+ * If there are too many Rs sharing same id, reset id for leftover Rs.
+ */
+static void collect_linked_regs(struct bpf_verifier_state *vstate, u32 id,
+				struct linked_regs *linked_regs)
+{
+	struct bpf_func_state *func;
+	struct bpf_reg_state *reg;
+	int i, j;
+
+	id = id & ~BPF_ADD_CONST;
+	for (i = vstate->curframe; i >= 0; i--) {
+		func = vstate->frame[i];
+		for (j = 0; j < BPF_REG_FP; j++) {
+			reg = &func->regs[j];
+			__collect_linked_regs(linked_regs, reg, id, i, j, true);
+		}
+		for (j = 0; j < func->allocated_stack / BPF_REG_SIZE; j++) {
+			if (!is_spilled_reg(&func->stack[j]))
+				continue;
+			reg = &func->stack[j].spilled_ptr;
+			__collect_linked_regs(linked_regs, reg, id, i, j, false);
+		}
+	}
+}
+
+/* For all R in linked_regs, copy known_reg range into R
+ * if R->id == known_reg->id.
+ */
+static void sync_linked_regs(struct bpf_verifier_state *vstate, struct bpf_reg_state *known_reg,
+			     struct linked_regs *linked_regs)
 {
 	struct bpf_reg_state fake_reg;
-	struct bpf_func_state *state;
 	struct bpf_reg_state *reg;
+	struct linked_reg *e;
+	int i;
 
-	bpf_for_each_reg_in_vstate(vstate, state, reg, ({
+	for (i = 0; i < linked_regs->cnt; ++i) {
+		e = &linked_regs->entries[i];
+		reg = e->is_reg ? &vstate->frame[e->frameno]->regs[e->regno]
+				: &vstate->frame[e->frameno]->stack[e->spi].spilled_ptr;
 		if (reg->type != SCALAR_VALUE || reg == known_reg)
 			continue;
 		if ((reg->id & ~BPF_ADD_CONST) != (known_reg->id & ~BPF_ADD_CONST))
···
 			copy_register_state(reg, known_reg);
 			/*
 			 * Must preserve off, id and add_const flag,
-			 * otherwise another find_equal_scalars() will be incorrect.
+			 * otherwise another sync_linked_regs() will be incorrect.
 			 */
 			reg->off = saved_off;
 
···
 			scalar_min_max_add(reg, &fake_reg);
 			reg->var_off = tnum_add(reg->var_off, fake_reg.var_off);
 		}
-	}));
+	}
 }
 
 static int check_cond_jmp_op(struct bpf_verifier_env *env,
···
 	struct bpf_reg_state *regs = this_branch->frame[this_branch->curframe]->regs;
 	struct bpf_reg_state *dst_reg, *other_branch_regs, *src_reg = NULL;
 	struct bpf_reg_state *eq_branch_regs;
+	struct linked_regs linked_regs = {};
 	u8 opcode = BPF_OP(insn->code);
 	bool is_jmp32;
 	int pred = -1;
···
 		return 0;
 	}
 
+	/* Push scalar registers sharing same ID to jump history,
+	 * do this before creating 'other_branch', so that both
+	 * 'this_branch' and 'other_branch' share this history
+	 * if parent state is created.
+	 */
+	if (BPF_SRC(insn->code) == BPF_X && src_reg->type == SCALAR_VALUE && src_reg->id)
+		collect_linked_regs(this_branch, src_reg->id, &linked_regs);
+	if (dst_reg->type == SCALAR_VALUE && dst_reg->id)
+		collect_linked_regs(this_branch, dst_reg->id, &linked_regs);
+	if (linked_regs.cnt > 1) {
+		err = push_jmp_history(env, this_branch, 0, linked_regs_pack(&linked_regs));
+		if (err)
+			return err;
+	}
+
 	other_branch = push_stack(env, *insn_idx + insn->off + 1, *insn_idx,
 				  false);
 	if (!other_branch)
···
 	if (BPF_SRC(insn->code) == BPF_X &&
 	    src_reg->type == SCALAR_VALUE && src_reg->id &&
 	    !WARN_ON_ONCE(src_reg->id != other_branch_regs[insn->src_reg].id)) {
-		find_equal_scalars(this_branch, src_reg);
-		find_equal_scalars(other_branch, &other_branch_regs[insn->src_reg]);
+		sync_linked_regs(this_branch, src_reg, &linked_regs);
+		sync_linked_regs(other_branch, &other_branch_regs[insn->src_reg], &linked_regs);
 	}
 	if (dst_reg->type == SCALAR_VALUE && dst_reg->id &&
 	    !WARN_ON_ONCE(dst_reg->id != other_branch_regs[insn->dst_reg].id)) {
-		find_equal_scalars(this_branch, dst_reg);
-		find_equal_scalars(other_branch, &other_branch_regs[insn->dst_reg]);
+		sync_linked_regs(this_branch, dst_reg, &linked_regs);
+		sync_linked_regs(other_branch, &other_branch_regs[insn->dst_reg], &linked_regs);
 	}
 
 	/* if one pointer register is compared to another pointer
···
 		 *
 		 * First verification path is [1-6]:
 		 * - at (4) same bpf_reg_state::id (b) would be assigned to r6 and r7;
-		 * - at (5) r6 would be marked <= X, find_equal_scalars() would also mark
+		 * - at (5) r6 would be marked <= X, sync_linked_regs() would also mark
 		 *   r7 <= X, because r6 and r7 share same id.
 		 * Next verification path is [1-4, 6].
 		 *
···
 			 * the current state.
 			 */
 			if (is_jmp_point(env, env->insn_idx))
-				err = err ? : push_jmp_history(env, cur, 0);
+				err = err ? : push_jmp_history(env, cur, 0, 0);
 			err = err ? : propagate_precision(env, &sl->state);
 			if (err)
 				return err;
···
 		}
 
 		if (is_jmp_point(env, env->insn_idx)) {
-			err = push_jmp_history(env, state, 0);
+			err = push_jmp_history(env, state, 0, 0);
 			if (err)
 				return err;
 		}

tools/testing/selftests/bpf/progs/verifier_subprog_precision.c

···
 __msg("mark_precise: frame0: regs=r6 stack= before 13: (bf) r1 = r7")
 __msg("mark_precise: frame0: regs=r6 stack= before 12: (27) r6 *= 4")
 __msg("mark_precise: frame0: regs=r6 stack= before 11: (25) if r6 > 0x3 goto pc+4")
-__msg("mark_precise: frame0: regs=r6 stack= before 10: (bf) r6 = r0")
+__msg("mark_precise: frame0: regs=r0,r6 stack= before 10: (bf) r6 = r0")
 __msg("mark_precise: frame0: regs=r0 stack= before 9: (85) call bpf_loop")
 /* State entering callback body popped from states stack */
 __msg("from 9 to 17: frame1:")

tools/testing/selftests/bpf/verifier/precise.c

···
 	.result = VERBOSE_ACCEPT,
 	.errstr =
 	"mark_precise: frame0: last_idx 26 first_idx 20\
-	mark_precise: frame0: regs=r2,r9 stack= before 25\
-	mark_precise: frame0: regs=r2,r9 stack= before 24\
-	mark_precise: frame0: regs=r2,r9 stack= before 23\
-	mark_precise: frame0: regs=r2,r9 stack= before 22\
-	mark_precise: frame0: regs=r2,r9 stack= before 20\
+	mark_precise: frame0: regs=r2 stack= before 25\
+	mark_precise: frame0: regs=r2 stack= before 24\
+	mark_precise: frame0: regs=r2 stack= before 23\
+	mark_precise: frame0: regs=r2 stack= before 22\
+	mark_precise: frame0: regs=r2 stack= before 20\
 	mark_precise: frame0: parent state regs=r2,r9 stack=:\
 	mark_precise: frame0: last_idx 19 first_idx 10\
 	mark_precise: frame0: regs=r2,r9 stack= before 19\
···
 	.errstr =
 	"26: (85) call bpf_probe_read_kernel#113\
 	mark_precise: frame0: last_idx 26 first_idx 22\
-	mark_precise: frame0: regs=r2,r9 stack= before 25\
-	mark_precise: frame0: regs=r2,r9 stack= before 24\
-	mark_precise: frame0: regs=r2,r9 stack= before 23\
-	mark_precise: frame0: regs=r2,r9 stack= before 22\
-	mark_precise: frame0: parent state regs=r2,r9 stack=:\
+	mark_precise: frame0: regs=r2 stack= before 25\
+	mark_precise: frame0: regs=r2 stack= before 24\
+	mark_precise: frame0: regs=r2 stack= before 23\
+	mark_precise: frame0: regs=r2 stack= before 22\
+	mark_precise: frame0: parent state regs=r2 stack=:\
 	mark_precise: frame0: last_idx 20 first_idx 20\
 	mark_precise: frame0: regs=r2,r9 stack= before 20\
 	mark_precise: frame0: parent state regs=r2,r9 stack=:\