[v6,bpf-next,17/17] bpf: generalize reg_set_min_max() to handle two sets of two registers

Commit Message

Andrii Nakryiko Nov. 2, 2023, 3:37 a.m. UTC

Change reg_set_min_max() to take FALSE/TRUE sets of two registers each,
instead of assuming that we are always comparing to a constant. For now
we still assume that right-hand side registers are constants (and make
sure that's the case by swapping src/dst regs, if necessary), but
subsequent patches will remove this limitation.

reg_set_min_max() is now called unconditionally for any register
comparison, so that might include pointer vs pointer. This makes it
consistent with is_branch_taken() generality. But we currently only
support adjustments based on SCALAR vs SCALAR comparisons, so
reg_set_min_max() has to guard itself againts pointers.

Taking two by two registers allows to further unify and simplify
check_cond_jmp_op() logic. We utilize fake register for BPF_K
conditional jump case, just like with is_branch_taken() part.

Acked-by: Eduard Zingerman <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
---
 kernel/bpf/verifier.c | 131 ++++++++++++++++++------------------------
 1 file changed, 56 insertions(+), 75 deletions(-)

Patch

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 438bf96b1c2d..2197385d91dc 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -14379,32 +14379,50 @@  static int is_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_state *reg
 	return is_scalar_branch_taken(reg1, reg2, opcode, is_jmp32);
 }
 
-/* Adjusts the register min/max values in the case that the dst_reg is the
- * variable register that we are working on, and src_reg is a constant or we're
- * simply doing a BPF_K check.
- * In JEQ/JNE cases we also adjust the var_off values.
+/* Adjusts the register min/max values in the case that the dst_reg and
+ * src_reg are both SCALAR_VALUE registers (or we are simply doing a BPF_K
+ * check, in which case we havea fake SCALAR_VALUE representing insn->imm).
+ * Technically we can do similar adjustments for pointers to the same object,
+ * but we don't support that right now.
  */
 static void reg_set_min_max(struct bpf_reg_state *true_reg1,
+			    struct bpf_reg_state *true_reg2,
 			    struct bpf_reg_state *false_reg1,
-			    u64 uval, u32 uval32,
+			    struct bpf_reg_state *false_reg2,
 			    u8 opcode, bool is_jmp32)
 {
-	struct tnum false_32off = tnum_subreg(false_reg1->var_off);
-	struct tnum false_64off = false_reg1->var_off;
-	struct tnum true_32off = tnum_subreg(true_reg1->var_off);
-	struct tnum true_64off = true_reg1->var_off;
-	s64 sval = (s64)uval;
-	s32 sval32 = (s32)uval32;
-
-	/* If the dst_reg is a pointer, we can't learn anything about its
-	 * variable offset from the compare (unless src_reg were a pointer into
-	 * the same object, but we don't bother with that.
-	 * Since false_reg1 and true_reg1 have the same type by construction, we
-	 * only need to check one of them for pointerness.
+	struct tnum false_32off, false_64off;
+	struct tnum true_32off, true_64off;
+	u64 uval;
+	u32 uval32;
+	s64 sval;
+	s32 sval32;
+
+	/* If either register is a pointer, we can't learn anything about its
+	 * variable offset from the compare (unless they were a pointer into
+	 * the same object, but we don't bother with that).
 	 */
-	if (__is_pointer_value(false, false_reg1))
+	if (false_reg1->type != SCALAR_VALUE || false_reg2->type != SCALAR_VALUE)
+		return;
+
+	/* we expect right-hand registers (src ones) to be constants, for now */
+	if (!is_reg_const(false_reg2, is_jmp32)) {
+		opcode = flip_opcode(opcode);
+		swap(true_reg1, true_reg2);
+		swap(false_reg1, false_reg2);
+	}
+	if (!is_reg_const(false_reg2, is_jmp32))
 		return;
 
+	false_32off = tnum_subreg(false_reg1->var_off);
+	false_64off = false_reg1->var_off;
+	true_32off = tnum_subreg(true_reg1->var_off);
+	true_64off = true_reg1->var_off;
+	uval = false_reg2->var_off.value;
+	uval32 = (u32)tnum_subreg(false_reg2->var_off).value;
+	sval = (s64)uval;
+	sval32 = (s32)uval32;
+
 	switch (opcode) {
 	/* JEQ/JNE comparison doesn't change the register equivalence.
 	 *
@@ -14541,22 +14559,6 @@  static void reg_set_min_max(struct bpf_reg_state *true_reg1,
 	}
 }
 
-/* Same as above, but for the case that dst_reg holds a constant and src_reg is
- * the variable reg.
- */
-static void reg_set_min_max_inv(struct bpf_reg_state *true_reg,
-				struct bpf_reg_state *false_reg,
-				u64 uval, u32 uval32,
-				u8 opcode, bool is_jmp32)
-{
-	opcode = flip_opcode(opcode);
-	/* This uses zero as "not present in table"; luckily the zero opcode,
-	 * BPF_JA, can't get here.
-	 */
-	if (opcode)
-		reg_set_min_max(true_reg, false_reg, uval, uval32, opcode, is_jmp32);
-}
-
 /* Regs are known to be equal, so intersect their min/max/var_off */
 static void __reg_combine_min_max(struct bpf_reg_state *src_reg,
 				  struct bpf_reg_state *dst_reg)
@@ -14881,53 +14883,32 @@  static int check_cond_jmp_op(struct bpf_verifier_env *env,
 		return -EFAULT;
 	other_branch_regs = other_branch->frame[other_branch->curframe]->regs;
 
-	/* detect if we are comparing against a constant value so we can adjust
-	 * our min/max values for our dst register.
-	 * this is only legit if both are scalars (or pointers to the same
-	 * object, I suppose, see the PTR_MAYBE_NULL related if block below),
-	 * because otherwise the different base pointers mean the offsets aren't
-	 * comparable.
-	 */
 	if (BPF_SRC(insn->code) == BPF_X) {
-		struct bpf_reg_state *src_reg = &regs[insn->src_reg];
+		reg_set_min_max(&other_branch_regs[insn->dst_reg],
+				&other_branch_regs[insn->src_reg],
+				dst_reg, src_reg, opcode, is_jmp32);
 
 		if (dst_reg->type == SCALAR_VALUE &&
-		    src_reg->type == SCALAR_VALUE) {
-			if (tnum_is_const(src_reg->var_off) ||
-			    (is_jmp32 &&
-			     tnum_is_const(tnum_subreg(src_reg->var_off))))
-				reg_set_min_max(&other_branch_regs[insn->dst_reg],
-						dst_reg,
-						src_reg->var_off.value,
-						tnum_subreg(src_reg->var_off).value,
-						opcode, is_jmp32);
-			else if (tnum_is_const(dst_reg->var_off) ||
-				 (is_jmp32 &&
-				  tnum_is_const(tnum_subreg(dst_reg->var_off))))
-				reg_set_min_max_inv(&other_branch_regs[insn->src_reg],
-						    src_reg,
-						    dst_reg->var_off.value,
-						    tnum_subreg(dst_reg->var_off).value,
-						    opcode, is_jmp32);
-			else if (!is_jmp32 &&
-				 (opcode == BPF_JEQ || opcode == BPF_JNE))
-				/* Comparing for equality, we can combine knowledge */
-				reg_combine_min_max(&other_branch_regs[insn->src_reg],
-						    &other_branch_regs[insn->dst_reg],
-						    src_reg, dst_reg, opcode);
-			if (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]);
-			}
-
-		}
-	} else if (dst_reg->type == SCALAR_VALUE) {
+		    src_reg->type == SCALAR_VALUE &&
+		    !is_jmp32 && (opcode == BPF_JEQ || opcode == BPF_JNE)) {
+			/* Comparing for equality, we can combine knowledge */
+			reg_combine_min_max(&other_branch_regs[insn->src_reg],
+					    &other_branch_regs[insn->dst_reg],
+					    src_reg, dst_reg, opcode);
+		}
+	} else /* BPF_SRC(insn->code) == BPF_K */ {
 		reg_set_min_max(&other_branch_regs[insn->dst_reg],
-					dst_reg, insn->imm, (u32)insn->imm,
-					opcode, is_jmp32);
+				src_reg /* fake one */,
+				dst_reg, src_reg /* same fake one */,
+				opcode, is_jmp32);
 	}
 
+	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]);
+	}
 	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);