bpf: generalize reg_set_min_max() to handle two sets of two registers

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 <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231102033759.2541186-18-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 40ed261d3489533a466ab5e9d3222d3adf71fbee..e801c50d385754896ae707cdddcf0cc3f1210dac 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -14400,32 +14400,50 @@ 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.
         *
@@ -14562,22 +14580,6 @@ 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)
@@ -14902,53 +14904,32 @@ 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);