[PATCH v5 bpf-next 17/23] bpf: generalize reg_set_min_max() to handle two sets of two registers

Andrii Nakryiko
2023-10-27T18:16:56Z

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.

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.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
kernel/bpf/verifier.c | 112 ++++++++++++++++++------------------------
1 file changed, 49 insertions(+), 63 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index dde04b17c3a3..522566699fbe 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -14387,26 +14387,43 @@ static int is_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_state *reg
* In JEQ/JNE cases we also adjust the var_off values.
*/
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 val, u32 val32,
+ 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)val;
- s32 sval32 = (s32)val32;
-
- /* 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 val;
+ u32 val32;
+ 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;
+ val = false_reg2->var_off.value;
+ val32 = (u32)tnum_subreg(false_reg2->var_off).value;
+ sval = (s64)val;
+ sval32 = (s32)val32;
+
switch (opcode) {
/* JEQ/JNE comparison doesn't change the register equivalence.
*
@@ -14543,22 +14560,6 @@ 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 val, u32 val32,
- 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, val, val32, 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)
@@ -14891,45 +14892,30 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
* 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]);
- }
-
+ 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 if (dst_reg->type == SCALAR_VALUE) {
- reg_set_min_max(&other_branch_regs[insn->dst_reg],
- dst_reg, insn->imm, (u32)insn->imm,
- opcode, is_jmp32);
+ reg_set_min_max(&other_branch_regs[insn->dst_reg], 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);
--
2.34.1