[PATCH bpf-next 02/13] bpf: generalize is_scalar_branch_taken() logic

Andrii Nakryiko
2023-11-03T00:08:44Z

Generalize is_branch_taken logic for SCALAR_VALUE register to handle
cases when both registers are not constants. Previously supported
<range> vs <scalar> cases are a natural subset of more generic <range>
vs <range> set of cases.

Generalized logic relies on straightforward segment intersection checks.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
kernel/bpf/verifier.c | 103 ++++++++++++++++++++++++++----------------
1 file changed, 63 insertions(+), 40 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 52934080042c..2627461164ed 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -14187,82 +14187,104 @@ static int is_scalar_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_sta
u8 opcode, bool is_jmp32)
{
struct tnum t1 = is_jmp32 ? tnum_subreg(reg1->var_off) : reg1->var_off;
+ struct tnum t2 = is_jmp32 ? tnum_subreg(reg2->var_off) : reg2->var_off;
u64 umin1 = is_jmp32 ? (u64)reg1->u32_min_value : reg1->umin_value;
u64 umax1 = is_jmp32 ? (u64)reg1->u32_max_value : reg1->umax_value;
s64 smin1 = is_jmp32 ? (s64)reg1->s32_min_value : reg1->smin_value;
s64 smax1 = is_jmp32 ? (s64)reg1->s32_max_value : reg1->smax_value;
- u64 uval = is_jmp32 ? (u32)tnum_subreg(reg2->var_off).value : reg2->var_off.value;
- s64 sval = is_jmp32 ? (s32)uval : (s64)uval;
+ u64 umin2 = is_jmp32 ? (u64)reg2->u32_min_value : reg2->umin_value;
+ u64 umax2 = is_jmp32 ? (u64)reg2->u32_max_value : reg2->umax_value;
+ s64 smin2 = is_jmp32 ? (s64)reg2->s32_min_value : reg2->smin_value;
+ s64 smax2 = is_jmp32 ? (s64)reg2->s32_max_value : reg2->smax_value;

switch (opcode) {
case BPF_JEQ:
- if (tnum_is_const(t1))
- return !!tnum_equals_const(t1, uval);
- else if (uval < umin1 || uval > umax1)
+ /* constants, umin/umax and smin/smax checks would be
+ * redundant in this case because they all should match
+ */
+ if (tnum_is_const(t1) && tnum_is_const(t2))
+ return t1.value == t2.value;
+ /* const ranges */
+ if (umin1 == umax1 && umin2 == umax2)
+ return umin1 == umin2;
+ if (smin1 == smax1 && smin2 == smax2)
+ return smin1 == smin2;
+ /* non-overlapping ranges */
+ if (umin1 > umax2 || umax1 < umin2)
return 0;
- else if (sval < smin1 || sval > smax1)
+ if (smin1 > smax2 || smax1 < smin2)
return 0;
break;
case BPF_JNE:
- if (tnum_is_const(t1))
- return !tnum_equals_const(t1, uval);
- else if (uval < umin1 || uval > umax1)
+ /* constants, umin/umax and smin/smax checks would be
+ * redundant in this case because they all should match
+ */
+ if (tnum_is_const(t1) && tnum_is_const(t2))
+ return t1.value != t2.value;
+ /* non-overlapping ranges */
+ if (umin1 > umax2 || umax1 < umin2)
return 1;
- else if (sval < smin1 || sval > smax1)
+ if (smin1 > smax2 || smax1 < smin2)
return 1;
break;
case BPF_JSET:
- if ((~t1.mask & t1.value) & uval)
+ if (!is_reg_const(reg2, is_jmp32)) {
+ swap(reg1, reg2);
+ swap(t1, t2);
+ }
+ if (!is_reg_const(reg2, is_jmp32))
+ return -1;
+ if ((~t1.mask & t1.value) & t2.value)
return 1;
- if (!((t1.mask | t1.value) & uval))
+ if (!((t1.mask | t1.value) & t2.value))
return 0;
break;
case BPF_JGT:
- if (umin1 > uval )
+ if (umin1 > umax2)
return 1;
- else if (umax1 <= uval)
+ else if (umax1 <= umin2)
return 0;
break;
case BPF_JSGT:
- if (smin1 > sval)
+ if (smin1 > smax2)
return 1;
- else if (smax1 <= sval)
+ else if (smax1 <= smin2)
return 0;
break;
case BPF_JLT:
- if (umax1 < uval)
+ if (umax1 < umin2)
return 1;
- else if (umin1 >= uval)
+ else if (umin1 >= umax2)
return 0;
break;
case BPF_JSLT:
- if (smax1 < sval)
+ if (smax1 < smin2)
return 1;
- else if (smin1 >= sval)
+ else if (smin1 >= smax2)
return 0;
break;
case BPF_JGE:
- if (umin1 >= uval)
+ if (umin1 >= umax2)
return 1;
- else if (umax1 < uval)
+ else if (umax1 < umin2)
return 0;
break;
case BPF_JSGE:
- if (smin1 >= sval)
+ if (smin1 >= smax2)
return 1;
- else if (smax1 < sval)
+ else if (smax1 < smin2)
return 0;
break;
case BPF_JLE:
- if (umax1 <= uval)
+ if (umax1 <= umin2)
return 1;
- else if (umin1 > uval)
+ else if (umin1 > umax2)
return 0;
break;
case BPF_JSLE:
- if (smax1 <= sval)
+ if (smax1 <= smin2)
return 1;
- else if (smin1 > sval)
+ else if (smin1 > smax2)
return 0;
break;
}
@@ -14341,28 +14363,28 @@ static int is_pkt_ptr_branch_taken(struct bpf_reg_state *dst_reg,
static int is_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_state *reg2,
u8 opcode, bool is_jmp32)
{
- u64 val;
-
if (reg_is_pkt_pointer_any(reg1) && reg_is_pkt_pointer_any(reg2) && !is_jmp32)
return is_pkt_ptr_branch_taken(reg1, reg2, opcode);

- /* try to make sure reg2 is a constant SCALAR_VALUE */
- if (!is_reg_const(reg2, is_jmp32)) {
- opcode = flip_opcode(opcode);
- swap(reg1, reg2);
- }
- /* for now we expect reg2 to be a constant to make any useful decisions */
- if (!is_reg_const(reg2, is_jmp32))
- return -1;
- val = reg_const_value(reg2, is_jmp32);
+ if (__is_pointer_value(false, reg1) || __is_pointer_value(false, reg2)) {
+ u64 val;
+
+ /* arrange that reg2 is a scalar, and reg1 is a pointer */
+ if (!is_reg_const(reg2, is_jmp32)) {
+ opcode = flip_opcode(opcode);
+ swap(reg1, reg2);
+ }
+ /* and ensure that reg2 is a constant */
+ if (!is_reg_const(reg2, is_jmp32))
+ return -1;

- if (__is_pointer_value(false, reg1)) {
if (!reg_not_null(reg1))
return -1;

/* If pointer is valid tests against zero will fail so we can
* use this to direct branch taken.
*/
+ val = reg_const_value(reg2, is_jmp32);
if (val != 0)
return -1;

@@ -14376,6 +14398,7 @@ static int is_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_state *reg
}
}

+ /* now deal with two scalars, but not necessarily constants */
return is_scalar_branch_taken(reg1, reg2, opcode, is_jmp32);
}

--
2.34.1