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bpf: propagate nullness information for reg to reg comparisons 

Propagate nullness information for branches of register to register
equality compare instructions. The following rules are used:
- suppose register A maybe null
- suppose register B is not null
- for JNE A, B, ... - A is not null in the false branch
- for JEQ A, B, ... - A is not null in the true branch

E.g. for program like below:

  r6 = skb->sk;
  r7 = sk_fullsock(r6);
  r0 = sk_fullsock(r6);
  if (r0 == 0) return 0;    (a)
  if (r0 != r7) return 0;   (b)
  *r7->type;                (c)
  return 0;

It is safe to dereference r7 at point (c), because of (a) and (b).

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20221115224859.2452988-2-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Eduard Zingerman 2022-11-16 00:48:58 +02:00 committed by Alexei Starovoitov
parent 32637e3300
commit befae75856
1 changed files with 33 additions and 2 deletions
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35
kernel/bpf/verifier.c
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@ -10267,6 +10267,7 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
struct bpf_verifier_state *other_branch;
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;
u8 opcode = BPF_OP(insn->code);
bool is_jmp32;
int pred = -1;
@ -10376,8 +10377,8 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
/* 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, but we don't support that right now), because
* otherwise the different base pointers mean the offsets aren't
* 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) {
@ -10426,6 +10427,36 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
find_equal_scalars(other_branch, &other_branch_regs[insn->dst_reg]);
}
/* if one pointer register is compared to another pointer
* register check if PTR_MAYBE_NULL could be lifted.
* E.g. register A - maybe null
* register B - not null
* for JNE A, B, ... - A is not null in the false branch;
* for JEQ A, B, ... - A is not null in the true branch.
*/
if (!is_jmp32 && BPF_SRC(insn->code) == BPF_X &&
__is_pointer_value(false, src_reg) && __is_pointer_value(false, dst_reg) &&
type_may_be_null(src_reg->type) != type_may_be_null(dst_reg->type)) {
eq_branch_regs = NULL;
switch (opcode) {
case BPF_JEQ:
eq_branch_regs = other_branch_regs;
break;
case BPF_JNE:
eq_branch_regs = regs;
break;
default:
/* do nothing */
break;
}
if (eq_branch_regs) {
if (type_may_be_null(src_reg->type))
mark_ptr_not_null_reg(&eq_branch_regs[insn->src_reg]);
else
mark_ptr_not_null_reg(&eq_branch_regs[insn->dst_reg]);
}
}
/* detect if R == 0 where R is returned from bpf_map_lookup_elem().
* NOTE: these optimizations below are related with pointer comparison
* which will never be JMP32.