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bpf: teach verifier to recognize imm += ptr pattern 

Humans don't write C code like:
  u8 *ptr = skb->data;
  int imm = 4;
  imm += ptr;
but from llvm backend point of view 'imm' and 'ptr' are registers and
imm += ptr may be preferred vs ptr += imm depending which register value
will be used further in the code, while verifier can only recognize ptr += imm.
That caused small unrelated changes in the C code of the bpf program to
trigger rejection by the verifier. Therefore teach the verifier to recognize
both ptr += imm and imm += ptr.
For example:
when R6=pkt(id=0,off=0,r=62) R7=imm22
after r7 += r6 instruction
will be R6=pkt(id=0,off=0,r=62) R7=pkt(id=0,off=22,r=62)

Fixes: 969bf05eb3 ("bpf: direct packet access")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Alexei Starovoitov 2016-05-19 18:17:14 -07:00 committed by David S. Miller
parent d91b28ed42
commit 1b9b69ecb3
1 changed files with 17 additions and 1 deletions
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18
kernel/bpf/verifier.c
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@ -1245,6 +1245,7 @@ static int check_packet_ptr_add(struct verifier_env *env, struct bpf_insn *insn)
struct reg_state *regs = env->cur_state.regs;
struct reg_state *dst_reg = &regs[insn->dst_reg];
struct reg_state *src_reg = &regs[insn->src_reg];
struct reg_state tmp_reg;
s32 imm;
if (BPF_SRC(insn->code) == BPF_K) {
@ -1267,6 +1268,19 @@ static int check_packet_ptr_add(struct verifier_env *env, struct bpf_insn *insn)
*/
dst_reg->off += imm;
} else {
if (src_reg->type == PTR_TO_PACKET) {
/* R6=pkt(id=0,off=0,r=62) R7=imm22; r7 += r6 */
tmp_reg = *dst_reg; /* save r7 state */
*dst_reg = *src_reg; /* copy pkt_ptr state r6 into r7 */
src_reg = &tmp_reg; /* pretend it's src_reg state */
/* if the checks below reject it, the copy won't matter,
* since we're rejecting the whole program. If all ok,
* then imm22 state will be added to r7
* and r7 will be pkt(id=0,off=22,r=62) while
* r6 will stay as pkt(id=0,off=0,r=62)
*/
}
if (src_reg->type == CONST_IMM) {
/* pkt_ptr += reg where reg is known constant */
imm = src_reg->imm;
@ -1565,7 +1579,9 @@ static int check_alu_op(struct verifier_env *env, struct bpf_insn *insn)
return 0;
} else if (opcode == BPF_ADD &&
BPF_CLASS(insn->code) == BPF_ALU64 &&
dst_reg->type == PTR_TO_PACKET) {
(dst_reg->type == PTR_TO_PACKET ||
(BPF_SRC(insn->code) == BPF_X &&
regs[insn->src_reg].type == PTR_TO_PACKET))) {
/* ptr_to_packet += K|X */
return check_packet_ptr_add(env, insn);
} else if (BPF_CLASS(insn->code) == BPF_ALU64 &&