Re: [PATCH v5 bpf-next 14/23] bpf: generalize is_branch_taken to handle all conditional jumps in one place

Eduard Zingerman
2023-10-31T15:38:33Z

On Fri, 2023-10-27 at 11:13 -0700, Andrii Nakryiko wrote:
> > Make is_branch_taken() a single entry point for branch pruning decision
> > making, handling both pointer vs pointer, pointer vs scalar, and scalar
> > vs scalar cases in one place. This also nicely cleans up check_cond_jmp_op().
> >
> > Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

Acked-by: Eduard Zingerman <eddyz87@gmail.com>

> > ---
> > kernel/bpf/verifier.c | 49 ++++++++++++++++++++++---------------------
> > 1 file changed, 25 insertions(+), 24 deletions(-)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 25b5234ebda3..fedd6d0e76e5 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -14169,6 +14169,19 @@ static void find_good_pkt_pointers(struct bpf_verifier_state *vstate,
> > }));
> > }
> >
> > +/* check if register is a constant scalar value */
> > +static bool is_reg_const(struct bpf_reg_state *reg, bool subreg32)
> > +{
> > + return reg->type == SCALAR_VALUE &&
> > + tnum_is_const(subreg32 ? tnum_subreg(reg->var_off) : reg->var_off);
> > +}
> > +
> > +/* assuming is_reg_const() is true, return constant value of a register */
> > +static u64 reg_const_value(struct bpf_reg_state *reg, bool subreg32)
> > +{
> > + return subreg32 ? tnum_subreg(reg->var_off).value : reg->var_off.value;
> > +}
> > +
> > /*
> > * <reg1> <op> <reg2>, currently assuming reg2 is a constant
> > */
> > @@ -14410,12 +14423,20 @@ 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)
> > {
> > - struct tnum reg2_tnum = is_jmp32 ? tnum_subreg(reg2->var_off) : reg2->var_off;
> > u64 val;
> >
> > - if (!tnum_is_const(reg2_tnum))
> > + 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 = reg2_tnum.value;
> > + val = reg_const_value(reg2, is_jmp32);
> >
> > if (__is_pointer_value(false, reg1)) {
> > if (!reg_not_null(reg1))
> > @@ -14896,27 +14917,7 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
> > }
> >
> > is_jmp32 = BPF_CLASS(insn->code) == BPF_JMP32;
> > -
> > - if (BPF_SRC(insn->code) == BPF_K) {
> > - pred = is_branch_taken(dst_reg, src_reg, opcode, is_jmp32);
> > - } else if (src_reg->type == SCALAR_VALUE &&
> > - is_jmp32 && tnum_is_const(tnum_subreg(src_reg->var_off))) {
> > - pred = is_branch_taken(dst_reg, src_reg, opcode, is_jmp32);
> > - } else if (src_reg->type == SCALAR_VALUE &&
> > - !is_jmp32 && tnum_is_const(src_reg->var_off)) {
> > - pred = is_branch_taken(dst_reg, src_reg, opcode, is_jmp32);
> > - } else if (dst_reg->type == SCALAR_VALUE &&
> > - is_jmp32 && tnum_is_const(tnum_subreg(dst_reg->var_off))) {
> > - pred = is_branch_taken(src_reg, dst_reg, flip_opcode(opcode), is_jmp32);
> > - } else if (dst_reg->type == SCALAR_VALUE &&
> > - !is_jmp32 && tnum_is_const(dst_reg->var_off)) {
> > - pred = is_branch_taken(src_reg, dst_reg, flip_opcode(opcode), is_jmp32);
> > - } else if (reg_is_pkt_pointer_any(dst_reg) &&
> > - reg_is_pkt_pointer_any(src_reg) &&
> > - !is_jmp32) {
> > - pred = is_pkt_ptr_branch_taken(dst_reg, src_reg, opcode);
> > - }
> > -
> > + pred = is_branch_taken(dst_reg, src_reg, opcode, is_jmp32);
> > if (pred >= 0) {
> > /* If we get here with a dst_reg pointer type it is because
> > * above is_branch_taken() special cased the 0 comparison.