On Fri, Nov 3, 2023 at 12:52 AM Shung-Hsi Yu <shung-hsi.yu@xxxxxxxx> wrote:
>
> On Thu, Nov 02, 2023 at 05:08:10PM -0700, Andrii Nakryiko wrote:
> > Generalize bounds adjustment logic of reg_set_min_max() to handle not
> > just register vs constant case, but in general any register vs any
> > register cases. For most of the operations it's trivial extension based
> > on range vs range comparison logic, we just need to properly pick
> > min/max of a range to compare against min/max of the other range.
> >
> > For BPF_JSET we keep the original capabilities, just make sure JSET is
> > integrated in the common framework. This is manifested in the
> > internal-only BPF_KSET + BPF_X "opcode" to allow for simpler and more
> ^ typo?
>
> Two more comments below
>
> > uniform rev_opcode() handling. See the code for details. This allows to
> > reuse the same code exactly both for TRUE and FALSE branches without
> > explicitly handling both conditions with custom code.
> >
> > Note also that now we don't need a special handling of BPF_JEQ/BPF_JNE
> > case none of the registers are constants. This is now just a normal
> > generic case handled by reg_set_min_max().
> >
> > To make tnum handling cleaner, tnum_with_subreg() helper is added, as
> > that's a common operator when dealing with 32-bit subregister bounds.
> > This keeps the overall logic much less noisy when it comes to tnums.
> >
> > Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> > ---
> > include/linux/tnum.h | 4 +
> > kernel/bpf/tnum.c | 7 +-
> > kernel/bpf/verifier.c | 327 ++++++++++++++++++++----------------------
> > 3 files changed, 165 insertions(+), 173 deletions(-)
> >
> > diff --git a/include/linux/tnum.h b/include/linux/tnum.h
> > index 1c3948a1d6ad..3c13240077b8 100644
> > --- a/include/linux/tnum.h
> > +++ b/include/linux/tnum.h
> > @@ -106,6 +106,10 @@ int tnum_sbin(char *str, size_t size, struct tnum a);
> > struct tnum tnum_subreg(struct tnum a);
> > /* Returns the tnum with the lower 32-bit subreg cleared */
> > struct tnum tnum_clear_subreg(struct tnum a);
> > +/* Returns the tnum with the lower 32-bit subreg in *reg* set to the lower
> > + * 32-bit subreg in *subreg*
> > + */
> > +struct tnum tnum_with_subreg(struct tnum reg, struct tnum subreg);
> > /* Returns the tnum with the lower 32-bit subreg set to value */
> > struct tnum tnum_const_subreg(struct tnum a, u32 value);
> > /* Returns true if 32-bit subreg @a is a known constant*/
> > diff --git a/kernel/bpf/tnum.c b/kernel/bpf/tnum.c
> > index 3d7127f439a1..f4c91c9b27d7 100644
> > --- a/kernel/bpf/tnum.c
> > +++ b/kernel/bpf/tnum.c
> > @@ -208,7 +208,12 @@ struct tnum tnum_clear_subreg(struct tnum a)
> > return tnum_lshift(tnum_rshift(a, 32), 32);
> > }
> >
> > +struct tnum tnum_with_subreg(struct tnum reg, struct tnum subreg)
> > +{
> > + return tnum_or(tnum_clear_subreg(reg), tnum_subreg(subreg));
> > +}
> > +
> > struct tnum tnum_const_subreg(struct tnum a, u32 value)
> > {
> > - return tnum_or(tnum_clear_subreg(a), tnum_const(value));
> > + return tnum_with_subreg(a, tnum_const(value));
> > }
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 2197385d91dc..52934080042c 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -14379,218 +14379,211 @@ static int is_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_state *reg
> > return is_scalar_branch_taken(reg1, reg2, opcode, is_jmp32);
> > }
> >
> > -/* Adjusts the register min/max values in the case that the dst_reg and
> > - * src_reg are both SCALAR_VALUE registers (or we are simply doing a BPF_K
> > - * check, in which case we havea fake SCALAR_VALUE representing insn->imm).
> > - * Technically we can do similar adjustments for pointers to the same object,
> > - * but we don't support that right now.
> > +/* Opcode that corresponds to a *false* branch condition.
> > + * E.g., if r1 < r2, then reverse (false) condition is r1 >= r2
> > */
> > -static void reg_set_min_max(struct bpf_reg_state *true_reg1,
> > - struct bpf_reg_state *true_reg2,
> > - struct bpf_reg_state *false_reg1,
> > - struct bpf_reg_state *false_reg2,
> > - u8 opcode, bool is_jmp32)
> > +static u8 rev_opcode(u8 opcode)
>
> Nit: rev_opcode and flip_opcode seems like a possible source of confusing
> down the line. Flip and reverse are often interchangable, i.e. "flip the
> order" and "reverse the order" is the same thing.
>
> Maybe "neg_opcode" or "neg_cond_opcode"?
neg has too strong connotation with BPF_NEG, so not really happy with
this one. In selftest I used "complement_op", but it's also quite
arbitrary.
>
> Or do it the otherway around, keep rev_opcode but rename flip_opcode.
how about flip_opcode -> swap_opcode? and then keep reg_opcode as is?
>
> One more comment about BPF_JSET below
>
please trim big chunks of code you are not commenting on to keep
emails a bit shorter
[...]
> > if (is_jmp32) {
> > - __mark_reg32_known(false_reg1, uval32);
> > - false_32off = tnum_subreg(false_reg1->var_off);
> > + if (opcode & BPF_X)
> > + t = tnum_and(tnum_subreg(reg1->var_off), tnum_const(~val));
> > + else
> > + t = tnum_or(tnum_subreg(reg1->var_off), tnum_const(val));
> > + reg1->var_off = tnum_with_subreg(reg1->var_off, t);
> > } else {
> > - ___mark_reg_known(false_reg1, uval);
> > - false_64off = false_reg1->var_off;
> > + if (opcode & BPF_X)
> > + reg1->var_off = tnum_and(reg1->var_off, tnum_const(~val));
> > + else
> > + reg1->var_off = tnum_or(reg1->var_off, tnum_const(val));
> > }
> > break;
>
> Since you're already adding a tnum helper, I think we can add one more
> for BPF_JSET here
>
> struct tnum tnum_neg(struct tnum a)
> {
> return TNUM(~a.value, a.mask);
> }
>
I'm not sure what tnum_neg() does (even if the correct
implementation), but either way I'd like to minimize touching tnum
stuff, it's too tricky :) we can address that as a separate patch if
you'd like
> So instead of getting a value out of tnum then putting the value back
> into tnum again
>
> u64 val;
> val = reg_const_value(reg2, is_jmp32);
> tnum_ops(..., tnum_const(val or ~val);
>
> Keep the value in tnum and process it as-is if possible
>
> tnum_ops(..., reg2->var_off or tnum_neg(reg2->var_off));
>
> And with that hopefully make this fragment short enough that we don't
> mind duplicate a bit of code to seperate the BPF_JSET case from the
> BPF_JSET | BPF_X case. IMO a conditional is_power_of_2 check followed by
> two level of branching is a bit too much to follow, it is better to have
> them seperated just like how you're doing it for the others already.
I can split those two cases without any new tnum helpers, the
duplicated part is just const checking, basically, no big deal
>
> I.e. something like the follow
>
> case BPF_JSET: {
> if (!is_reg_const(reg2, is_jmp32))
> swap(reg1, reg2);
> if (!is_reg_const(reg2, is_jmp32))
> break;
> /* comment */
> if (!is_power_of_2(reg_const_value(reg2, is_jmp32))
> break;
>
> if (is_jmp32) {
> t = tnum_or(tnum_subreg(reg1->var_off), tnum_subreg(reg2->var_off));
> reg1->var_off = tnum_with_subreg(reg1->var_off, t);
> } else {
> reg1->var_off = tnum_or(reg1->var_off, reg2->var_off);
> }
> break;
> }
> case BPF_JSET | BPF_X: {
> if (!is_reg_const(reg2, is_jmp32))
> swap(reg1, reg2);
> if (!is_reg_const(reg2, is_jmp32))
> break;
>
> if (is_jmp32) {
> /* a slightly long line ... */
> t = tnum_and(tnum_subreg(reg1->var_off), tnum_neg(tnum_subreg(reg2->var_off)));
> reg1->var_off = tnum_with_subreg(reg1->var_off, t);
> } else {
> reg1->var_off = tnum_and(reg1->var_off, tnum_neg(reg2->var_off));
> }
> break;
> }
>
> > ...
