On Sun, Nov 5, 2023 at 6:11 PM Shung-Hsi Yu <shung-hsi.yu@xxxxxxxx> wrote: > > Note: Andrii' patch mentioned in the Link tag isn't merge yet, I'll > resend this along with the proposed refactoring once it is merged. > For now, sending the patch as RFC for feedback and review. > > While the BPF instruction set does not contain a bitwise-NOT > instruction, the verifier may still need to compute the bitwise-NOT > result for the value tracked in the register. One such case reference in > the link below is > > u64 val; > val = reg_const_value(reg2, is_jmp32); > tnum_ops(..., tnum_const(~val); > > Where the value is extract of out tnum, operated with bitwise-NOT, then > simply turned back into tnum again; plus it has the limitation of only > working on constant. This commit adds the tnum_not() helper that compute > the bitwise-NOT result for all the values tracked within the tnum, that > allow us to simplify the above code to > > tnum_ops(..., tnum_not(reg2->var_off)); > > without being limited to constant, and is general enough to be reused > and composed with other tnum operations. > > Link: https://lore.kernel.org/bpf/ZUSwQtfjCsKpbWcL@u94a/ > Signed-off-by: Shung-Hsi Yu <shung-hsi.yu@xxxxxxxx> > --- > > include/linux/tnum.h | 2 ++ > kernel/bpf/tnum.c | 5 +++++ > 2 files changed, 7 insertions(+) > > diff --git a/include/linux/tnum.h b/include/linux/tnum.h > index 1c3948a1d6ad..817065df1297 100644 > --- a/include/linux/tnum.h > +++ b/include/linux/tnum.h > @@ -46,6 +46,8 @@ struct tnum tnum_and(struct tnum a, struct tnum b); > struct tnum tnum_or(struct tnum a, struct tnum b); > /* Bitwise-XOR, return @a ^ @b */ > struct tnum tnum_xor(struct tnum a, struct tnum b); > +/* Bitwise-NOT, return ~@a */ > +struct tnum tnum_not(struct tnum a); > /* Multiply two tnums, return @a * @b */ > struct tnum tnum_mul(struct tnum a, struct tnum b); > > diff --git a/kernel/bpf/tnum.c b/kernel/bpf/tnum.c > index 3d7127f439a1..b4f4a4beb0c9 100644 > --- a/kernel/bpf/tnum.c > +++ b/kernel/bpf/tnum.c > @@ -111,6 +111,11 @@ struct tnum tnum_xor(struct tnum a, struct tnum b) > return TNUM(v & ~mu, mu); > } > > +struct tnum tnum_not(struct tnum a) > +{ > + return TNUM(~a.value & ~a.mask, a.mask); > +} > + In isolation this does look like it's implementing the tnum version of ~x, so I have no objections to this. But I'm not sure it actually simplifies anything in my patches. But let's see, once it lands, please send a follow up applying this tnum_not(). > /* Generate partial products by multiplying each bit in the multiplier (tnum a) > * with the multiplicand (tnum b), and add the partial products after > * appropriately bit-shifting them. Instead of directly performing tnum addition > > base-commit: 1a119e269dc69e82217525d92a93e082c4424fc8 > -- > 2.42.0 >
