On Mon, Nov 06, 2023 at 11:56:22AM -0800, Andrii Nakryiko wrote: > 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> > > --- [...] > > 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(). Okay, will send once it lands. [...]
