On Thu, Nov 2, 2023 at 7:40 AM Shung-Hsi Yu <shung-hsi.yu@xxxxxxxx> wrote: > > On Wed, Nov 01, 2023 at 08:37:49PM -0700, Andrii Nakryiko wrote: > > Add a few interesting cases in which we can tighten 64-bit bounds based > > on newly learnt information about 32-bit bounds. E.g., when full u64/s64 > > registers are used in BPF program, and then eventually compared as > > u32/s32. The latter comparison doesn't change the value of full > > register, but it does impose new restrictions on possible lower 32 bits > > of such full registers. And we can use that to derive additional full > > register bounds information. > > > > Acked-by: Eduard Zingerman <eddyz87@xxxxxxxxx> > > Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx> > > Acked-by: Shung-Hsi Yu <shung-hsi.yu@xxxxxxxx> > > One question below > > > --- > > kernel/bpf/verifier.c | 44 +++++++++++++++++++++++++++++++++++++++++++ > > 1 file changed, 44 insertions(+) > > > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > > index 08888784cbc8..d0d0a1a1b662 100644 > > --- a/kernel/bpf/verifier.c > > +++ b/kernel/bpf/verifier.c > > @@ -2536,10 +2536,54 @@ static void __reg64_deduce_bounds(struct bpf_reg_state *reg) > > } > > } > > > > +static void __reg_deduce_mixed_bounds(struct bpf_reg_state *reg) > > +{ > > + /* Try to tighten 64-bit bounds from 32-bit knowledge, using 32-bit > > + * values on both sides of 64-bit range in hope to have tigher range. > > + * E.g., if r1 is [0x1'00000000, 0x3'80000000], and we learn from > > + * 32-bit signed > 0 operation that s32 bounds are now [1; 0x7fffffff]. > > + * With this, we can substitute 1 as low 32-bits of _low_ 64-bit bound > > + * (0x100000000 -> 0x100000001) and 0x7fffffff as low 32-bits of > > + * _high_ 64-bit bound (0x380000000 -> 0x37fffffff) and arrive at a > > + * better overall bounds for r1 as [0x1'000000001; 0x3'7fffffff]. > > + * We just need to make sure that derived bounds we are intersecting > > + * with are well-formed ranges in respecitve s64 or u64 domain, just > > + * like we do with similar kinds of 32-to-64 or 64-to-32 adjustments. > > + */ > > + __u64 new_umin, new_umax; > > + __s64 new_smin, new_smax; > > + > > + /* u32 -> u64 tightening, it's always well-formed */ > > + new_umin = (reg->umin_value & ~0xffffffffULL) | reg->u32_min_value; > > + new_umax = (reg->umax_value & ~0xffffffffULL) | reg->u32_max_value; > > + reg->umin_value = max_t(u64, reg->umin_value, new_umin); > > + reg->umax_value = min_t(u64, reg->umax_value, new_umax); > > + /* u32 -> s64 tightening, u32 range embedded into s64 preserves range validity */ > > + new_smin = (reg->smin_value & ~0xffffffffULL) | reg->u32_min_value; > > + new_smax = (reg->smax_value & ~0xffffffffULL) | reg->u32_max_value; > > + reg->smin_value = max_t(s64, reg->smin_value, new_smin); > > + reg->smax_value = min_t(s64, reg->smax_value, new_smax); > > + > > + /* if s32 can be treated as valid u32 range, we can use it as well */ > > + if ((u32)reg->s32_min_value <= (u32)reg->s32_max_value) { > > + /* s32 -> u64 tightening */ > > + new_umin = (reg->umin_value & ~0xffffffffULL) | (u32)reg->s32_min_value; > > + new_umax = (reg->umax_value & ~0xffffffffULL) | (u32)reg->s32_max_value; > > + reg->umin_value = max_t(u64, reg->umin_value, new_umin); > > + reg->umax_value = min_t(u64, reg->umax_value, new_umax); > > + /* s32 -> s64 tightening */ > > + new_smin = (reg->smin_value & ~0xffffffffULL) | (u32)reg->s32_min_value; > > + new_smax = (reg->smax_value & ~0xffffffffULL) | (u32)reg->s32_max_value; > > + reg->smin_value = max_t(s64, reg->smin_value, new_smin); > > + reg->smax_value = min_t(s64, reg->smax_value, new_smax); > > + } > > +} > > + > > Guess this might be something you've considered already, but I think it > won't hurt to ask: > > All verifier.c patches up to till this point all use a lot of > > reg->min_value = max_t(typeof(reg->min_value), reg->min_value, new_min); > reg->max_value = min_t(typeof(reg->max_value), reg->max_value, new_max); > > where min_value/max_value is one of umin, smin, u32, or s32. Could we > refactor those out with some form of > > reg_bounds_intersect(reg, new_min, new_max) > > The point of this is not really about reducing the line of code, but to > reduce the cognitive load of juggling all the min_t and max_t. With > something reg_bounds_intersect() we only need to check that > new_min/new_max pair is valid and trust the macro/function itself to > handle the rest correctly. Yes, I thought about that. And it should be doable with macro and a bunch of refactoring. I decided to leave it to future follow ups, as there is already plenty of refactoring happing. > > > static void __reg_deduce_bounds(struct bpf_reg_state *reg) > > { > > __reg32_deduce_bounds(reg); > > __reg64_deduce_bounds(reg); > > + __reg_deduce_mixed_bounds(reg); > > } > > > > /* Attempts to improve var_off based on unsigned min/max information */ > > -- > > 2.34.1 > >