Re: [PATCH v4 bpf-next 3/7] bpf: enhance subregister bounds deduction logic

[Shung-Hsi Yu <shung-hsi.yu@suse.com>]

On Mon, Oct 23, 2023 at 11:20:46AM +0800, Shung-Hsi Yu wrote:
> On Sun, Oct 22, 2023 at 01:57:39PM -0700, Andrii Nakryiko wrote:
> > Add handling of a bunch of possible cases which allows deducing extra
> > information about subregister bounds, both u32 and s32, from full register
> > u64/s64 bounds.
> > 
> > Also add smin32/smax32 bounds derivation from corresponding umin32/umax32
> > bounds, similar to what we did with smin/smax from umin/umax derivation in
> > previous patch.
> > 
> > Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> > 
> > ---
> >  kernel/bpf/verifier.c | 52 +++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 52 insertions(+)
> > 
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 885dd4a2ff3a..3fc9bd5e72b8 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -2130,6 +2130,58 @@ static void __update_reg_bounds(struct bpf_reg_state *reg)
> >  /* Uses signed min/max values to inform unsigned, and vice-versa */
> >  static void __reg32_deduce_bounds(struct bpf_reg_state *reg)
> >  {
> > +	/* if upper 32 bits of u64/s64 range don't change,
> > +	 * we can use lower 32 bits to improve our u32/s32 boundaries
> > +	 */
> > +	if ((reg->umin_value >> 32) == (reg->umax_value >> 32)) {
> > +		/* u64 to u32 casting preserves validity of low 32 bits as
> > +		 * a range, if upper 32 bits are the same
> > +		 */
> > +		reg->u32_min_value = max_t(u32, reg->u32_min_value, (u32)reg->umin_value);
> > +		reg->u32_max_value = min_t(u32, reg->u32_max_value, (u32)reg->umax_value);
> > +
> > +		if ((s32)reg->umin_value <= (s32)reg->umax_value) {
> > +			reg->s32_min_value = max_t(s32, reg->s32_min_value, (s32)reg->umin_value);
> > +			reg->s32_max_value = min_t(s32, reg->s32_max_value, (s32)reg->umax_value);
> > +		}
> > +	}
> > +	if ((reg->smin_value >> 32) == (reg->smax_value >> 32)) {
> > +		/* low 32 bits should form a proper u32 range */
> > +		if ((u32)reg->smin_value <= (u32)reg->smax_value) {
> > +			reg->u32_min_value = max_t(u32, reg->u32_min_value, (u32)reg->smin_value);
> > +			reg->u32_max_value = min_t(u32, reg->u32_max_value, (u32)reg->smax_value);
> > +		}
> > +		/* low 32 bits should form a proper s32 range */
> > +		if ((s32)reg->smin_value <= (s32)reg->smax_value) {
> > +			reg->s32_min_value = max_t(s32, reg->s32_min_value, (s32)reg->smin_value);
> > +			reg->s32_max_value = min_t(s32, reg->s32_max_value, (s32)reg->smax_value);
> > +		}
> > +	}
> > +	/* Special case where upper bits form a small sequence of two
> > +	 * sequential numbers (in 32-bit unsigned space, so 0xffffffff to
> > +	 * 0x00000000 is also valid), while lower bits form a proper s32 range
> > +	 * going from negative numbers to positive numbers.
> > +	 * E.g.: [0xfffffff0ffffff00; 0xfffffff100000010]. Iterating
> > +	 * over full 64-bit numbers range will form a proper [-16, 16]
> > +	 * ([0xffffff00; 0x00000010]) range in its lower 32 bits.
> > +	 */

Oops, scratch that, these below is not entirely incorrect.

> Not sure if we want ascii art here but though it'd be useful to share. It
> took a while to wrap my head around this concept until I look at this as
> number lines.
> 
> Say we've got umin, umax tracked like so (asterisk * marks the sequence of
> numbers we believe is possible to occur).
> 
>               u64            
>   |--------***--------------|
>    {  32-bits }{  32-bits  }
> 
> And s32_min, s32_max tracked liked so.
> 
>                            s32
>                      |***---------|
> 
> The above u64 range can be mapped into two possible s32 range when we've
> removed the upper 32-bits.

The u64 range can be mapped into 2^32 possible s32 ranges. So the s32 ranges
view has been enlarged 2^32 here.

And I'm also missing the condition that it crosses U32_MAX in u32 range.

I will redo the graphs.

>               u64               same u64 wrapped
>   |--------***--------------|-----...
>            |||
>         |--***-------|------------|
>               s32          s32
>  
> Since both s32 range are possible, we take the union between then, and the
> s32 range we're already tracking
> 
>         |------------|
>         |--***-------|
>         |***---------|
> 
> And arrives at the final s32 range.
> 
>         |*****-------|
> 
> Taking this (wrapped) number line view and operates them with set operations
> (latter is similar to what tnum does) is quite useful and I think hints that
> we may be able to unify signed and unsigned range tracking. I'll look into
> this a bit more and send a follow up.
> 
> > +	if ((u32)(reg->umin_value >> 32) + 1 == (u32)(reg->umax_value >> 32) &&
> > +	    (s32)reg->umin_value < 0 && (s32)reg->umax_value >= 0) {
> > +		reg->s32_min_value = max_t(s32, reg->s32_min_value, (s32)reg->umin_value);
> > +		reg->s32_max_value = min_t(s32, reg->s32_max_value, (s32)reg->umax_value);
> > +	}
> > +	if ((u32)(reg->smin_value >> 32) + 1 == (u32)(reg->smax_value >> 32) &&
> > +	    (s32)reg->smin_value < 0 && (s32)reg->smax_value >= 0) {
> > +		reg->s32_min_value = max_t(s32, reg->s32_min_value, (s32)reg->smin_value);
> > +		reg->s32_max_value = min_t(s32, reg->s32_max_value, (s32)reg->smax_value);
> > +	}
> > +	/* if u32 range forms a valid s32 range (due to matching sign bit),
> > +	 * try to learn from that
> > +	 */
> > +	if ((s32)reg->u32_min_value <= (s32)reg->u32_max_value) {
> > +		reg->s32_min_value = max_t(s32, reg->s32_min_value, reg->u32_min_value);
> > +		reg->s32_max_value = min_t(s32, reg->s32_max_value, reg->u32_max_value);
> > +	}
> >  	/* Learn sign from signed bounds.
> >  	 * If we cannot cross the sign boundary, then signed and unsigned bounds
> >  	 * are the same, so combine.  This works even in the negative case, e.g.
> > -- 
> > 2.34.1
> >