Re: [Bug Report] bpf: incorrectly pruning runtime execution path

[Eduard Zingerman]

On Wed, 2023-12-13 at 15:40 -0800, Andrii Nakryiko wrote:
[...]
> >     24: (18) r2 = 0x4                     ; R2_w=4
> >     26: (7e) if w8 s>= w0 goto pc+5
> >     mark_precise: frame0: last_idx 26 first_idx 22 subseq_idx -1
> >     mark_precise: frame0: regs=r5,r8 stack= before 24: (18) r2 = 0x4
> >     ...                   ^^^^^^^^^^
> >                           ^^^^^^^^^^
> > Here w8 == 15, w0 in range [0, 2], so the jump is being predicted,
> > but for some reason R0 is not among the registers that would be marked precise.
> 
> It is, as a second step. There are two concatenated precision logs:
> 
> mark_precise: frame0: last_idx 26 first_idx 22 subseq_idx -1
> mark_precise: frame0: regs=r0 stack= before 24: (18) r2 = 0x4
> mark_precise: frame0: regs=r0 stack= before 23: (bf) r5 = r8
> mark_precise: frame0: regs=r0 stack= before 22: (67) r4 <<= 2
> 
> 
> The issue is elsewhere, see my last email.

Oh, right, there are two calls to mark_chain_precision in a row, thanks