Re: [syzbot ci] Re: bpf: Use tnums for JEQ/JNE is_branch_taken logic

[Paul Chaignon <paul.chaignon@gmail.com>]

On Wed, Aug 20, 2025 at 12:37:46PM -0700, Eduard Zingerman wrote:
> On Wed, 2025-08-20 at 13:34 +0200, Paul Chaignon wrote:
> 
> [...]
> 
> > I have a patch to potentially fix this, but I'm still testing it and
> > would prefer to send it separately as it doesn't really relate to my
> > current patchset.
> 
> I'd like to bring this point again: this is a cat-and-mouse game.
> is_scalar_branch_taken() and regs_refine_cond_op() are essentially
> same operation and should be treated as such: produce register states
> for both branches and prune those that result in an impossible state.
> There is nothing wrong with this logically and we haven't got a single
> real bug from the invariant violations check if I remember correctly.
> 
> Comparing the two functions, it looks like tricky cases are BPF_JE/JNE
> and BPF_JSET/JSET|BPF_X. However, given that regs_refine_cond_op() is
> called for a false branch with opcode reversed it looks like there is
> no issues with these cases.
> 
> I'll give this a try.

Hi Eduard,

Did you get a chance to look into this? syzkaller came back (finally)
complaining about the remaining invariant violations:
https://lore.kernel.org/bpf/68bacb3e.050a0220.192772.018d.GAE@google.com/
If not, I can have a look at the end of the week.

Paul

> 
> [...]