Re: [bpf, xdp] headroom - was: Re: Question about to KMSAN: uninit-value in can_receive

[Andrii Nakryiko]

On Sat, Jan 3, 2026 at 4:21 AM Oliver Hartkopp <socketcan@xxxxxxxxxxxx> wrote:
>
> Hello Jakub,
>
> thanks for stepping in!
>
> On 02.01.26 21:04, Jakub Kicinski wrote:
>
> > You're asking the wrong person, IIUC Andrii is tangentially involved
> > in XDP (via bpf links?):
> >
> (..)
> >
> > Without looking too deeply - XDP has historically left the new space
> > uninitialized after push, expecting programs to immediately write the
> > headers in that space. syzbot had run into this in the past but I can't
> > find any references to past threads quickly :(
>
> To identify Andrii I mainly looked into the code with 'git blame' that

Hey, sorry for a late response, I've been out on vacation for the past
~2 weeks. But as Jakub correctly pointed out, I'm probably not the
right person to help with this, I touched XDP bits only superficially
to wire up some generic BPF infrastructure, while the issue at hand
goes deeper than that. I'll let you guys figure this out.

> led to this problematic call chain:
>
>    pskb_expand_head+0x226/0x1a60 net/core/skbuff.c:2275
>    netif_skb_check_for_xdp net/core/dev.c:5081 [inline]
>    netif_receive_generic_xdp net/core/dev.c:5112 [inline]
>    do_xdp_generic+0x9e3/0x15a0 net/core/dev.c:5180
>
> Having in mind that the syzkaller refers to
> 6.13.0-rc7-syzkaller-00039-gc3812b15000c I wonder if we can leave this
> report as-is, as the problem might be solved in the meantime??
>
> In any case I wonder, if we should add some code to re-check if the
> headroom of the CAN-related skbs is still consistent and not changed in
> size by other players. And maybe add some WARN_ON_ONCE() before dropping
> the skb then.
>
> When the skb headroom is not safe to be used we need to be able to
> identify and solve it.
>
> Best regards,
> Oliver
>