Re: [PATCH net] netdevsim: Initialize all fields of ip header when building dummy sk_buff

[Nikola Z. Ivanov]

On 4/21/26 11:19 AM, Breno Leitao wrote:
> On Tue, Apr 21, 2026 at 10:37:38AM +0300, Nikola Z. Ivanov wrote:
> > Syzbot reports a KMSAN uninit-value originating from
> > nsim_dev_trap_skb_build, with the allocation also
> > being performed in the same function.
> >
> > The cause of the KMSAN warning is a missing assignment of
> > the tos and id fields of the ip header.
> >
> > Fix this by calling skb_put_zero instead of skb_put to
> > guarantee null initialization.
> > Additionally remove the now redundant zero assignments
> > and reorder the remaining ones so that they more closely
> > match the order of the fields as they appear in the ip header.
> >
> > Closes: https://syzkaller.appspot.com/bug?extid=23d7fcd204e3837866ff
> How do you check in the report above that the missig un-initialized
> fields are "tos" and "id"?
>
> Thanks for the fix,
> --breno
Hi Breno,

I don't think it is visible here, my guess would
be because the checksum calculator walks the
header in small chunks instead of referencing
its fields.

The whole "KMSAN: uninit-value in irqentry_exit_to_kernel_mode_preempt"
doesn't really sound quite right.

Thank you!