On Mon, Nov 17, 2025 at 02:58:37PM +0100, Markus Armbruster wrote:
> Markus Armbruster <[email protected]> writes:
>
> > Daniel P. Berrangé <[email protected]> writes:
> >
> >> On Thu, Aug 07, 2025 at 03:14:56PM +0200, Markus Armbruster wrote:
> >>> Three functions in ebpf_rss.h take an Error ** argument and return bool.
> >>> Good.
> >>>
> >>> They can all fail without setting an error. Not good.
> >>>
> >>> The failures without error are:
> >>>
> >>> * All three stubs in ebpf_rss-stub.c always. Oversight?
> >>
> >> Opps, yes, we really should have added error_setg() calls for diagnosis
> >> if someone tries to use eBPF when QEMU build has it disabled.
>
> Easy enough, but...
>
> > Some stubs exist only to mollify the linker. They are not meant to be
> > called. They should abort(), optionally with lipstick.
> >
> > Other stubs are called and should fail nicely.
> >
> > Can you tell me offhand which kind these are?
>
> If calling these stubs is possible, I'd like to know how I can get them
> called, so I can test the errors I add.
>
> If calling is not possible, I'd rather add abort()s.
>
> I tried to figure out whether calling is possible, but it ended in
> confusion. Can you help?
* ebpf_rss_set_all
Is called from virtio_net_attach_ebpf_rss
The call is unreachable if ebpf_rss_is_loaded returns false
Stub for ebpf_rss_is_loaded always returns false
=> ebpf_rss_set_all stub is unreachable
* ebpf_rss_load_fds, ebpf_rss_load
Is called from virtio_net_load_ebpf_fds, which is called from
virtio_net_load_ebpf
The call to virtio_net_load_ebpf_fds is unreachable if
virtio_net_attach_ebpf_to_backend fails
virtio_net_attach_ebpf_to_backend fails if set_steering_ebpf
fails
set_steering_ebpf fails if ioctl(fd, TUNSETSTEERINGEBPF...)
fails on Linux; all non-Linux impls of ebpf_rss_load_fds
return -1
It is theoretically p9ossible to build QEMU without EBPF
while both glibc & the kernel support TUNSETSTEERINGEBPF ioctl
=> ebpf_rss_load_fds, ebpf_rss_load are reachable in stubs
> >>> * Non-stub ebpf_rss_load() when ebpf_rss_is_loaded(). Are these
> >>> reachable?
> >>
> >> This scenario should never happen, and we should add a call like
> >>
> >> error_setg(errp, "eBPF program is already loaded");
> >>
> >> to report it correctly.
> >
> > Is it a programming error when it happens?
>
> This question is still open as well.
I'd consider it a programming error. I don't think we have a code
path that could trigger it currently.
With regards,
Daniel
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