Re: ebpf functions can fail without setting an error

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Nov 17, 2025 at 02:58:37PM +0100, Markus Armbruster wrote:
> Markus Armbruster <armbru@redhat.com> writes:
> 
> > Daniel P. Berrangé <berrange@redhat.com> 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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