Switching to vmlinux.h definitions seems to make the verifier very
unhappy with bitfield accesses. The error is:
; md.u.md2.dir = direction;
33: (69) r1 = *(u16 *)(r2 +11)
misaligned stack access off (0x0; 0x0)+-64+11 size 2
It looks like disabling CO-RE relocations seem to make the error go
away.
Signed-off-by: Daniel Xu <[email protected]>
---
tools/testing/selftests/bpf/progs/test_tunnel_kern.c | 1 +
1 file changed, 1 insertion(+)
diff --git a/tools/testing/selftests/bpf/progs/test_tunnel_kern.c b/tools/testing/selftests/bpf/progs/test_tunnel_kern.c
index 3065a716544d..ec7e04e012ae 100644
--- a/tools/testing/selftests/bpf/progs/test_tunnel_kern.c
+++ b/tools/testing/selftests/bpf/progs/test_tunnel_kern.c
@@ -6,6 +6,7 @@
* modify it under the terms of version 2 of the GNU General Public
* License as published by the Free Software Foundation.
*/
+#define BPF_NO_PRESERVE_ACCESS_INDEX
#include "vmlinux.h"
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_endian.h>
--
2.42.0
On Fri, Oct 27, 2023 at 11:46 AM Daniel Xu <[email protected]> wrote:
>
> Switching to vmlinux.h definitions seems to make the verifier very
> unhappy with bitfield accesses. The error is:
>
> ; md.u.md2.dir = direction;
> 33: (69) r1 = *(u16 *)(r2 +11)
> misaligned stack access off (0x0; 0x0)+-64+11 size 2
>
> It looks like disabling CO-RE relocations seem to make the error go
> away.
>
for accessing bitfields libbpf provides
BPF_CORE_READ_BITFIELD_PROBED() and BPF_CORE_READ_BITFIELD() macros
> Signed-off-by: Daniel Xu <[email protected]>
> ---
> tools/testing/selftests/bpf/progs/test_tunnel_kern.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/tools/testing/selftests/bpf/progs/test_tunnel_kern.c b/tools/testing/selftests/bpf/progs/test_tunnel_kern.c
> index 3065a716544d..ec7e04e012ae 100644
> --- a/tools/testing/selftests/bpf/progs/test_tunnel_kern.c
> +++ b/tools/testing/selftests/bpf/progs/test_tunnel_kern.c
> @@ -6,6 +6,7 @@
> * modify it under the terms of version 2 of the GNU General Public
> * License as published by the Free Software Foundation.
> */
> +#define BPF_NO_PRESERVE_ACCESS_INDEX
> #include "vmlinux.h"
> #include <bpf/bpf_helpers.h>
> #include <bpf/bpf_endian.h>
> --
> 2.42.0
>
On Fri, Oct 27, 2023 at 01:33:09PM -0700, Andrii Nakryiko wrote:
> On Fri, Oct 27, 2023 at 11:46 AM Daniel Xu <[email protected]> wrote:
> >
> > Switching to vmlinux.h definitions seems to make the verifier very
> > unhappy with bitfield accesses. The error is:
> >
> > ; md.u.md2.dir = direction;
> > 33: (69) r1 = *(u16 *)(r2 +11)
> > misaligned stack access off (0x0; 0x0)+-64+11 size 2
> >
> > It looks like disabling CO-RE relocations seem to make the error go
> > away.
> >
>
> for accessing bitfields libbpf provides
> BPF_CORE_READ_BITFIELD_PROBED() and BPF_CORE_READ_BITFIELD() macros
In this case the code in question is:
__u8 direction = 0;
md.u.md2.dir = direction;
IOW the problem is assigning to bitfields, not reading from them.
Is that something that libbpf needs to support as well?
Thanks,
Daniel
On Sun, Oct 29, 2023 at 4:22 PM Daniel Xu <[email protected]> wrote:
>
> On Fri, Oct 27, 2023 at 01:33:09PM -0700, Andrii Nakryiko wrote:
> > On Fri, Oct 27, 2023 at 11:46 AM Daniel Xu <[email protected]> wrote:
> > >
> > > Switching to vmlinux.h definitions seems to make the verifier very
> > > unhappy with bitfield accesses. The error is:
> > >
> > > ; md.u.md2.dir = direction;
> > > 33: (69) r1 = *(u16 *)(r2 +11)
> > > misaligned stack access off (0x0; 0x0)+-64+11 size 2
> > >
> > > It looks like disabling CO-RE relocations seem to make the error go
> > > away.
> > >
> >
> > for accessing bitfields libbpf provides
> > BPF_CORE_READ_BITFIELD_PROBED() and BPF_CORE_READ_BITFIELD() macros
>
> In this case the code in question is:
>
> __u8 direction = 0;
> md.u.md2.dir = direction;
>
> IOW the problem is assigning to bitfields, not reading from them.
>
> Is that something that libbpf needs to support as well?
Ah, I missed that this is a write into a struct. I think we can
support BPF_CORE_WRITE_BITFIELD() (not the PROBED version, though)
using all the same CO-RE relocations. It's probably a very niche case,
but BPF_CORE_READ_BITFIELD() is niche as well (though an absolute
necessity when the need does come up).
>
> Thanks,
> Daniel