On Wed, Oct 07, 2015 at 06:11:55PM -0300, Marcelo Ricardo Leitner wrote:
> Hi Jakub,
>
> On Tue, Oct 06, 2015 at 01:04:43PM +0200, Jakub Tomalak wrote:
> > Greetings,
> >
> > I have discovered an alignment issue when using SCTP in a 32-bit
> > binary on a 64-bit host.
> >
> > Essentially, the problem is that struct sctp_assoc_stats contains a
> > sockaddr_storage field and is at a different offset for 32-bit and
> > 64-bit architectures (well, Intel/AMD anyway).
> >
> > Essentially when a 32-bit binary makes a getsockopt call with
> > sctp_assoc_stats againts a 64-bit kernel, the kernel copies the data
> > in with a different alignment, causing the 32-bit binary to receive
> > incorrect data for all of the chunk counters (fields after the
> > sockaddr field)..
> >
> > The fix is relatively simple for i386 and x86_64 architectures. I am
> > not sure about MIPS64 and ARM, since a colleague mentioned they do not
> > support unaligned memory access and also mentioned that MIPS64 doesn't
> > allow alignment with word sizes smaller than 8 bytes.
> >
> > Here is the diff I tested against both 32-bit and 64-bit RHEL and
> > Ubuntu systems (with kernels ranging from 3.12.x to 3.16.y).
> >
> > diff --git a/include/uapi/linux/sctp.h b/include/uapi/linux/sctp.h
> > index ce70fe6..41aa566 100644
> > --- a/include/uapi/linux/sctp.h
> > +++ b/include/uapi/linux/sctp.h
> > @@ -873,7 +873,7 @@ struct sctp_assoc_stats {
> > __u64 sas_iodchunks; /* Ordered data chunks received */
> > __u64 sas_octrlchunks; /* Control chunks sent */
> > __u64 sas_ictrlchunks; /* Control chunks received */
> > -};
> > +} __attribute__((packed, aligned(4)));
> >
> > /*
> > * 8.1 sctp_bindx()
> > @@ -900,6 +900,6 @@ struct sctp_paddrthlds {
> > struct sockaddr_storage spt_address;
> > __u16 spt_pathmaxrxt;
> > __u16 spt_pathpfthld;
> > -};
> > +} __attribute__((packed, aligned(4)));
> >
> > #endif /* _UAPI_SCTP_H */
> >
> > I have no idea what the official process is for submitting patches to
> > the SCTP module, and since this is probably a very edge case scenario,
> > I thought I would at least mention it in case anyone else comes across
> > similar problems.
>
> We can say that it starts by sharing your thoughts like you did. And
> when you are confident with a patch, you can submit it to netdev@
> directly while keeping this ML Cc'ed. Dave Miller is the one who
> integrates sctp patches.
>
> Now about your patch. I think it's just too late to fix that. That
> issue is exposed to user-space for too long so that I'm afraid it has
> already become a "feature". If we fix that, it will break applications
> that are handling that in some other way.
>
> Marcelo
>
> --
For the life of me I can't recall how to do this, but can we add a thunk layer
to the syscall to detect the arch of the calling process and fix up the
alignment while inside the syscall, and translate back in the epiloge?
Neil
> To unsubscribe from this list: send the line "unsubscribe linux-sctp" in
> the body of a message to majordomo@xxxxxxxxxxxxxxx
> More majordomo info at http://vger.kernel.org/majordomo-info.html
>
--
To unsubscribe from this list: send the line "unsubscribe linux-sctp" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
