Message-ID: <20210616235351.ay3vj6gk36bpatgy@apollo>
Date:   Thu, 17 Jun 2021 05:23:55 +0530
From:   Kumar Kartikeya Dwivedi <memxor@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH v2] libbpf: add request buffer type for netlink messages

On Thu, Jun 17, 2021 at 04:48:08AM IST, Andrii Nakryiko wrote:
> On Wed, Jun 16, 2021 at 10:04 AM Kumar Kartikeya Dwivedi
> <memxor@...il.com> wrote:
> >
> > Coverity complains about OOB writes to nlmsghdr. There is no OOB as we
> > write to the trailing buffer, but static analyzers and compilers may
> > rightfully be confused as the nlmsghdr pointer has subobject provenance
> > (and hence subobject bounds).
> >
> > Remedy this by using an explicit request structure, but we also need to
> > start the buffer in case of ifinfomsg without any padding. The alignment
> > on netlink wire protocol is 4 byte boundary, so we just insert explicit
>
> struct ifinfomsg has unsigned field in it, which makes it
> automatically 4-byte aligned because the struct is not packed. Do we
> really need that _pad[4] thing?.. Even if we do, I'm still not sure
> how it helps with alignment... If anything, explicit
> __attribute__((aligned(4))) would be better.
>

What I meant was that reusing the same struct for both means that the trailing
buffer where attributes are added starts right after struct tcmsg/struct
ifinfomsg. Since tcmsg is 20 bytes, ifinfomsg is 16. I didn't want it to trigger
if it ends up tracking the active member of the union (or effective type). Poor
wording I guess. Everything is aligned properly, just wanted to explain why
_pad[4] is there.

> > 4 byte buffer to avoid compilers throwing off on read and write from/to
> > padding.
> >
> > Also switch nh_tail (renamed to req_tail) to cast req * to char * so
>
> it probably should use (void *) everywhere, instead of (char *), but I
> see that existing code is using char * exclusively, so it's probably
> for another patch
>

I'll fix it in the resend.

> > that it can be understood as arithmetic on pointer to the representation
> > array (hence having same bound as request structure), which should
> > further appease analyzers.
> >
> > As a bonus, callers don't have to pass sizeof(req) all the time now, as
> > size is implicitly obtained using the pointer. While at it, also reduce
> > the size of attribute buffer to 128 bytes (132 for ifinfomsg using
> > functions due to the need to align buffer after it).
>
> Sorry if it's a stupid question, but why it's safe to reduce the
> buffer size from 128 to 256?
>

We just need something big enough, we already check the size everytime we add an
attribute to make sure we don't run out of space. It was a remnant from previous
versions where a lot of attributes were added. They're pretty limited now so I
just changed to a small safe value that works fine for both.

> >
> > Summary of problem:
> >   Even though C standard allows interconveritility of pointer to first
>
> s/interconveritility/interconvertibility/ ?
>
> >   member and pointer to struct, for the purposes of alias analysis it
> >   would still consider the first as having pointer value "pointer to T"
> >   where T is type of first member hence having subobject bounds,
> >   allowing analyzers within reason to complain when object is accessed
> >   beyond the size of pointed to object.
> >
> >   The only exception to this rule may be when a char * is formed to a
> >   member subobject. It is not possible for the compiler to be able to
> >   tell the intent of the programmer that it is a pointer to member
> >   object or the underlying representation array of the containing
> >   object, so such diagnosis is supressed.
>
> typo: suppressed
>

Thanks.

> >
> > Fixes: 715c5ce454a6 ("libbpf: Add low level TC-BPF management API")
> > Signed-off-by: Kumar Kartikeya Dwivedi <memxor@...il.com>
> > ---
> > Changelog:
> > v1 -> v2:
> >  * Add short summary instead of links about the underlying issue (Daniel)
> > ---
> >  tools/lib/bpf/netlink.c | 107 +++++++++++++++-------------------------
> >  tools/lib/bpf/nlattr.h  |  37 +++++++++-----
> >  2 files changed, 65 insertions(+), 79 deletions(-)
> >
>
> [...]

--
Kartikeya

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