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Date:   Fri, 24 Apr 2020 17:12:29 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Quentin Monnet <quentin@...valent.com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next 07/10] bpftool: expose attach_type-to-string
 array to non-cgroup code

On Fri, Apr 24, 2020 at 10:08 AM Quentin Monnet <quentin@...valent.com> wrote:
>
> 2020-04-24 09:27 UTC-0700 ~ Andrii Nakryiko <andrii.nakryiko@...il.com>
> > On Fri, Apr 24, 2020 at 3:32 AM Quentin Monnet <quentin@...valent.com> wrote:
> >>
> >> 2020-04-23 22:35 UTC-0700 ~ Andrii Nakryiko <andriin@...com>
> >>> Move attach_type_strings into main.h for access in non-cgroup code.
> >>> bpf_attach_type is used for non-cgroup attach types quite widely now. So also
> >>> complete missing string translations for non-cgroup attach types.
> >>>
> >>> Signed-off-by: Andrii Nakryiko <andriin@...com>
> >>> ---
> >>>  tools/bpf/bpftool/cgroup.c | 28 +++-------------------------
> >>>  tools/bpf/bpftool/main.h   | 32 ++++++++++++++++++++++++++++++++
> >>>  2 files changed, 35 insertions(+), 25 deletions(-)
> >>>
> >>> diff --git a/tools/bpf/bpftool/cgroup.c b/tools/bpf/bpftool/cgroup.c
> >>> index 62c6a1d7cd18..d1fd9c9f2690 100644
> >>> --- a/tools/bpf/bpftool/cgroup.c
> >>> +++ b/tools/bpf/bpftool/cgroup.c
> >>> @@ -31,35 +31,13 @@
> >>>
> >>>  static unsigned int query_flags;
> >>>
> >>> -static const char * const attach_type_strings[] = {
> >>> -     [BPF_CGROUP_INET_INGRESS] = "ingress",
> >>> -     [BPF_CGROUP_INET_EGRESS] = "egress",
> >>> -     [BPF_CGROUP_INET_SOCK_CREATE] = "sock_create",
> >>> -     [BPF_CGROUP_SOCK_OPS] = "sock_ops",
> >>> -     [BPF_CGROUP_DEVICE] = "device",
> >>> -     [BPF_CGROUP_INET4_BIND] = "bind4",
> >>> -     [BPF_CGROUP_INET6_BIND] = "bind6",
> >>> -     [BPF_CGROUP_INET4_CONNECT] = "connect4",
> >>> -     [BPF_CGROUP_INET6_CONNECT] = "connect6",
> >>> -     [BPF_CGROUP_INET4_POST_BIND] = "post_bind4",
> >>> -     [BPF_CGROUP_INET6_POST_BIND] = "post_bind6",
> >>> -     [BPF_CGROUP_UDP4_SENDMSG] = "sendmsg4",
> >>> -     [BPF_CGROUP_UDP6_SENDMSG] = "sendmsg6",
> >>> -     [BPF_CGROUP_SYSCTL] = "sysctl",
> >>> -     [BPF_CGROUP_UDP4_RECVMSG] = "recvmsg4",
> >>> -     [BPF_CGROUP_UDP6_RECVMSG] = "recvmsg6",
> >>> -     [BPF_CGROUP_GETSOCKOPT] = "getsockopt",
> >>> -     [BPF_CGROUP_SETSOCKOPT] = "setsockopt",
> >>> -     [__MAX_BPF_ATTACH_TYPE] = NULL,
> >>
> >> So you removed the "[__MAX_BPF_ATTACH_TYPE] = NULL" from the new array,
> >> if I understand correctly this is because all attach type enum members
> >> are now in the new attach_type_name[] so we're safe by looping until we
> >> reach __MAX_BPF_ATTACH_TYPE. Sounds good in theory but...
> >>
> >
> > Well, NULL is default value, so having [__MAX_BPF_ATTACH_TYPE] = NULL
> > just increases ARRAY_SIZE(attach_type_names) by one. Which is
> > generally not needed, because we do proper < ARRAY_SIZE() checks
> > everywhere... except for one place. show_bpf_prog in cgroup.c looks up
> > name directly and can pass NULL into jsonw_string_field which will
> > crash.
> >
> > I can fix that by setting [__MAX_BPF_ATTACH_TYPE] to "unknown" or
> > adding extra check in show_bpf_prog() code? Any preferences?
>
> Maybe add the extra check, so we remove this [__MAX_BPF_ATTACH_TYPE]
> indeed. It will be more consistent with the array with program names,
> and as you say, all other places loop on ARRAY_SIZE() just fine.

Sounds good.

>
> Maybe we could print the integer value for the type if we don't know the
> name? Not sure if this is good for JSON though.

We do that in a bunch of places, I'll see if that's easy to do.

>
> >
> >>> -};
> >>> -
> >>>  static enum bpf_attach_type parse_attach_type(const char *str)
> >>>  {
> >>>       enum bpf_attach_type type;
> >>>
> >>>       for (type = 0; type < __MAX_BPF_ATTACH_TYPE; type++) {
> >>> -             if (attach_type_strings[type] &&
> >>> -                 is_prefix(str, attach_type_strings[type]))
> >>> +             if (attach_type_name[type] &&
> >>> +                 is_prefix(str, attach_type_name[type]))
> >>>                       return type;
> >>>       }
> >>
> >> ... I'm concerned the "attach_type_name[type]" here could segfault if we
> >> add a new attach type to the kernel, but don't report it immediately to
> >> bpftool's array.
> >
> > I don't think so. Here we'll iterate over all possible bpf_attach_type
> > (as far as our copy of UAPI header is concerned, of course). If some
> > of the values don't have entries in attach_type_name array, we'll get
> > back NULL (same as with explicit [__MAX_BPF_ATTACH_TYPE] = NULL, btw),
> > which will get handled properly in the loop. And caller will get back
> > __MAX_BPF_ATTACH_TYPE as bpf_attach_type value. So unless I'm still
> > missing something, it seems to be working exactly the same as before?
> >
> >>
> >> Is there any drawback with keeping the "[__MAX_BPF_ATTACH_TYPE] = NULL"?
> >> Or change here to loop on ARRAY_SIZE(), as you do in your own patch for
> >> link?
> >
> > ARRAY_SIZE() == __MAX_BPF_ATTACH_TYPE, isn't it? Previously ARRAY_SIZE
> > was (__MAX_BPF_ATTACH_TYPE + 1), but I don't think it's necessary?
>
> ARRAY_SIZE() /should/ be equal to __MAX_BPF_ATTACH_TYPE, the concern is
> only if new attach types get added to UAPI header and we forget to add
> them to the array. In that case, the assumption is not longer valid and
> we risk reading out of the array in parse_attach_type(). That was not
> the case before, because we knew that the array was always big enough.
> There was no risk to read beyond index __MAX_BPF_ATTACH_TYPE, there is
> one now to read beyond index BPF_LSM_MAC when new types are added. Or am
> I the one missing something?

Ah, I see what you are saying... I can just declare array as

const char *attach_type_strings[__MAX_BPF_ATTACH_TYPE] = { ... }

to prevent this. There is still this issue of potentially getting back
NULL pointer. But that warrants separate "audit" of the code usage and
fixing appropriately, I don't think it belongs in this patch set.

>
> >
> > The only difference is show_bpf_prog() which now is going to do out of
> > array reads, while previously it would get NULL. But both cases are
> > bad and needs fixing.
> >
>
> Right, nice catch, this needs a fix.

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