linux-kernel - Re: [PATCH bpf-next v2] bpftool: Check map name length when map create

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Message-ID: <CAEf4Bzb9qWbF=H0L7ZF+nfztH8KkkfBJL00XgaKCeNpj25o7xA@mail.gmail.com>
Date: Tue, 11 Feb 2025 13:07:59 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Quentin Monnet <qmo@...nel.org>
Cc: Rong Tao <rtoax@...mail.com>, ast@...nel.org, daniel@...earbox.net, 
	rongtao@...tc.cn, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, 
	"open list:BPF [TOOLING] (bpftool)" <bpf@...r.kernel.org>, open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2] bpftool: Check map name length when map create

On Tue, Feb 11, 2025 at 2:48 AM Quentin Monnet <qmo@...nel.org> wrote:
>
> 2025-02-11 18:38 UTC+0800 ~ Rong Tao <rtoax@...mail.com>
> > From: Rong Tao <rongtao@...tc.cn>
> >
> > The size of struct bpf_map::name is BPF_OBJ_NAME_LEN (16).
> >
> > bpf(2) {
> >   map_create() {
> >     bpf_obj_name_cpy(map->name, attr->map_name, sizeof(attr->map_name));
> >   }
> > }
> >
> > When specifying a map name using bpftool map create name, no error is
> > reported if the name length is greater than 15.
> >
> >     $ sudo bpftool map create /sys/fs/bpf/12345678901234567890 \
> >         type array key 4 value 4 entries 5 name 12345678901234567890
> >
> > Users will think that 12345678901234567890 is legal, but this name cannot
> > be used to index a map.
> >
> >     $ sudo bpftool map show name 12345678901234567890
> >     Error: can't parse name
> >
> >     $ sudo bpftool map show
> >     ...
> >     1249: array  name 123456789012345  flags 0x0
> >       key 4B  value 4B  max_entries 5  memlock 304B
> >
> >     $ sudo bpftool map show name 123456789012345
> >     1249: array  name 123456789012345  flags 0x0
> >       key 4B  value 4B  max_entries 5  memlock 304B
> >
> > The map name provided in the command line is truncated, but no error is
> > reported. This submission checks the length of the map name.
> >
> > Signed-off-by: Rong Tao <rongtao@...tc.cn>
>
>
> Reviewed-by: Quentin Monnet <qmo@...nel.org>
>

Would it make sense to just warn but proceed with a truncated name?
libbpf truncates the name when creating a map, but preserves the
original name in BTF (and in memory, fetchable through
bpf_map__name()). So from the user's perspective that map is still
named "blah-blah-something-long", even if the kernel records just a
prefix of that.

Basically, instead of forcing users to count the first 15 characters,
warn, but do the right thing anyways?

> Thank you!