netdev - Re: [PATCH bpf-next 2/2] tools: bpftool: add "bpftool map freeze" subcommand

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Date:   Wed, 21 Aug 2019 15:08:50 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Quentin Monnet <quentin.monnet@...ronome.com>,
        Alexei Starovoitov <ast@...nel.org>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        oss-drivers@...ronome.com
Subject: Re: [PATCH bpf-next 2/2] tools: bpftool: add "bpftool map freeze"
 subcommand

On 8/21/19 2:58 PM, Quentin Monnet wrote:
> 2019-08-21 13:40 UTC+0200 ~ Daniel Borkmann <daniel@...earbox.net>
>> On 8/21/19 10:52 AM, Quentin Monnet wrote:
>>> Add a new subcommand to freeze maps from user space.
>>>
>>> Signed-off-by: Quentin Monnet <quentin.monnet@...ronome.com>
>>> Reviewed-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
>>> ---
>>>    .../bpf/bpftool/Documentation/bpftool-map.rst |  9 +++++
>>>    tools/bpf/bpftool/bash-completion/bpftool     |  4 +--
>>>    tools/bpf/bpftool/map.c                       | 34 ++++++++++++++++++-
>>>    3 files changed, 44 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/tools/bpf/bpftool/Documentation/bpftool-map.rst
>>> b/tools/bpf/bpftool/Documentation/bpftool-map.rst
>>> index 61d1d270eb5e..1c0f7146aab0 100644
>>> --- a/tools/bpf/bpftool/Documentation/bpftool-map.rst
>>> +++ b/tools/bpf/bpftool/Documentation/bpftool-map.rst
>>> @@ -36,6 +36,7 @@ MAP COMMANDS
>>>    |    **bpftool** **map pop**        *MAP*
>>>    |    **bpftool** **map enqueue**    *MAP* **value** *VALUE*
>>>    |    **bpftool** **map dequeue**    *MAP*
>>> +|    **bpftool** **map freeze**     *MAP*
>>>    |    **bpftool** **map help**
>>>    |
>>>    |    *MAP* := { **id** *MAP_ID* | **pinned** *FILE* }
>>> @@ -127,6 +128,14 @@ DESCRIPTION
>>>        **bpftool map dequeue**  *MAP*
>>>              Dequeue and print **value** from the queue.
>>>    +    **bpftool map freeze**  *MAP*
>>> +          Freeze the map as read-only from user space. Entries from a
>>> +          frozen map can not longer be updated or deleted with the
>>> +          **bpf\ ()** system call. This operation is not reversible,
>>> +          and the map remains immutable from user space until its
>>> +          destruction. However, read and write permissions for BPF
>>> +          programs to the map remain unchanged.
>>
>> That is not correct, programs that are loaded into the system /after/
>> the map
>> has been frozen cannot modify values either, thus read-only from both
>> sides.
> 
> Are you entirely sure about it? I could not find the relevant
> restriction in the code, the checks seem to be on map flags
> (BPF_F_RDONLY) which do not seem to be modified by the "frozen" status
> in map_freeze()? And tests I ran on my side seem to indicate the map can
> still be updated by new programs. Did I miss something?
> 
> Tested on 5.3.0-rc1:
> 
> 1. Create hash map
> 2. Load BPF program foo, using map
> 3. Test-run program foo - map is updated
> 4. Freeze map - update effectively becomes impossible from user space
> 5. Load BPF program bar, using same map
> 6. Test-run program bar - map is still updated

Looks like I need some more coffee. ;-) Indeed, the program side was via
BPF_F_RDONLY_PROG flag.