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

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Date:   Wed, 21 Aug 2019 13:58:26 +0100
From:   Quentin Monnet <quentin.monnet@...ronome.com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        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

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.
> 
> Thanks,
> Daniel

Hi Daniel,

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

Best regards,
Quentin