netdev - Re: [PATCH bpf] libbpf: Sanitise internal map names so they are not rejected by the kernel

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Date:   Tue, 18 Feb 2020 23:54:34 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Yonghong Song <yhs@...com>, ast@...com
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH bpf] libbpf: Sanitise internal map names so they are not
 rejected by the kernel

On 2/18/20 5:42 PM, Toke Høiland-Jørgensen wrote:
> Yonghong Song <yhs@...com> writes:
>> On 2/18/20 6:40 AM, Daniel Borkmann wrote:
>>> On 2/17/20 6:17 PM, Toke Høiland-Jørgensen wrote:
>>>> The kernel only accepts map names with alphanumeric characters,
>>>> underscores
>>>> and periods in their name. However, the auto-generated internal map names
>>>> used by libbpf takes their prefix from the user-supplied BPF object name,
>>>> which has no such restriction. This can lead to "Invalid argument" errors
>>>> when trying to load a BPF program using global variables.
>>>>
>>>> Fix this by sanitising the map names, replacing any non-allowed
>>>> characters
>>>> with underscores.
>>>>
>>>> Fixes: d859900c4c56 ("bpf, libbpf: support global data/bss/rodata
>>>> sections")
>>>> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
>>>
>>> Makes sense to me, applied, thanks! I presume you had something like '-'
>>> in the
>>> global var leading to rejection?
>>
>> The C global variable cannot have '-'. I saw a complain in bcc mailing
>> list sometimes back like: if an object file is a-b.o, then we will
>> generate a map name like a-b.bss for the bss ELF section data. The
>> map name "a-b.bss" name will be rejected by the kernel. The workaround
>> is to change object file name. Not sure whether this is the only
>> issue which may introduce non [a-zA-Z0-9_] or not. But this patch indeed
>> should fix the issue I just described.

Yep, meant object file name, just realized too late after sending. :/

> Yes, this was exactly my problem; my object file is called
> 'xdp-dispatcher.o'. Fun error to track down :P
> 
> Why doesn't the kernel allow dashes in the name anyway?

Commit cb4d2b3f03d8 ("bpf: Add name, load_time, uid and map_ids to bpf_prog_info")
doesn't state a specific reason, and we did later extend it via 3e0ddc4f3ff1 ("bpf:
allow . char as part of the object name"). My best guess right now is potentially
not to confuse BPF's kallsyms handling with dashes etc.