Bug Report: Unaligned Access in BGP Code on ARMv7 Platforms

[nick]

Can you give me a quick guide on how to enable the debug output?
I tried for example setting in the config file:

> log "/tmp/birddebug.log" all;
> debug protocols all;

I am not able to see any output from ALIGN.

On 12/11/24 4:40 AM, Ondrej Zajicek wrote:
> On Wed, Dec 11, 2024 at 02:14:39AM +0100, nick wrote:
>> Thank you! I just ran a quick test and encountered the same crash at the
>> same line. I’ll have more time to investigate tomorrow and can provide
>> additional details then. Do you have any other ideas I could try in the
>> meantime?
> A part of my previous explanation was wrong, the issue is not related to
> net_copy(), as the crash happened even before.
>
> For some reason, struct bgp_prefix is u64-aligned. That makes sense in
> the original code, where u64 alignment is forced by struct net_addr, but
> i have no idea why this is true even after the patch removing the forced
> alignment.
>
> The slab allocator in sl_alloc() just returns u32-aligned memory, because
> it is configured for sizeof(struct bgp_prefix) + sizeof(net_addr_ip6) and
> the second is 20 as net_addr_ip6 is u32-aligned. The sl_alloc() internally
> only enforces word-size alignment, wich is u32 in your case.
>
> Please disregard the previous patch. Can you try the first attached patch
> (log-alignment.patch) to log alignment of different structures with or
> without the second patch (net-addr-u32-align.patch) and report me results?
>
>
>> On 12/11/24 1:41 AM, Ondrej Zajicek wrote:
>>> On Tue, Dec 10, 2024 at 09:15:46PM +0100, nick via Bird-users wrote:
>>>> I also uploaded the coredumpfile:
>>>> https://github.com/PolynomialDivision/coredumpupload/blob/main/bird_coredump
>>> Thanks. This seems like an interesting issue. In BIRD, generic net_addr
>>> structure is explicitly u64-aligned (to accomodate VPN variants), while
>>> specific net_addr_ip4 and net_addr_ip6 are just u32-aligned. In this case
>>> net_addr_ip6 is allocated with u32 alignment, but then copied with
>>> net_copy(), which assumes generic net_addr for arguments, and compiler
>>> probably used some u64-optimized copying, which required 64-bit alignment
>>> despite being on 32-bit platform,
>>>
>>> For starters, try the attached patch. But it is preliminary, we will revisit
>>> alignment of these structures.
>>>
>>>
>>>>>>> The root cause appears to be insufficient alignment of memory
>>>>>>> allocated for
>>>>>>> structures, specifically in this line:
>>>>>>>
>>>>>>> ```c
>>>>>>> px = mb_alloc(c->pool, sizeof(struct bgp_prefix) + net->length);
>>>>>>> ```
>>> Note that it is really allocated two lines above, here:
>>>
>>>     px = sl_alloc(c->prefix_slab);
>>>