Re: [PATCH v3 5/8] gdbserver: Transmit target description ID in thread list and stop reply

Andrew Burgess via Gdb-patches
2023-02-03T16:30:16Z

Thiago Jung Bauermann <thiago.bauermann@linaro.org> writes:

> Simon Marchi <simark@simark.ca> writes:
>
>> On 2/2/23 14:52, Thiago Jung Bauermann wrote:
>>>
>>> Simon Marchi <simark@simark.ca> writes:
>>>
>>>>> Having read some of the later patches, I have some additional thoughts
>>>>> here:
>>>>>
>>>>> I think we should make it explicit here that IDs are connection wide,
>>>>> not per-process. We should also make it clear that GDB might[1] cache
>>>>> target descriptions per remote connection, and so a remote target should
>>>>> not reuse a target description ID except where the target description is
>>>>> identical.
>>>
>>> Ah, good point. I will make that clarification.
>>>
>>>>> [1] I say "GDB might" here because if we say "GDB will" then this would
>>>>> imply each target description will only be asked for once. And I
>>>>> figure, why be overly restrictive.
>>>>
>>>> Thanks for pointing this out, I had the same thought while reading the
>>>> patch.
>>>>
>>>> In my original idea, I imagined that target description IDs could be
>>>> some hashes computed from the XML content (a bit like git hashes or ELF
>>>> build IDs), such that a given target description would always have the
>>>> same ID. This would give clients the possibility to cache target
>>>> descriptions locally, a bit like the index cache. It did sound nice,
>>>> but perhaps it's not really important.
>>>
>>> Ah, sorry I misunderstood this part of your suggestion. I thought that
>>> the caching was supposed to be limited to the duration of the connection,
>>> and thus the m_tdescs map in struct remote state would be enough to
>>> provide that functionality. Do you mean that the cache should be on
>>> disk, so that it survives GDB quitting? I can look into that if you want
>>> and implement it, if not complicated.
>>
>> To be clear, I'm not asking you to implement an on-disk cache, I'm just
>> trying to think about what the limitations of the proposed solution are.
>> Because mistakes or shortcomings introduced in the remote protocol (like
>> any API / ABI meant to be stable) are difficult to fix afterwards. If
>> there is a chance we want to do an on-disk cache (or share a cache
>> between multiple concurrent connections), we should think about that
>> now.
>
> Indeed, that's a good idea.
>
>> On one hand, perhaps we consider that target descriptions are small
>> enough that there's no point in having a persistent cache like that.
>> The time to fetch each target description once per connection is
>> probably insignificant.
>
> I don't have a good intuition about that. My uneducated guess is that
> it's not worth it, but as you say I think it's a good idea to leave the
> door open if we can.
>
>> On the other hand, perhaps doing the hash approach is easy enough that
>> we might as well do it, and that just leaves more doors open. In my
>> mind, as a start, we could just pass the XML of the tdesc through a
>> sha256 function, possibly doing something more sophisticated later (this
>> means the hash of a given tdesc could change over time, but still two
>> identical hashes mean identical tdescs). However, I don't think there
>> are sha256 functions built in on the OSes we want to support, so we'd
>> have to depend on some external library. And that complicates things
>> quite a bit... We already have a CRC32 function in gdbserver, but I
>> don't know if that's good enough to be confident there won't be
>> collisions.
>
> I tried to research this a bit, but I'm not confident enough in my
> knowledge of the subject to reach a conclusion. I only know enough about
> hashes to be dangerous. I did find a hash testsuite called SMhasher
> which seems to be well regarded, and its website¹ has
> reports for CRC32², which shows that it fails many collision and
> distribution bias tests. So it doesn't look like it is very reliable for
> using as a hash.
>
> We do support libxxhash as an optional dependency, so we could use that
> if gdbserver is linked with it. Its SMhasher report³ looks good, IUUC
> (but not the 32 bits version though⁴).
>
> We could also vendor coreutils's lib/sha256.[ch] files. They're 660
> lines in total.
>
>>> In this case, I think the tdesc ID should be of the format
>>> <prefix>:<hex number>, so that <prefix> can be “sha1”, “sha256” etc to
>>> indicate that <hex number> is a hash with the given algo, or even
>>> “number” to indicate that it's a simple integer like it is today.
>>>
>>> Perhaps we can do the prefix thing even if not implementing the cache,
>>> to leave the possibility of adding it in the future?
>>
>> I think I would choose between hash and number, but not do both, I don't
>> see the need to have both.
>
> I suggested a prefix to designate an integer ID as a way to implement
> only this possibility for now, but which would allow implementing hash
> IDs later, as additional prefixes. Or another possibility is that if we
> later decide to support hashes, we could add a separate protocol feature
> to signal that.

So, I'd also be in favour of supporting the simple "just a number"
scheme. Not everything that talks to GDB is our GDBServer, and not
every target might have the right hashing library. I'd hate to force
folk to have to implement some hashing code, just to use this feature of
the remote protocol.

I have a proposal for how to negotiate different ID types without
adding a prefix to every ID sent from gdbserver...

We don't need to use a prefix in the actual ID. I believe you're
already adding a qSupported feature for per-thread-tdesc. The
qSupported mechanism already supports value passing.

So, we could pass a value back and forth in the qSupported negotiation
where GDB and GDBServer can agree on a numbering scheme.

If GDB sends out:

per-thread-tdesc=value

Where `value` is a comma separated list of letters, each letter
representing a scheme that GDB understands:

N: Each ID is a number,

H: Each ID is a hash of the target description contents.

Then GDBServer replies with:

per-thread-tdesc=value

Where `value` is a single letter drawn from the set of letters that GDB
sent out, this is the scheme that GDBServer will be using. Thus,
GDBServer might reply with _one_ of the following:

per-thread-tdesc=N
per-thread-tdesc=H

If GDB only offers a scheme that GDBServer doesn't understand, e.g. GDB
sends:

per-thread-tdesc=X

Then GDBServer just doesn't send this feature back, effectively claiming
it doesn't support 'per-thread-tdesc'.

The great thing about this is that the hard work here is all in writing
the documentation.

To begin with you _only_ need to support the 'N' scheme. GDB always
sends out just 'per-thread-tdesc=N', GDBServer just checks that the
value is 'N', and replies 'per-thread-tdesc=N'. GDB checks that the
return value is also 'N', and then we carry on as before. BUT, we now
have a planned route to add more complex schemes in the future.

Thanks,
Andrew