Re: [PATCH 0/3] aarch64: Update ld.so for vector abi

[Carlos O'Donell <carlos at redhat dot com>]

On 08/02/2018 09:26 AM, Szabolcs Nagy wrote:
> On 02/08/18 14:17, Carlos O'Donell wrote:
>> On 08/02/2018 06:44 AM, Szabolcs Nagy wrote:
>>> ok (ld audit is another reason to avoid plt for vector functions..)
>>
>> If you avoid the PLT with no way to add back a call through the PLT
>> this completely breaks developer tooling that uses the PLT, like
>> LD_AUDIT, latrace, and custom tooling. Be aware that there are large
>> customers in various industry verticals that use LD_AUDIT extensively.
>>
> 
> ld_audit cannot possibly work correctly (because variadic
> functions with large arguments are broken, it can cause
> runtime crashes) so it is a 'best effort hack' that sometimes
> happens to work.

If it works for 100% of the cases the developer is interested in
supporting then it is perfect for the developer and the user can
use it without any problem.

Don't fall into the engineers fallacy that because you can't make
it 100% provably correct that it is not useful or not valuable.

Everything we do, to some extent, is a "best effort hack" within
the limits of the machines we operate under, particularly the
constraints of memory buses, addressing, and reasonable performance.

The lack of unlimited variadic argument support is a corner case
where each architecture could document the maximum limit (of the 
stack copying we do) or we could implement something *newer* that
operates *like* LD_AUDIT but works differently.

> so i'm sad that users depend on it.. possibly because of
> false advertisement somewhere that this is a reliable mechanism. 
No one has made any false advertisements. What is happening is that
silicon vendors are looking for performance by removing the PLT.
The PLT has been around for long enough that there is developer
tooling that considers it a part of the ELF ABI. If you remove support
for it without providing a way to add it back, like we are discussing
with H.J. for x86_64, you will negatively impact your architectures
ability to compete on a feature that developers use and deploy into
production. The feature, LD_AUDIT in particular, has been around since
Solaris implemented it, so it has become a relevant feature for some
large companies.

On x86_64 with -fno-plt, we hope to leave in the PLT and the required
relocations to route the GOT-direct calls through the PLT if LD_AUDIT
is enabled. If you can't do that, then you will have to document that
calls with the new vector ABI do not support LD_AUDIT, and again this
will impact some customers.

-- 
Cheers,
Carlos.