Re: [RFC patch 2/5] RISC-V: Reserve about 5K space in mcontext_t to support future ISA expansion.

[Florian Weimer <fweimer@redhat.com>]

* DJ Delorie via Libc-alpha:

> Vincent Chen <vincent.chen@sifive.com> writes:
>> I am not familiar with the mechanism of LD_AUDIT, so I actually do not
>> know if this modification may have any effect on LD_AUDIT. If
>> possible, could you briefly introduce the issues for me? Thank you
>> very much.
>
> In general, when function foo() calls DSO function bar(), and bar() is
> in an object that needs to be loaded from disk, the loader needs to save
> foo()'s context, do a bunch of work, restore the context, and call
> bar().
>
> The LD_AUDIT feature adds a lot more "do a bunch of work" both on the
> foo->bar call, and on the bar->foo return, typically calling some third
> party functions to process the audit messages.
>
> However, if the "do a bunch of work" changes registers that aren't saved
> in the context, and aren't agreed on as "call clobbered" and thus
> changeable, problems happen.  If foo() expects a register to be
> preserved across the call to bar(), and the loader and audit functions
> don't know that and clobber it, foo() breaks.

One point of clarification:

The issue is with register usage for passing argument and return values.
It's more or less unrelated to whether registers are callee-saved or
caller-saved.  So you need special LD_AUDIT support as soon it's
possible to pass vector arguments and return values in registers (as
opposed to memory).

Thanks,
Florian