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Re: [PATCH v4][C][ADA] use function descriptors instead of trampolines in C


Am Dienstag, den 18.12.2018, 17:24 +0100 schrieb Jakub Jelinek:
> On Tue, Dec 18, 2018 at 09:03:41AM -0700, Jeff Law wrote:
> > Right.  This is the classic example and highlights the ABI concerns.  If
> > we use the low bit to distinguish between a normal function pointer and
> > a pointer to a descriptor and qsort doesn't know about it, then we lose.
> > 
> > One way around this is to make *all* function pointers be some kind of
> > descriptor and route all indirect calls through a resolver.  THen you
> 
> Either way, you are creating a new ABI for calling functions through
> function pointers.  Because of how rarely GNU C nested functions are used
> these days, if we want to do anything I'd think it might be better to use
> trampolines, just don't place them on the stack, say have a mmaped page of
> trampolines perhaps with some pointer encryption to where they jump to, so
> it isn't a way to circumvent non-executable stack, and have some register
> and unregister function you'd call to get or release the trampoline.
> If more trampolines are needed than currently available, the library could
> just mmap another such page.  A problem is how it should interact with
> longjmp or similar APIs, because then we could leak some trampolines (no
> "destructor" for the trampoline would be called.  The leaking could be
> handled e.g. through remembering the thread and frame pointer for which it
> has been allocated and if you ask for a new trampoline with a frame pointer
> above the already allocated one, release those entries or reuse them,
> instead of allocating a new one.  And somehow deal with thread exit.

Yes, something like this. If the trampolines are pre-allocated, this could
even avoid the need to clear the cache on archs where this is needed.

Best,
Martin

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