jose.march...@oracle.com (Jose E. Marchesi) writes: > > +#undef TARGET_PASS_BY_REFERENCE > > +#define TARGET_PASS_BY_REFERENCE bpf_pass_by_reference > > I might have misunderstood, but I thought from an earlier (IRC?) > message, it wasn't possible for the callee to access the caller's > frame, which was why you had the error about running out of argument > registers. If so, won't passing by reference make the argument > inaccessible in practice? I don't see what you gain by defining > the hook, since I'd have assumed (at least after the fix above) > that it would be better to pass by value and get an error about > having no argument registers left. > > Yes. I added that hook before I had the restriction of number of > arguments in place. Removing it. > > Happy auto correction :) > > A colleague (who actually _uses_ eBPF extensively, ahem) tells me that > the kernel verifier allows to pass addresses of the caller's stack > frame, tracking that it is a ptr to a stack location, and it knows which > stack it came from. So it is indeed possible for the callee to access > the caller's frame, and therefore to pass arguments by reference. > > On the downside, it is not possible for a callee to access the caller's > frame applying an offset to its frame pointer, because the stacks are > disjoint. This means that most probably I will have to dedicate a real, > not eliminable register to act as the arg pointer, if I want to get rid > of the annoying limitation on the number of arguments... and in order > to keep ABI compatibility with llvm built objects, this register is > gonna have to be %r5, i.e. the last register usable to pass arguments, > but it should be only used for that purpose if the function gets more > than 5 arguments... sounds messy, but there is hope, yay! > > However, unless someone comes with a magical macro to define or an > existing target doing the same thing, I am deferring attacking this > problem for later (TM) and for the time being I will keep both the > ability of passing aggregates and other big arguments by reference, and > the limit on number of arguments (this is what clang does.) > > I hope that's ok for you people.
Sounds good :-)
