On Mon, 8 Oct 2018 09:29:56 -0700 Andy Lutomirski <l...@amacapital.net> wrote:
> > On Oct 8, 2018, at 8:57 AM, Peter Zijlstra <pet...@infradead.org> wrote: > > > > On Mon, Oct 08, 2018 at 01:33:14AM -0700, Andy Lutomirski wrote: > >>> Can't we hijack the relocation records for these functions before they > >>> get thrown out in the (final) link pass or something? > >> > >> I could be talking out my arse here, but I thought we could do this, > >> too, then changed my mind. The relocation records give us the > >> location of the call or jump operand, but they don’t give the address > >> of the beginning of the instruction. > > > > But that's like 1 byte before the operand, right? We could even double check > > this by reading back that byte and ensuring it is in fact 0xE8 (CALL). > > > > AFAICT there is only the _1_ CALL encoding, and that is the 5 byte: E8 > > <PLT32>, > > so if we have the PLT32 location, we also have the instruction location. Or > > am > > I missing something? > > There’s also JMP and Jcc, any of which can be used for rail calls, but those > are also one byte. I suppose GCC is unlikely to emit a prefixed form of any > of these. So maybe we really can assume they’re all one byte. > > But there is a nasty potential special case: anything that takes the > function’s address. This includes jump tables, computed gotos, and plain old > function pointers. And I suspect that any of these could have one of the > rather large number of CALL/JMP/Jcc bytes before the relocation by > coincidence. > FYI, your email client is horrible to read from decent email clients :-p Anyway, I'd like to have these "dynamic functions" be "special" where they can't be added to tables or what not. If you need to add one to a table or function pointer, then you need to have a wrapper function that does the call. I think we can come up with some kind of wrapper or linker magic to enforce this too. -- Steve
