On Mon, Oct 08, 2018 at 09:29:56AM -0700, Andy Lutomirski 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.
I'm pretty sure only a basic JMP is used for tail calls. > 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. But those special cases aren't in a text section, right? If we just make sure the relocations are applied to a text section, and that they're preceded by the CALL or JMP byte, wouldn't that be sufficient? I'm not really convinced we need objtool for this, maybe I'll try whipping up a POC. -- Josh
