* Jann Horn: > On Thu, Jul 30, 2020 at 6:02 PM Florian Weimer <fweimer@xxxxxxxxxx> wrote: >> Functions no longer start with the ENDBR64 prefix. Instead, the link >> editor produces a PLT entry with an ENDBR64 prefix if it detects any >> address-significant relocation for it. The PLT entry performs a NOTRACK >> jump to the target address. This assumes that the target address is >> subject to RELRO, of course, so that redirection is not possible. >> Without address-significant relocations, the link editor produces a PLT >> entry without the ENDBR64 prefix (but still with the NOTRACK jump), or >> perhaps no PLT entry at all. > > How would this interact with function pointer comparisons? As in, if > library A exports a function func1 without referencing it, and > libraries B and C both take references to func1, would they end up > with different function pointers (pointing to their respective PLT > entries)? Same as today. ELF already deals with this by picking one canonical function address per process. Some targets already need PLTs for inter-DSO calls, so the problem is not new. It happens even on x86 because the main program can refer to its PLT stubs without run-time relocations, so those determine the canonical address of those functions, and not the actual implementation in a shared object. > Would this mean that the behavior of a program that compares > function pointers obtained through different shared libraries might > change? Hopefully not, because that would break things quite horribly (as it's sometimes possible to observe if the RTLD_DEEPBIND flag is used). Both the canonicalization and the fact in order to observe the function pointer, you need to take its address should take care of this. > I guess you could maybe canonicalize function pointers somehow, but > that'd probably at least break dlclose(), right? Ahh, dlclose. I think in this case, my idea to generate a PLT stub locally in the address-generating DSO will not work because the canonical address must survive dlclose if it refers to another DSO. There are two ways to deal with this: do not unload the PLT stub until the target DSO is also unloaded (but make sure that the DSO can be reloaded at a different address; probably not worth the complexity), or use the dlsym hack I sketched for regular symbol binding as well. Even more room for experiments, I guess. Thanks, Florian
