It would be nice to have a way for new programs to declare that they don't need vsyscalls. What's the right way to do this? An ELF header entry in the loader? An ELF header entry in the program? A new arch_prctl? As background, there's an old part of the x86_64 ABI that allows programs to do gettimeofday, clock_gettime, and getcpu by calling to fixed addresses of the form 0xffffffffff600n00 where n indicates which of those three syscalls is being invoked. This is a security issue. Since Linux 3.1, vsyscalls are emulated using NX and page faults. As a result, vsyscalls no longer offer any performance advantage over normal syscalls; in fact, they're much slower. As far as I know, nothing newer than 2012 will attempt to use vsyscalls if a vdso is present. (Sadly, a lot of things will still fall back to the vsyscall page if there is no vdso, but that shouldn't matter, since there is always a vdso.) Despite the emulation, they could still be used as a weird form of ROP gadget that lives at a fixed address. I'd like to offer a way for new runtimes to indicate that they don't use vsyscalls so that the kernel can selectively disable emulation and remove the fixed-address executable code issue. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
