On Tue, Aug 25, 2020 at 9:12 AM Andy Lutomirski <[email protected]> wrote: > I don’t like this at all. Your behavior really shouldn’t depend on > whether the new instructions are available. Also, some day I would > like to change Linux to have the new behavior even if FSGSBASE > instructions are not available, and this will break rr again. (The > current !FSGSBASE behavior is an ugly optimization of dubious value. > I would not go so far as to describe it as correct.)
Ok. > I would suggest you do one of the following things: > > 1. Use int $0x80 directly to load 32-bit regs into a child. This > might dramatically simplify your code and should just do the right > thing. I don't know what that means. > 2. Something like your patch but make it unconditional. > > 3. Ask for, and receive, real kernel support for setting FS and GS in > the way that 32-bit code expects. I think the easiest way forward for us would be a PTRACE_GET/SETREGSET like operation that operates on the regsets according to the *tracee*'s bitness (rather than the tracer, as it works currently). Does that sound workable? - Kyle
