On Tue, Aug 25, 2020 at 9:46 AM Andy Lutomirski <l...@kernel.org> wrote: > > On Tue, Aug 25, 2020 at 9:32 AM Kyle Huey <m...@kylehuey.com> wrote: > > > > On Tue, Aug 25, 2020 at 9:12 AM Andy Lutomirski <l...@amacapital.net> 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. > > This is untested, but what I mean is: > > static int ptrace32(int req, pid_t pid, int addr, int data) { > int ret; > /* new enough kernels won't clobber r8, etc. */ > asm volatile ("int $0x80" : "=a" (ret) : "a" (26 /* ptrace */), "b" > (req), "c" (pid), "d" (addr), "S" (data) : "flags", "r8", "r9", "r10", > "r11"); > return ret; > } > > with a handful of caveats: > > - This won't compile with -fPIC, I think. Instead you'll need to > write a little bit of asm to set up and restore ebx yourself. gcc is > silly like this. > > - Note that addr is an int. You'll need to mmap(..., MAP_32BIT, ...) > to get a buffer that can be pointed to with an int. > > The advantage is that this should work on all kernels that support > 32-bit mode at all. > > > > > > 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? > > > > Strictly speaking, on Linux, there is no unified concept of a task's > bitness, so "set all these registers according to the target's > bitness" is not well defined. We could easily give you a > PTRACE_SETREGS_X86_32, etc, though.
In the process of responding to this I spent some time doing code inspection and discovered a subtlety in the ptrace API that I was previously unaware of. PTRACE_GET/SETREGS use the regset views corresponding to the tracer but PTRACE_GET/SETREGSET use the regset views corresponding to the tracee. This means it is possible for us today to set FS/GS "the old way" with a 64 bit tracer/32 bit tracee combo, as long as we use PTRACE_SETREGSET with NT_PRSTATUS instead of PTRACE_SETREGS. - Kyle