Messages in this thread | | | Date | Tue, 19 May 2020 09:15:51 +0100 | From | Will Deacon <> | Subject | Re: arm64: Register modification during syscall entry/exit stop |
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Hi Keno,
On Mon, May 18, 2020 at 09:05:30PM -0400, Keno Fischer wrote: > Continuing my theme of "weird things I encounter > while trying to use ptrace on arm64", I ran into the > effect of the following code in the syscall entry/exit > reporting: > > ``` > /* > * A scratch register (ip(r12) on AArch32, x7 on AArch64) is > * used to denote syscall entry/exit: > */ > regno = (is_compat_task() ? 12 : 7); > saved_reg = regs->regs[regno]; > regs->regs[regno] = dir; > ``` > > This seems very weird to me. I can't think of any > other architecture that does something similar > (other than unicore32 apparently, but the ptrace > support there seems like it might have just been > copied from ARM). I'm able to work around this > in my application, but it adds another stumbling block.
Yes, we inherited this from ARM and I think strace relies on it. In hindsight, it is a little odd, although x7 is a parameter register in the PCS and so it won't be live on entry to a system call.
> Some examples of things that happen: > - Writes to x7 during syscall exit stops are ignored, so > if the ptracer tries to emulate a setjmp-type thing, it > might miss this register (ptracers sometimes like to do > this to manually serialize execution between different > threads, puppeteering a single thread of execution > between different register states). > - Reads from x7 are incorrect, so if the ptracer saves > a register state and later tries to set it back to the task, > it may get x7 incorrect, but user space may be expecting > the register to be preserved (when might this happen? - > consider a ptracer that wants to modify some syscall > arguments, it modifies the arguments, restarts the syscall > but then incurs a signal, so it tries to restore the original > registers to let userspace deal with the signal without > being confused - expect signal traps don't ignore x7 > modifications, so x7 may have been unexpectedly > modified). > - We now have seccomp traps, which kind of look and > act like syscall-entry traps, but don't have this behavior, > so it's not particularly reliable for ptracers to use. > > Furthermore, it seems unnecessary to me on modern > kernels. We now have PTRACE_GET_SYSCALL_INFO, > which exposes this information without lying to the ptracer > about the tracee's registers. > > I understand, we can't just change this, since people may > be relying on it, but I would like to propose adding a ptrace > option (PTRACE_O_ARM_REGSGOOD?) that turns this > behavior off. Now, I don't think we currently have any other > arch-specific ptrace options, so maybe there is a different > option that would be preferable (e.g. could be a different > regset), but I do think it would be good to have a way to > operate on the real x7 value. As I said, I can work around it, > but hopefully I will be able to save a future implementer > some headache.
I'm not opposed to extending ptrace so that we can try to wean people off this interface, but I think we need some concrete situations where the current behaviour actually causes a problem. Although the examples you've listed above are interesting, I don't see why x7 is important in any of them (and we only support up to 6 system call arguments).
Will
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