[Bug 1663287] Re: Illegal delay slot code causes abort on mips64

[martin short]

I found the exact same bug. Tested on several hosts and qemu releases.
The newest one I tested was on FreeBSD 12.1 host and qemu-4.1.1_1 built
from ports.

Instructions:
  4000d0:       0320f809        jalr    t9
  4000d4:       45454545        0x45454545         # bc1any4t $fcc1,0x800101f8

I was running qemu-mips as:

qemu-system-mipsel -s -m 1024 -M malta \
        -kernel vmlinux-3.16.0-6-4kc-malta -initrd 
initrd.img-3.16.0-6-4kc-malta \
        -device virtio-blk-pci,drive=hd0 -drive 
if=none,id=hd0,format=qcow2,file=debian_wheezy_mipsel_standard.qcow2    \
        -append "root=/dev/vda1" \
        -device virtio-net-pci,netdev=net0 \
        -netdev user,id=net0,hostfwd=tcp::1666-:22,ipv6=off  \
        -curses 

abort() was in target/mips/translate.c:12945, in gen_branch().

Doesn't really matter if the instruction is supported on given CPU, user
can crash the qemu within guest.

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https://bugs.launchpad.net/bugs/1663287

Title:
  Illegal delay slot code causes abort on mips64

Status in QEMU:
  New

Bug description:
  During some randomised testing of an experimental MIPS implementation
  I found an instruction sequence that also causes aborts on mainline
  qemu's MIPS support.  The problem is triggered by an MSA branch
  instruction appearing in a delay slot when emulating a processor
  without MSA support.

  For example, with the current repository HEAD
  (f073cd3a2bf1054135271b837c58a7da650dd84b) configured for
  mips64-softmmu, if I run the attached binary using

      mips64-softmmu/qemu-system-mips64 -bios ../abort2.bin -machine
  mipssim -nographic

  it will report

      unknown branch 0x13000
      Aborted (core dumped)

  The binary contains the following two instructions:

      00200008 jr at
      47081e61 bz.b       w8,0xffffffffbfc0798c

  The jr sets up a jump, and hflags is set accordingly in
  gen_compute_branch (in target/mips/translate.c).  When processing the
  bz.b, check_insn generates an exception because the instruction isn't
  support, but gen_msa_branch skips the usual delay slot check for the
  same reason, and sets more bits in hflags, leading to an abort in
  gen_branch because the hflags are now invalid.

  I suspect the best fix is to remove the instruction set condition from
  the delay slot check in gen_msa_branch.

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