[Bug 1883984] Re: QEMU S/390x sqxbr (128-bit IEEE 754 square root) crashes qemu-system-s390x

[Christian Ehrhardt ]

Note: final upstream commit link
https://git.qemu.org/?p=qemu.git;a=commit;h=9bf728a09bf7509b27543664f9cca6f4f337f608

** Changed in: qemu (Ubuntu)
     Assignee: Christian Ehrhardt  (paelzer) => (unassigned)

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Title:
  QEMU S/390x sqxbr (128-bit IEEE 754 square root) crashes qemu-system-
  s390x

Status in QEMU:
  Fix Committed
Status in qemu package in Ubuntu:
  Fix Released
Status in qemu source package in Focal:
  Triaged

Bug description:
  [Impact]

   * An instruction was described wrong so that on usage the program would 
     crash.

  [Test Case]

   * Run s390x in emulation and there use this program:
     For simplicity and speed you can use KVM guest as usual on s390x, that 
     after prep&install&compile of the test you run in qemu-tcg like:

     $ sudo qemu-system-s390x -machine s390-ccw-virtio,accel=tcg -cpu 
max,zpci=on -serial mon:stdio -display none -m 4096 -nic 
user,model=virtio,hostfwd=tcp::2222-:22 -drive 
file=/var/lib/uvtool/libvirt/images/focal-sqxbr.qcow,if=none,id=drive-virtio-disk0,format=qcow2,cache=none
 -device 
virtio-blk-ccw,devno=fe.0.0001,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1,scsi=off
     Obviously is you have no s390x access you need to use emulation right 
     away.

   * Build and run failing program
     $ sudo apt install clang
     $ cat > bug-sqrtl-one-line.c << EOF
  int main(void) { volatile long double x, r; x = 4.0L; __asm__ 
  __volatile__("sqxbr %0, %1" : "=f" (r) : "f" (x)); return (0);}
  EOF
     $ cc bug-sqrtl-one-line.c
     $ ./a.out
     Segmentation fault (core dumped)

     qemu is dead by now as long as the bug is present

  [Regression Potential]

   * The change only modifies 128 bit square root on s390x so regressions
     should be limited to exactly that - which formerly before this fix was 
     a broken instruction.

  [Other Info]
   
   * n/a

  ---

  In porting software to guest Ubuntu 18.04 and 20.04 VMs for S/390x, I 
discovered
  that some of my own numerical programs, and also a GNU configure script for at
  least one package with CC=clang, would cause an instant crash of the VM, 
sometimes
  also destroying recently opened files, and producing long strings of NUL 
characters
  in /var/log/syslog in the S/390 guest O/S.

  Further detective work narrowed the cause of the crash down to a single IBM 
S/390
  instruction: sqxbr (128-bit IEEE 754 square root).  Here is a one-line program
  that when compiled and run on a VM hosted on QEMUcc emulator version 4.2.0
  (Debian 1:4.2-3ubuntu6.1) [hosted on Ubuntu 20.04 on a Dell Precision 7920
  workstation with an Intel Xeon Platinum 8253 CPU],  and also on QEMU emulator
  version 5.0.0, reproducibly produces a VM crash under qemu-system-s390x.

  % cat bug-sqrtl-one-line.c
  int main(void) { volatile long double x, r; x = 4.0L; __asm__ 
__volatile__("sqxbr %0, %1" : "=f" (r) : "f" (x)); return (0);}

  % cc bug-sqrtl-one-line.c && ./a.out
  Segmentation fault (core dumped)

  The problem code may be the function float128_sqrt() defined in 
qemu-5.0.0/fpu/softfloat.c
  starting at line 7619.  I have NOT attempted to run the qemu-system-s390x 
executable
  under a debugger.  However, I observe that S/390 is the only CPU family that 
I know of,
  except possibly for a Fujitsu SPARC-64, that has a 128-bit square root in 
hardware.
  Thus, this instruction bug may not have been seen before.

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