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

[Nelson H F Beebe]

Public bug reported:

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.

** Affects: qemu
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1883984

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

Status in QEMU:
  New

Bug description:
  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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