old version sudo apt install qemu-system-s390x=1:4.2-3ubuntu6.4 ...test as listed in the test instructions ...
ubuntu@focal-sqxbr:~$ ./a.out Segmentation fault (qemu is dead at this point) $ sudo apt install qemu-system-s390x=1:4.2-3ubuntu6.5 Reading package lists... Done Building dependency tree Reading state information... Done The following packages will be upgraded: qemu-system-s390x 1 upgraded, 0 newly installed, 0 to remove and 315 not upgraded. Need to get 2334 kB of archives. After this operation, 4096 B of additional disk space will be used. Get:1 http://ports.ubuntu.com focal-proposed/main s390x qemu-system-s390x s390x 1:4.2-3ubuntu6.5 [2334 kB] Fetched 2334 kB in 1s (3927 kB/s) (Reading database ... 203254 files and directories currently installed.) Preparing to unpack .../qemu-system-s390x_1%3a4.2-3ubuntu6.5_s390x.deb ... Unpacking qemu-system-s390x (1:4.2-3ubuntu6.5) over (1:4.2-3ubuntu6.4) ... Setting up qemu-system-s390x (1:4.2-3ubuntu6.5) ... Processing triggers for man-db (2.9.3-2) ... ubuntu@s1lp05:~$ ubuntu@focal-sqxbr:~$ ./a.out (no crash) Setting verified ** Tags removed: verification-needed verification-needed-focal ** Tags added: verification-done verification-done-focal -- You received this bug notification because you are a member of qemu- devel-ml, which is subscribed to QEMU. https://bugs.launchpad.net/bugs/1883984 Title: QEMU S/390x sqxbr (128-bit IEEE 754 square root) crashes qemu-system- s390x Status in QEMU: Fix Released Status in qemu package in Ubuntu: Fix Released Status in qemu source package in Focal: Fix Committed 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. To manage notifications about this bug go to: https://bugs.launchpad.net/qemu/+bug/1883984/+subscriptions