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[Bug 1883984] Re: QEMU S/390x sqxbr (128-bit IEEE 754 square root) crash
From: |
Christian Ehrhardt |
Subject: |
[Bug 1883984] Re: QEMU S/390x sqxbr (128-bit IEEE 754 square root) crashes qemu-system-s390x |
Date: |
Wed, 26 Aug 2020 12:19:48 -0000 |
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
--
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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:
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.
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- [Bug 1883984] Re: QEMU S/390x sqxbr (128-bit IEEE 754 square root) crashes qemu-system-s390x, Christian Ehrhardt , 2020/08/03
- [Bug 1883984] Re: QEMU S/390x sqxbr (128-bit IEEE 754 square root) crashes qemu-system-s390x, Launchpad Bug Tracker, 2020/08/03
- [Bug 1883984] Re: QEMU S/390x sqxbr (128-bit IEEE 754 square root) crashes qemu-system-s390x, Christian Ehrhardt , 2020/08/03
- [Bug 1883984] Re: QEMU S/390x sqxbr (128-bit IEEE 754 square root) crashes qemu-system-s390x, Launchpad Bug Tracker, 2020/08/07
- [Bug 1883984] Re: QEMU S/390x sqxbr (128-bit IEEE 754 square root) crashes qemu-system-s390x, Christian Ehrhardt , 2020/08/19
- [Bug 1883984] Re: QEMU S/390x sqxbr (128-bit IEEE 754 square root) crashes qemu-system-s390x, Christian Ehrhardt , 2020/08/19
- [Bug 1883984] Re: QEMU S/390x sqxbr (128-bit IEEE 754 square root) crashes qemu-system-s390x, Launchpad Bug Tracker, 2020/08/19
- [Bug 1883984] Re: QEMU S/390x sqxbr (128-bit IEEE 754 square root) crashes qemu-system-s390x, Thomas Huth, 2020/08/20
- [Bug 1883984] Re: QEMU S/390x sqxbr (128-bit IEEE 754 square root) crashes qemu-system-s390x, Timo Aaltonen, 2020/08/21
- [Bug 1883984] Re: QEMU S/390x sqxbr (128-bit IEEE 754 square root) crashes qemu-system-s390x,
Christian Ehrhardt <=