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[Qemu-devel] [Bug 1815024] Re: SIGILL on instruction "stck" under qemu-s

From: Thomas Huth
Subject: [Qemu-devel] [Bug 1815024] Re: SIGILL on instruction "stck" under qemu-s390x in user mode
Date: Fri, 22 Feb 2019 15:24:04 -0000

Fix has been merged:

** Changed in: qemu
       Status: New => Fix Committed

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  SIGILL on instruction "stck" under qemu-s390x in user mode

Status in QEMU:
  Fix Committed

Bug description:
  qemu-s390x in user mode crashes with SIGILL (under host architecture
  x86_64, running Debian unstable) when executing target instruction
  "stck" ("STORE CLOCK", see
  which is basically a kind of equivalent of Intel "rdtsc". The same
  instruction works fine under qemu-s390x in system mode. The bug is
  reproducible with both the qemu version distributed in Debian unstable
  and with the latest upstream master (commit

  This bug manifested itself as a crash of ssh-keygen program, which
  uses "stck" to obtain some bits of randomness during key creation.
  Bisection of the code led to the attached minimal example. Compile
  with (inside an s390x system):

   $ gcc -c -o test.o test.c
   $ gcc -c -o rdtsc.o rdtsc.S
   $ gcc -o test test.o rdtsc.o

  Then run test. It will crash with SIGILL in user mode and run fine in
  system mode. Also, compare with the original file at
  (there the instruction "stckf" is also used; it is probable that it
  has the same problem if it is supported altogether, but it did not
  test for this).

  Running qemu-s390x with options -d
  in_asm,out_asm,op,op_opt,exec,nochain,cpu gives the trace attached in

  Thanks, Giovanni.

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