kern/52424: Panic running usr.bin/mixerctl/t_mixerctl test

[gson%gson.org@localhost (Andreas Gustafsson)]

>Number:         52424
>Category:       kern
>Synopsis:       Panic running usr.bin/mixerctl/t_mixerctl test
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Jul 22 22:05:00 +0000 2017
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current
>Organization:
>Environment:
System: NetBSD 
Architecture: i386
Machine: i386
>Description:

Recently, several runs of the automated i386 ATF test runs on
babylon5.netbsd.org have ended in a kernel panic during the
usr.bin/mixerctl/t_mixerctl test.  For example, the log file

  http://releng.netbsd.org/b5reports/i386/build/2017.07.20.18.17.25/test.log

contains:

  usr.bin/mixerctl/t_mixerctl (522/716): 4 test cases
      nflag: pad0: outputs: 44100Hz, 16-bit, stereo
  audio0 at pad0: half duplex, playback, capture, mmap
  pad0: Virtual format configured - Format SLINEAR, precision 16, channels 2, frequency 44100
  audio0: detached
  pad0: detached
  [4.551921s] Passed.
      noargs_usage: pad0: outputs: 44100Hz, 16-bit, stereo
  audio0 at pad0: half duplex, playback, capture, mmap
  pad0: Virtual format configured - Format SLINEAR, precision 16, channels 2, frequency 44100
  audio0: detached
  pad0: detached
  [2.913791s] Passed.
      nonexistant_device: [1.826129s] Passed.
      showvalue: pad0: outputs: 44100Hz, 16-bit, stereo
  audio0 at pad0: half duplex, playback, capture, mmap
  pad0: Virtual format configured - Format SLINEAR, precision 16, channels 2, frequency 44100
  uvm_fault(0xc1309d00, 0xc8d60000, 1) -> 0xe
  fatal page fault in supervisor mode
  trap type 6 code 0 eip 0xc06f6393 cs 0x8 eflags 0x297 cr2 0xc8d60288 ilevel 0 esp 0xc2627000
  curlwp 0xc1cdf2a0 pid 16394 lid 1 lowest kstack 0xc8c6c2c0
  panic: trap
  cpu0: Begin traceback...
  vpanic(c0f7a55a,c8c6ec44,c8c6ecc0,c012074f,c0f7a55a,c8c6eccc,c8c6eccc,1,c8c6c2c0,297) at netbsd:vpanic+0x121
  snprintf(c0f7a55a,c8c6eccc,c8c6eccc,1,c8c6c2c0,297,c8d60288,0,c2627000,0) at netbsd:snprintf
  trap() at netbsd:trap+0xd90
  --- trap (number 6) ---
  audio_query_devinfo(c2216008,c32c4d02,c2148408,3,c1cdf2a0,c0943ec5,32c,1,c1e30280,c2216008) at netbsd:audio_query_devinfo+0x51
  audioioctl(c1aba4c0,c32c4d02,c2148408,0,0,c8c6ee9c,3,ffffffff,32c,c2148408) at netbsd:audioioctl+0x118
  sys_ioctl(c1cdf2a0,c8c6ef68,c8c6ef60,c1e30280,b081c000,c8c6ef60,c8c6ef68,36,0,0) at netbsd:sys_ioctl+0xfc
  syscall() at netbsd:syscall+0x1ed
  --- syscall (number 54) ---
  b081c327:
  cpu0: End traceback...

Grepping the logs for 2017 shows nine such crashes, all in July:

  babylon5.netbsd.org$ zgrep query_devinfo 2017*/test.log.gz
  2017.07.02.16.06.42/test.log.gz:audio_query_devinfo(c2118408,c32c4d02,c1b35008,3,c2309540,c0943365,32c,1,c2110280,c2118408) at netbsd:audio_query_devinfo+0x51
  2017.07.03.09.08.35/test.log.gz:audio_query_devinfo(c2124808,c32c4d02,c2222008,3,c2374d20,c0943765,32c,1,c21f9500,c2124808) at netbsd:audio_query_devinfo+0x51
  2017.07.08.18.38.57/test.log.gz:audio_query_devinfo(c211dc08,c32c4d02,c221b408,3,c21b5a80,c0943445,32c,1,c2258ad0,c211dc08) at netbsd:audio_query_devinfo+0x51
  2017.07.09.19.37.33/test.log.gz:audio_query_devinfo(c2135808,c32c4d02,c212f008,3,c1ce6aa0,c0943475,32c,1,c21126a4,c2135808) at netbsd:audio_query_devinfo+0x51
  2017.07.09.22.04.04/test.log.gz:audio_query_devinfo(c212f408,c32c4d02,c2216c08,3,c22422a0,c0943465,32c,1,c19ae27c,c212f408) at netbsd:audio_query_devinfo+0x51
  2017.07.13.16.46.43/test.log.gz:audio_query_devinfo(c1e11c08,c32c4d02,c2116808,3,c2385d40,c09439a5,32c,1,c21fe42c,c1e11c08) at netbsd:audio_query_devinfo+0x51
  2017.07.17.19.55.13/test.log.gz:audio_query_devinfo(c222ec08,c32c4d02,c2216408,3,c255a7e0,c0943a15,32c,1,c21f5c74,c222ec08) at netbsd:audio_query_devinfo+0x51
  2017.07.20.18.17.25/test.log.gz:audio_query_devinfo(c2216008,c32c4d02,c2148408,3,c1cdf2a0,c0943ec5,32c,1,c1e30280,c2216008) at netbsd:audio_query_devinfo+0x51
  2017.07.21.08.49.05/test.log.gz:audio_query_devinfo(c1af8c08,c32c4d02,c1add008,3,c2377800,c0943725,32c,1,c19af424,c1af8c08) at netbsd:audio_query_devinfo+0x51

>How-To-Repeat:

Run the usr.bin/mixerctl/t_mixerctl test repeatedly.

>Fix: