[PATCH V2] kernel, add bug_on_warn

Wed Oct 22 03:13:59 PDT 2014

On 10/22/2014 12:27 AM, Rusty Russell wrote:
> Prarit Bhargava <prarit at redhat.com> writes:
>> There have been several times where I have had to rebuild a kernel to
>> cause a panic when hitting a WARN() in the code in order to get a crash
>> dump from a system.  Sometimes this is easy to do, other times (such as
>> in the case of a remote admin) it is not trivial to send new images to the
>> user.
>>
>> A much easier method would be a switch to change the WARN() over to a
>> BUG().  This makes debugging easier in that I can now test the actual
>> image the WARN() was seen on and I do not have to engage in remote
>> debugging.
>>
>> This patch adds a bug_on_warn kernel parameter, which calls BUG() in the
>> warn_slowpath_common() path.  The function will still print out the
>> location of the warning.
>>
>> An example of the bug_on_warn output:
>>
>> The first line below is from the WARN_ON() to output the WARN_ON()'s location.
>> After that the new BUG() call is displayed.
>>
>>  WARNING: CPU: 27 PID: 3204 at
>> /home/rhel7/redhat/debug/dummy-module/dummy-module.c:25 init_dummy+0x28/0x30
>> [dummy_module]()
>>  bug_on_warn set, calling BUG()...
>>  ------------[ cut here ]------------
>>  kernel BUG at kernel/panic.c:434!
>>  invalid opcode: 0000 [#1] SMP
>>  Modules linked in: dummy_module(OE+) sg nfsv3 rpcsec_gss_krb5 nfsv4
>> dns_resolver nfs fscache cfg80211 rfkill x86_pkg_temp_thermal intel_powerclamp
>> coretemp kvm_intel kvm crct10dif_pclmul crc32_pclmul crc32c_intel
>> ghash_clmulni_intel igb iTCO_wdt aesni_intel iTCO_vendor_support lrw gf128mul
>> sb_edac ptp edac_core glue_helper lpc_ich ioatdma pcspkr ablk_helper pps_core
>> i2c_i801 mfd_core cryptd dca shpchp ipmi_si wmi ipmi_msghandler acpi_cpufreq
>> nfsd auth_rpcgss nfs_acl lockd grace sunrpc xfs libcrc32c sr_mod cdrom sd_mod
>> mgag200 syscopyarea sysfillrect sysimgblt i2c_algo_bit drm_kms_helper isci ttm
>> drm libsas ahci libahci scsi_transport_sas libata i2c_core dm_mirror
>> dm_region_hash dm_log dm_mod
>>  CPU: 27 PID: 3204 Comm: insmod Tainted: G           OE  3.17.0+ #19
>>  Hardware name: Intel Corporation S2600CP/S2600CP, BIOS
>> RMLSDP.86I.00.29.D696.1311111329 11/11/2013
>>  task: ffff880034e75160 ti: ffff8807fc5ac000 task.ti: ffff8807fc5ac000
>>  RIP: 0010:[<ffffffff81076b81>]  [<ffffffff81076b81>] warn_slowpath_common+0xc1/0xd0
>>  RSP: 0018:ffff8807fc5afc68  EFLAGS: 00010246
>>  RAX: 0000000000000021 RBX: ffff8807fc5afcb0 RCX: 0000000000000000
>>  RDX: 0000000000000000 RSI: ffff88081efee5f8 RDI: ffff88081efee5f8
>>  RBP: ffff8807fc5afc98 R08: 0000000000000096 R09: 0000000000000000
>>  R10: 0000000000000711 R11: ffff8807fc5af93e R12: ffffffffa0424070
>>  R13: 0000000000000019 R14: ffffffffa0423068 R15: 0000000000000009
>>  FS:  00007f2d4b034740(0000) GS:ffff88081efe0000(0000) knlGS:0000000000000000
>>  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>  CR2: 00007f2d4a99f3c0 CR3: 00000007fd88b000 CR4: 00000000001407e0
>>  Stack:
>>   ffff8807fc5afcb8 ffffffff8199f020 ffff88080e396160 0000000000000000
>>   ffffffffa0423040 ffffffffa0425000 ffff8807fc5afd08 ffffffff81076be5
>>   0000000000000008 ffffffffa0424053 ffff880700000018 ffff8807fc5afd18
>>  Call Trace:
>>   [<ffffffffa0423040>] ? dummy_greetings+0x40/0x40 [dummy_module]
>>   [<ffffffff81076be5>] warn_slowpath_fmt+0x55/0x70
>>   [<ffffffffa0423068>] init_dummy+0x28/0x30 [dummy_module]
>>   [<ffffffff81002144>] do_one_initcall+0xd4/0x210
>>   [<ffffffff811b52c2>] ? __vunmap+0xc2/0x110
>>   [<ffffffff810f8889>] load_module+0x16a9/0x1b30
>>   [<ffffffff810f3d30>] ? store_uevent+0x70/0x70
>>   [<ffffffff810f49b9>] ? copy_module_from_fd.isra.44+0x129/0x180
>>   [<ffffffff810f8ec6>] SyS_finit_module+0xa6/0xd0
>>   [<ffffffff8166ce29>] system_call_fastpath+0x12/0x17
>>  Code: c4 08 5b 41 5c 41 5d 41 5e 41 5f 5d c3 48 c7 c7 20 42 8a 81 31 c0 e8 fc
>> 80 5e 00 eb 80 48 c7 c7 78 42 8a 81 31 c0 e8 ec 80 5e 00 <0f> 0b 66 66 66 66 2e
>> 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 55
>>  RIP  [<ffffffff81076b81>] warn_slowpath_common+0xc1/0xd0
>>   RSP <ffff8807fc5afc68>
>>  ---[ end trace 428218934a12088b ]---
>>
>> Successfully tested by me.
>>
>> Cc: Jonathan Corbet <corbet at lwn.net>
>> Cc: Andrew Morton <akpm at linux-foundation.org>
>> Cc: Rusty Russell <rusty at rustcorp.com.au>
>> Cc: "H. Peter Anvin" <hpa at zytor.com>
>> Cc: Andi Kleen <ak at linux.intel.com>
>> Cc: Masami Hiramatsu <masami.hiramatsu.pt at hitachi.com>
>> Cc: Fabian Frederick <fabf at skynet.be>
>> Cc: vgoyal at redhat.com
>> Cc: isimatu.yasuaki at jp.fujitsu.com
>> Cc: linux-doc at vger.kernel.org
>> Cc: kexec at lists.infradead.org
>> Cc: linux-api at vger.kernel.org
>> Signed-off-by: Prarit Bhargava <prarit at redhat.com>
>>
>> [v2]: add /proc/sys/kernel/bug_on_warn, additional documentation, modify
>>       !slowpath cases
>> ---
>>  Documentation/kdump/kdump.txt       |    7 +++++++
>>  Documentation/kernel-parameters.txt |    3 +++
>>  Documentation/sysctl/kernel.txt     |   12 ++++++++++++
>>  include/asm-generic/bug.h           |   12 ++++++++++--
>>  include/linux/kernel.h              |    1 +
>>  include/uapi/linux/sysctl.h         |    1 +
>>  kernel/panic.c                      |   21 ++++++++++++++++++++-
>>  kernel/sysctl.c                     |    7 +++++++
>>  kernel/sysctl_binary.c              |    1 +
>>  9 files changed, 62 insertions(+), 3 deletions(-)
>>
>> diff --git a/Documentation/kdump/kdump.txt b/Documentation/kdump/kdump.txt
>> index 6c0b9f2..a04ed72 100644
>> --- a/Documentation/kdump/kdump.txt
>> +++ b/Documentation/kdump/kdump.txt
>> @@ -471,6 +471,13 @@ format. Crash is available on Dave Anderson's site at the following URL:
>>  
>>     http://people.redhat.com/~anderson/
>>  
>> +Trigger Kdump on WARN()
>> +=======================
>> +
>> +The kernel parameter, bug_on_warn, calls BUG() in all WARN() paths.  This
>> +will cause a kdump to occur at the BUG() call.  In cases where a user
>> +wants to specify this during runtime, /proc/sys/kernel/bug_on_warn can be
>> +set to 1 to achieve the same behaviour.
> 
> What about during early boot?

Hi Rusty,

I really don't have a use case for this in early boot.  The kernel boots, the
initramfs, and then we run whatever init (systemd in my case).  A systemd script
configures kexec for kdump and that point kdump is "armed".  Doing a bug_on_warn
before this will simply result in a panicked system.  I don't get any "new"
information FWIW as I get a stack trace, etc., in both the WARN() and BUG() cases.

> 
> I'd recommend you use core_param().  Less code, and can be set on
> commandline.

Is that a general request, or is it dependent on the answer above?  Of course I
have no problem doing it either way.

P.