Chapter 20. Analyzing a core dump To identify the cause of the system crash, you can use the crash utility, which provides an interactive prompt similar to the GNU Debugger (GDB). By using crash , you can analyze a core dump created by kdump , netdump , diskdump , or xendump and a running Linux system. Alternatively, you can use the Kernel Oops Analyzer or the Kdump Helper tool. 20.1. Installing the crash utility With the provided information, understand the required packages and the procedure to install the crash utility. The crash utility might not be installed by default on your RHEL 8 systems. crash is a tool to interactively analyze a system's state while it is running or after a kernel crash occurs and a core dump file is created. The core dump file is also known as the vmcore file. Procedure Enable the relevant repositories: Install the crash package: Install the kernel-debuginfo package: The package kernel-debuginfo will correspond to the running kernel and provides the data necessary for the dump analysis. 20.2. Running and exiting the crash utility The crash utility is a powerful tool for analyzing kdump . By running crash on a crash dump file, you can gain insights into the system's state at the time of the crash, identify the root cause of the issue, and troubleshoot kernel-related problems. Prerequisites Identify the currently running kernel (for example 4.18.0-5.el8.x86_64 ). Procedure To start the crash utility, pass the following two necessary parameters: The debug-info (a decompressed vmlinuz image), for example /usr/lib/debug/lib/modules/4.18.0-5.el8.x86_64/vmlinux provided through a specific kernel-debuginfo package. The actual vmcore file, for example /var/crash/127.0.0.1-2018-10-06-14:05:33/vmcore . The resulting crash command will be as follows: Use the same <kernel> version that was captured by kdump . Example 20.1. Running the crash utility The following example shows analyzing a core dump created on October 6 2018 at 14:05 PM, using the 4.18.0-5.el8.x86_64 kernel. To exit the interactive prompt and stop crash , type exit or q . Note The crash command is also utilized as a powerful tool for debugging a live system. However, you must use it with caution to avoid system-level issues. Additional resources A Guide to Unexpected System Restarts 20.3. Displaying various indicators in the crash utility Use the crash utility to display various indicators, such as a kernel message buffer, a backtrace, a process status, virtual memory information and open files. Displaying the message buffer To display the kernel message buffer, type the log command at the interactive prompt: Type help log for more information about the command usage. Note The kernel message buffer includes the most essential information about the system crash. It is always dumped first in to the vmcore-dmesg.txt file. If you fail to obtain the full vmcore file, for example, due to insufficient space on the target location, you can obtain the required information from the kernel message buffer. By default, vmcore-dmesg.txt is placed in the /var/crash/ directory. Displaying a backtrace To display the kernel stack trace, use the bt command. Type bt <pid> to display the backtrace of a specific process or type help bt for more information about bt usage. Displaying a process status To display the status of processes in the system, use the ps command. Use ps <pid> to display the status of a single specific process. Use help ps for more information about ps usage. Displaying virtual memory information To display basic virtual memory information, type the vm command at the interactive prompt. Use vm <pid> to display information about a single specific process, or use help vm for more information about vm usage. Displaying open files To display information about open files, use the files command. Use files <pid> to display files opened by only one selected process, or use help files for more information about files usage. 20.4. Using Kernel Oops Analyzer The Kernel Oops Analyzer tool analyzes the crash dump by comparing the oops messages with known issues in the knowledge base. Prerequisites An oops message is secured to feed the Kernel Oops Analyzer. Procedure Access the Kernel Oops Analyzer tool. To diagnose a kernel crash issue, upload a kernel oops log generated in vmcore . Alternatively, you can diagnose a kernel crash issue by providing a text message or a vmcore-dmesg.txt as an input. Click DETECT to compare the oops message based on information from the makedumpfile against known solutions. Additional resources The Kernel Oops Analyzer article 20.5. The Kdump Helper tool The Kdump Helper tool helps to set up the kdump using the provided information. Kdump Helper generates a configuration script based on your preferences. Initiating and running the script on your server sets up the kdump service. Additional resources Kdump Helper

Commands:
subscription-manager repos --enable baseos repository
subscription-manager repos --enable appstream repository
subscription-manager repos --enable rhel-8-for-x86_64-baseos-debug-rpms
yum install crash
yum install kernel-debuginfo
crash /usr/lib/debug/lib/modules/4.18.0-5.el8.x86_64/vmlinux /var/crash/127.0.0.1-2018-10-06-14:05:33/vmcore
WARNING: kernel relocated [202MB]: patching 90160 gdb minimal_symbol values KERNEL: /usr/lib/debug/lib/modules/4.18.0-5.el8.x86_64/vmlinux DUMPFILE: /var/crash/127.0.0.1-2018-10-06-14:05:33/vmcore [PARTIAL DUMP] CPUS: 2 DATE: Sat Oct 6 14:05:16 2018 UPTIME: 01:03:57 LOAD AVERAGE: 0.00, 0.00, 0.00 TASKS: 586 NODENAME: localhost.localdomain RELEASE: 4.18.0-5.el8.x86_64 VERSION: #1 SMP Wed Aug 29 11:51:55 UTC 2018 MACHINE: x86_64 (2904 Mhz) MEMORY: 2.9 GB PANIC: "sysrq: SysRq : Trigger a crash" PID: 10635 COMMAND: "bash" TASK: ffff8d6c84271800 [THREAD_INFO: ffff8d6c84271800] CPU: 1 STATE: TASK_RUNNING (SYSRQ) crash>
crash> exit ~]#
crash> log ... several lines omitted EIP: 0060:[<c068124f>] EFLAGS: 00010096 CPU: 2 EIP is at sysrq_handle_crash+0xf/0x20 EAX: 00000063 EBX: 00000063 ECX: c09e1c8c EDX: 00000000 ESI: c0a09ca0 EDI: 00000286 EBP: 00000000 ESP: ef4dbf24 DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068 Process bash (pid: 5591, ti=ef4da000 task=f196d560 task.ti=ef4da000) Stack: c068146b c0960891 c0968653 00000003 00000000 00000002 efade5c0 c06814d0 <0> fffffffb c068150f b7776000 f2600c40 c0569ec4 ef4dbf9c 00000002 b7776000 <0> efade5c0 00000002 b7776000 c0569e60 c051de50 ef4dbf9c f196d560 ef4dbfb4 Call Trace: [<c068146b>] ? __handle_sysrq+0xfb/0x160 [<c06814d0>] ? write_sysrq_trigger+0x0/0x50 [<c068150f>] ? write_sysrq_trigger+0x3f/0x50 [<c0569ec4>] ? proc_reg_write+0x64/0xa0 [<c0569e60>] ? proc_reg_write+0x0/0xa0 [<c051de50>] ? vfs_write+0xa0/0x190 [<c051e8d1>] ? sys_write+0x41/0x70 [<c0409adc>] ? syscall_call+0x7/0xb Code: a0 c0 01 0f b6 41 03 19 d2 f7 d2 83 e2 03 83 e0 cf c1 e2 04 09 d0 88 41 03 f3 c3 90 c7 05 c8 1b 9e c0 01 00 00 00 0f ae f8 89 f6 <c6> 05 00 00 00 00 01 c3 89 f6 8d bc 27 00 00 00 00 8d 50 d0 83 EIP: [<c068124f>] sysrq_handle_crash+0xf/0x20 SS:ESP 0068:ef4dbf24 CR2: 0000000000000000
crash> bt PID: 5591 TASK: f196d560 CPU: 2 COMMAND: "bash" #0 [ef4dbdcc] crash_kexec at c0494922 #1 [ef4dbe20] oops_end at c080e402 #2 [ef4dbe34] no_context at c043089d #3 [ef4dbe58] bad_area at c0430b26 #4 [ef4dbe6c] do_page_fault at c080fb9b #5 [ef4dbee4] error_code (via page_fault) at c080d809 EAX: 00000063 EBX: 00000063 ECX: c09e1c8c EDX: 00000000 EBP: 00000000 DS: 007b ESI: c0a09ca0 ES: 007b EDI: 00000286 GS: 00e0 CS: 0060 EIP: c068124f ERR: ffffffff EFLAGS: 00010096 #6 [ef4dbf18] sysrq_handle_crash at c068124f #7 [ef4dbf24] __handle_sysrq at c0681469 #8 [ef4dbf48] write_sysrq_trigger at c068150a #9 [ef4dbf54] proc_reg_write at c0569ec2 #10 [ef4dbf74] vfs_write at c051de4e #11 [ef4dbf94] sys_write at c051e8cc #12 [ef4dbfb0] system_call at c0409ad5 EAX: ffffffda EBX: 00000001 ECX: b7776000 EDX: 00000002 DS: 007b ESI: 00000002 ES: 007b EDI: b7776000 SS: 007b ESP: bfcb2088 EBP: bfcb20b4 GS: 0033 CS: 0073 EIP: 00edc416 ERR: 00000004 EFLAGS: 00000246
crash> ps PID PPID CPU TASK ST %MEM VSZ RSS COMM > 0 0 0 c09dc560 RU 0.0 0 0 [swapper] > 0 0 1 f7072030 RU 0.0 0 0 [swapper] 0 0 2 f70a3a90 RU 0.0 0 0 [swapper] > 0 0 3 f70ac560 RU 0.0 0 0 [swapper] 1 0 1 f705ba90 IN 0.0 2828 1424 init ... several lines omitted 5566 1 1 f2592560 IN 0.0 12876 784 auditd 5567 1 2 ef427560 IN 0.0 12876 784 auditd 5587 5132 0 f196d030 IN 0.0 11064 3184 sshd > 5591 5587 2 f196d560 RU 0.0 5084 1648 bash
crash> vm PID: 5591 TASK: f196d560 CPU: 2 COMMAND: "bash" MM PGD RSS TOTAL_VM f19b5900 ef9c6000 1648k 5084k VMA START END FLAGS FILE f1bb0310 242000 260000 8000875 /lib/ld-2.12.so f26af0b8 260000 261000 8100871 /lib/ld-2.12.so efbc275c 261000 262000 8100873 /lib/ld-2.12.so efbc2a18 268000 3ed000 8000075 /lib/libc-2.12.so efbc23d8 3ed000 3ee000 8000070 /lib/libc-2.12.so efbc2888 3ee000 3f0000 8100071 /lib/libc-2.12.so efbc2cd4 3f0000 3f1000 8100073 /lib/libc-2.12.so efbc243c 3f1000 3f4000 100073 efbc28ec 3f6000 3f9000 8000075 /lib/libdl-2.12.so efbc2568 3f9000 3fa000 8100071 /lib/libdl-2.12.so efbc2f2c 3fa000 3fb000 8100073 /lib/libdl-2.12.so f26af888 7e6000 7fc000 8000075 /lib/libtinfo.so.5.7 f26aff2c 7fc000 7ff000 8100073 /lib/libtinfo.so.5.7 efbc211c d83000 d8f000 8000075 /lib/libnss_files-2.12.so efbc2504 d8f000 d90000 8100071 /lib/libnss_files-2.12.so efbc2950 d90000 d91000 8100073 /lib/libnss_files-2.12.so f26afe00 edc000 edd000 4040075 f1bb0a18 8047000 8118000 8001875 /bin/bash f1bb01e4 8118000 811d000 8101873 /bin/bash f1bb0c70 811d000 8122000 100073 f26afae0 9fd9000 9ffa000 100073 ... several lines omitted
crash> files PID: 5591 TASK: f196d560 CPU: 2 COMMAND: "bash" ROOT: / CWD: /root FD FILE DENTRY INODE TYPE PATH 0 f734f640 eedc2c6c eecd6048 CHR /pts/0 1 efade5c0 eee14090 f00431d4 REG /proc/sysrq-trigger 2 f734f640 eedc2c6c eecd6048 CHR /pts/0 10 f734f640 eedc2c6c eecd6048 CHR /pts/0 255 f734f640 eedc2c6c eecd6048 CHR /pts/0