如何使用crash分析vmcore - 之基础思路case1
dmesg查看内核日志
[2493382.671020] systemd-shutdown[1]: Sending SIGKILL to PID 28975 (docker-containe).
[2493382.671078] systemd-shutdown[1]: Sending SIGKILL to PID 29015 (systemd).
[2493420.208723] EXT4-fs (nvme0n1p1): sb orphan head is 140906170
[2493420.209198] sb_info orphan list:
[2493420.209663] inode nvme0n1p1:140906170 at ffff88490edabfb8: mode 100666, nlink 0, next 149423507
[2493420.210129] inode nvme0n1p1:149423507 at ffff8801b99391a8: mode 100666, nlink 0, next 17567381
[2493420.210583] inode nvme0n1p1:17567381 at ffff8806d4a26998: mode 100744, nlink 0, next 17570510
[2493420.211050] inode nvme0n1p1:17570510 at ffff886387f82ef8: mode 100644, nlink 0, next 17570503
[2493420.211508] inode nvme0n1p1:17570503 at ffff886a1f15bfb8: mode 100644, nlink 0, next 241700498
[2493420.211966] inode nvme0n1p1:241700498 at ffff8877481800e8: mode 100644, nlink 0, next 243138756
[2493420.212431] inode nvme0n1p1:243138756 at ffff88761ad10518: mode 100644, nlink 0, next 241565954
[2493420.212900] inode nvme0n1p1:241565954 at ffff8870d64bbfb8: mode 100755, nlink 0, next 241566333
[2493420.213366] inode nvme0n1p1:241566333 at ffff88721ae74c48: mode 100644, nlink 0, next 241050093
[2493420.213833] inode nvme0n1p1:241050093 at ffff887704958948: mode 100755, nlink 0, next 241567324
[2493420.214545] ------------[ cut here ]------------
[2493420.219336] kernel BUG at fs/ext4/super.c:879! <<<======这里指明BUG的代码位置
[2493420.223948] invalid opcode: 0000 [#1] SMP
[2493420.228133] Modules linked in: kpatch_D751550(OE) kpatch_D631237(OE) unix_diag(E) af_packet_diag(E) netlink_diag(E) dccp_diag(E) dccp(E) tcp_diag(E) udp_diag(E) inet_diag(E) [last unloaded: aisqos_hotfixes]
[2493420.246846] CPU: 58 PID: 1 Comm: systemd-shutdow Tainted: G W OE K 4.9.79-009.ali3000.alios7.x86_64 #1
[2493420.257009] Hardware name: Inventec AliServer Thor01-2U /TB800G4-G1 , BIOS A1.20 03/06/2018
[2493420.267339] task: ffff887e45918000 task.stack: ffffc90000014000
[2493420.273425] RIP: 0010:[<ffffffffa031a8df>] [<ffffffffa031a8df>] ext4_put_super+0x36f/0x3c0 [ext4] <<<=======这里指明BUG的代码位置
[2493420.282593] RSP: 0018:ffffc90000017de8 EFLAGS: 00010206
[2493420.288079] RAX: ffff88490edabf50 RBX: ffff887e43299000 RCX: 00000001949b336d
[2493420.295384] RDX: 0000000000000000 RSI: 0000000000000206 RDI: 0000000000000206
[2493420.302682] RBP: ffffc90000017e18 R08: 00000000000081a4 R09: 0000000000000000
[2493420.309988] R10: 0000000000000cb8 R11: 0000000000001e92 R12: ffff887e43299278
[2493420.317293] R13: ffff887e43298800 R14: ffff887e43299278 R15: ffffffffa034ff88
[2493420.324598] FS: 00007f3241ccf840(0000) GS:ffff887e78480000(0000) knlGS:0000000000000000
[2493420.332850] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[2493420.338767] CR2: 00007f5e1372fbd0 CR3: 00000004daa52000 CR4: 00000000007606f0
[2493420.346065] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[2493420.353361] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[2493420.360660] PKRU: 55555554
[2493420.363536] Stack:
[2493420.365721] 9cbae75a00000000 ffff887e43298800 ffffffffa034a5e0 ffff887e3818c7b8
[2493420.373365] 0000000000000000 ffff887e45918bb0 ffffc90000017e38 ffffffff81244aaf
[2493420.380991] 0000000000000083 ffff887e357b8680 ffffc90000017e58 ffffffff81244e37
[2493420.388617] Call Trace:
[2493420.391239] [<ffffffff81244aaf>] generic_shutdown_super+0x6f/0x100
[2493420.397676] [<ffffffff81244e37>] kill_block_super+0x27/0x70
[2493420.403508] [<ffffffff81244f73>] deactivate_locked_super+0x43/0x70
[2493420.409945] [<ffffffff8124547a>] deactivate_super+0x5a/0x60
[2493420.415770] [<ffffffff81264b2f>] cleanup_mnt+0x3f/0x90
[2493420.421169] [<ffffffff81264bc2>] __cleanup_mnt+0x12/0x20
[2493420.426733] [<ffffffff810a7b50>] task_work_run+0x80/0xa0
[2493420.432306] [<ffffffff810032ba>] exit_to_usermode_loop+0xaa/0xb0
[2493420.438572] [<ffffffff81003baa>] syscall_return_slowpath+0xaa/0xb0
[2493420.445011] [<ffffffff8171a783>] entry_SYSCALL_64_fastpath+0xc3/0xc5
[2493420.451623] Code: 60 04 00 00 48 8b 80 e0 00 00 <0f> 0b 49 c7 c7 88 ff 34 a0 49 8b
[2493420.459829] RIP [<ffffffffa031a8df>] ext4_put_super+0x36f/0x3c0 [ext4]
[2493420.466633] RSP <ffffc90000017de8>
crash>
通过dmesg日志，我们可以通过两个方法判断 bug的代码位置：
1. [2493420.219336] kernel BUG at fs/ext4/super.c:879!
2. [2493420.273425] RIP: 0010:[<ffffffffa031a8df>] [<ffffffffa031a8df>] ext4_put_super+0x36f/0x3c0 [ext4]
其中（0x36f代表和ext4_put_super函数入口的偏移量，0x3c0是基准地址 ）
从2找到代码crash的具体位置：
(gdb) p 0x36f
$11 = 879
反汇编函数，找到位置
crash> dis -l ext4_put_super
在crash中查看代码
crash本身是可以查看代码的，前提是你需要加载模块， 比如：
加载模块ext4：
crash> mod -s ext4
crash> mod <<----列出所有的模块
第879行：
crash> l *ext4_put_super+0x36f
0xffffffffa031a8df is in ext4_put_super (fs/ext4/super.c:879).
874 * isn't empty. The on-disk one can be non-empty if we've
875 * detected an error and taken the fs readonly, but the
876 * in-memory list had better be clean by this point. */
877 if (!list_empty(&sbi->s_orphan))
878 dump_orphan_list(sb, sbi);
879 J_ASSERT(list_empty(&sbi->s_orphan));
880
881 sync_blockdev(sb->s_bdev);
882 invalidate_bdev(sb->s_bdev);
883 if (sbi->journal_bdev && sbi->journal_bdev != sb->s_bdev) {
只有当我们找到具体的代码，才能进一步分析代码，究竟为什么会crash，比如，这个函数的参数（可能是某个struct）的值到底是什么？
bt打印栈
bt栈[exception RIP: ext4_put_super+879] 有可以看到是在 函数ext4_put_super 的第879行
crash> bt
PID: 1 TASK: ffff887e45918000 CPU: 58 COMMAND: "systemd-shutdow"
#0 [ffffc90000017a58] machine_kexec at ffffffff810603e8
#1 [ffffc90000017ab8] __crash_kexec at ffffffff811211cd
#2 [ffffc90000017b80] __crash_kexec at ffffffff811212a5
#3 [ffffc90000017b98] crash_kexec at ffffffff811212eb
#4 [ffffc90000017bb8] oops_end at ffffffff81030905
#5 [ffffc90000017be0] die at ffffffff81030ddb
#6 [ffffc90000017c10] do_trap at ffffffff8102df02
#7 [ffffc90000017c60] do_error_trap at ffffffff8102e2d9
#8 [ffffc90000017d20] do_invalid_op at ffffffff8102e830
#9 [ffffc90000017d30] invalid_op at ffffffff8171b63e
[exception RIP: ext4_put_super+879]
RIP: ffffffffa031a8df RSP: ffffc90000017de8 RFLAGS: 00010206
RAX: ffff88490edabf50 RBX: ffff887e43299000 RCX: 00000001949b336d
RDX: 0000000000000000 RSI: 0000000000000206 RDI: 0000000000000206
RBP: ffffc90000017e18 R8: 00000000000081a4 R9: 0000000000000000
R10: 0000000000000cb8 R11: 0000000000001e92 R12: ffff887e43299278
R13: ffff887e43298800 R14: ffff887e43299278 R15: ffffffffa034ff88
ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0018
#10 [ffffc90000017de0] ext4_put_super at ffffffffa031a91c [ext4]
#11 [ffffc90000017e20] generic_shutdown_super at ffffffff81244aaf
#12 [ffffc90000017e40] kill_block_super at ffffffff81244e37
#13 [ffffc90000017e60] deactivate_locked_super at ffffffff81244f73
#14 [ffffc90000017e80] deactivate_super at ffffffff8124547a
#15 [ffffc90000017e98] cleanup_mnt at ffffffff81264b2f
#16 [ffffc90000017eb0] __cleanup_mnt at ffffffff81264bc2
#17 [ffffc90000017ec0] task_work_run at ffffffff810a7b50
#18 [ffffc90000017f00] exit_to_usermode_loop at ffffffff810032ba
#19 [ffffc90000017f30] syscall_return_slowpath at ffffffff81003baa
#20 [ffffc90000017f50] entry_SYSCALL_64_fastpath at ffffffff8171a783
RIP: 00007f3241195c47 RSP: 00007fffb3db5438 RFLAGS: 00000246
RAX: 0000000000000000 RBX: 0000560b87fbd920 RCX: 00007f3241195c47
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000560b87fbdd10
RBP: 0000560b87fbda00 R8: 0000000000000000 R9: 00007f32410e416d
R10: 0000000000000021 R11: 0000000000000246 R12: 0000560b87fbdd10
R13: 00007fffb3db5538 R14: 00007fffb3db5523 R15: 0000000000000000
ORIG_RAX: 00000000000000a6 CS: 0033 SS: 002b
crash>
反汇编上下函数
当我们，分析到了出错的具体的代码行，下一步需要分析，传入的参数和struct
首先，我们需要看下 函数 ext4_put_super的原型，发现是static void ext4_put_super(struct super_block *sb),只有一个参数， 而且是一个结构体struct super_block, 现在我们需要知道 *sb 指针的地址是多少呢？ 那这个地址肯定是 上个函数 generic_shutdown_super 传递给它的.
现在分析的关键是，我们需要知道,当generic_shutdown_super在 ffffffff81244aaf 处，调用到 ext4_put_super的时候，传给 ext4_put_super 的指针地址是多少？
首先，需要 反汇编 函数generic_shutdown_super 找到地址ffffffff81244aaf
crash> dis -l generic_shutdown_super
/usr/src/debug/kernel-4.9.79-009.ali3000/linux-4.9.79-009.ali3000.alios7.x86_64/fs/super.c: 436
0xffffffff81244aa0 <generic_shutdown_super+96>: mov 0x30(%r12),%rax
0xffffffff81244aa5 <generic_shutdown_super+101>: test %rax,%rax
0xffffffff81244aa8 <generic_shutdown_super+104>: je 0xffffffff81244aaf <generic_shutdown_super+111>
/usr/src/debug/kernel-4.9.79-009.ali3000/linux-4.9.79-009.ali3000.alios7.x86_64/fs/super.c: 437
0xffffffff81244aaa <generic_shutdown_super+106>: mov %rbx,%rdi <===rbx 和 rdi 数据一致
0xffffffff81244aad <generic_shutdown_super+109>: callq *%rax <===在这里调用下个函数
/usr/src/debug/kernel-4.9.79-009.ali3000/linux-4.9.79-009.ali3000.alios7.x86_64/include/linux/compiler.h: 243
0xffffffff81244aaf <generic_shutdown_super+111>: mov 0x608(%rbx),%rax
/usr/src/debug/kernel-4.9.79-009.ali3000/linux-4.9.79-009.ali3000.alios7.x86_64/fs/super.c: 439
0xffffffff81244ab6 <generic_shutdown_super+118>: lea 0x608(%rbx),%rdx
0xffffffff81244abd <generic_shutdown_super+125>: cmp %rax,%rdx
0xffffffff81244ac0 <generic_shutdown_super+128>: jne 0xffffffff81244b1f <generic_shutdown_super+223>
接着，反汇编ext4_put_super , 你会发现push了很多的寄存器的值到stack
crash> dis -l ext4_put_super
/usr/src/debug/kernel-4.9.79-009.ali3000/linux-4.9.79-009.ali3000.alios7.x86_64/fs/ext4/super.c: 824
0xffffffffa031a570 <ext4_put_super>: nopl 0x0(%rax,%rax,1) [FTRACE NOP]
0xffffffffa031a575 <ext4_put_super+5>: push %rbp
0xffffffffa031a576 <ext4_put_super+6>: mov %rsp,%rbp
0xffffffffa031a579 <ext4_put_super+9>: push %r15 <===第1个寄存器入栈
0xffffffffa031a57b <ext4_put_super+11>: push %r14 <===第2个寄存器入栈
0xffffffffa031a57d <ext4_put_super+13>: push %r13 <===第3个寄存器入栈
0xffffffffa031a57f <ext4_put_super+15>: push %r12 <===第4个寄存器入栈
0xffffffffa031a581 <ext4_put_super+17>: mov %rdi,%r13
0xffffffffa031a584 <ext4_put_super+20>: push %rbx <===第5个寄存器入栈（rbx是在上个函数的时候，就有值的，所以，ext4_put_super函数的第一个参数的指针的地址就是这个寄存器的值）
0xffffffffa031a585 <ext4_put_super+21>: sub $0x8,%rsp
0xffffffffa031a589 <ext4_put_super+25>: mov 0x460(%rdi),%rbx
/usr/src/debug/kernel-4.9.79-009.ali3000/linux-4.9.79-009.ali3000.alios7.x86_64/fs/ext4/super.c: 826
0xffffffffa031a590 <ext4_put_super+32>: mov 0xe0(%rbx),%r14
/usr/src/debug/kernel-4.9.79-009.ali3000/linux-4.9.79-009.ali3000.alios7.x86_64/fs/ext4/super.c: 830
0xffffffffa031a597 <ext4_put_super+39>: callq 0xffffffffa03133f0 <ext4_unregister_li_request>
crash> bt -f
#10 [ffffc90000017de0] ext4_put_super at ffffffffa031a91c [ext4]
ffffc90000017de8: 9cbae75a00000000( ) ffff887e43298800(第5个寄存器的值)
ffffc90000017df8: ffffffffa034a5e0(第4个寄存器的值) ffff887e3818c7b8(第3个寄存器的值)
ffffc90000017e08: 0000000000000000(第2个寄存器的值) ffff887e45918bb0(第1个寄存器的值)
ffffc90000017e18: ffffc90000017e38 ffffffff81244aaf(这两个是不代表寄存器的)
#11 [ffffc90000017e20] generic_shutdown_super at ffffffff81244aaf
ffffc90000017e28: 0000000000000083 ffff887e357b8680
ffffc90000017e38: ffffc90000017e58 ffffffff81244e37
crash> struct super_block ffff887e43298800
struct super_block {
s_list = {
next = 0xffffffff81cb3db0 <super_blocks>, <=======这里也验证了，就是地址ffff887e43298800表示的就是 struct super_block
prev = 0xffff887e43968800
},
s_dev = 271581185,
s_blocksize_bits = 12 'f',
s_blocksize = 4096,
s_maxbytes = 17592186040320,
s_type = 0xffffffffa03589c0 <ext4_fs_type>,
s_op = 0xffffffffa034a5e0 <ext4_sops>,
dq_op = 0xffffffffa034a720 <ext4_quota_operations>,
s_qcop = 0xffffffff81843f60 <dquot_quotactl_sysfile_ops>,
s_export_op = 0xffffffffa034a580 <ext4_export_ops>,
s_flags = 805371904,
s_iflags = 1,
s_magic = 61267,
s_root = 0x0,
s_umount = {
count = {
counter = -4294967295
},
wait_list = {
next = 0xffff887e43298878,
prev = 0xffff887e43298878
},
wait_lock = {
raw_lock = {
val = {
counter = 0
}
}