本文基於Linux-4.0，根據一個crash現場的實例，根據堆棧中的數據，反推整個函數調用流程，由於本例子存在oops，也會直接打印出backtrace，最終可以與我們的分析結果做一下比較，看看分析是否正確。
/ # echo c > /proc/sysrq-trigger
sysrq: SysRq : Trigger a crash
Unable to handle kernel NULL pointer dereference at virtual address 00000000
pgd = ffff8000779f9000
[00000000] *pgd=00000000b79fb003, *pud=00000000b7cb3003, *pmd=0000000000000000
Internal error: Oops: 94000046 [#1] PREEMPT SMP
Modules linked in:
CPU: 1 PID: 642 Comm: sh Not tainted 4.0.0 #1
Hardware name: linux,dummy-virt (DT)
task: ffff800077f46300 ti: ffff800077804000 task.ti: ffff800077804000
PC is at sysrq_handle_crash+0x14/0x1c
LR is at __handle_sysrq+0x124/0x194
pc : [<ffff800000377cd4>] lr : [<ffff800000378688>] pstate: 60000145
sp : ffff800077807dd0
x29: ffff800077807dd0 x28: ffff800077804000
x27: ffff80000056d000 x26: 0000000000000040
x25: 000000000000011a x24: 0000000000000015
x23: 0000000000000000 x22: 0000000000000007
x21: 0000000000000063 x20: ffff8000007ae000
x19: ffff8000007c18e8 x18: 00000000005fd000
x17: 0000000000602000 x16: ffff80000019705c
x15: 0000000000001000 x14: 0ffffffffffffffe
x13: 0000000000000038 x12: 0101010101010101
x11: ffff8000007ae000 x10: 000000000000007c
x9 : 0000000000000002 x8 : 0000000000000001
x7 : 000000000000007c x6 : 0000000000000030
x5 : 0000000000002208 x4 : 0000000000000000
x3 : 0000000000000000 x2 : ffff800077804000
x1 : 0000000000000000 x0 : 0000000000000001
Process sh (pid: 642, stack limit = 0xffff800077804028)
Stack: (0xffff800077807dd0 to 0xffff800077808000)
7dc0: 77807e10 ffff8000 00378b04 ffff8000
7de0: 00000002 00000000 fffffffb ffffffff 77807ec8 ffff8000 1c84b550 00000000
7e00: 60000000 00000000 1c84b550 00000000 77807e30 ffff8000 001f2f5c ffff8000
7e20: 785b8c00 ffff8000 1c84b550 00000000 77807e50 ffff8000 00196650 ffff8000
7e40: 78607d00 ffff8000 00000002 00000000 77807e90 ffff8000 001970a0 ffff8000
7e60: 78607d00 ffff8000 78607d00 ffff8000 1c84b550 00000000 00000002 00000000
7e80: 60000000 00000000 00000000 00000000 da4cd8c0 0000ffff 00085c30 ffff8000
7ea0: 00000000 00000000 1c84b550 00000000 ffffffff ffffffff 00409398 00000000
7ec0: 00200200 00000000 00000000 00000000 00000001 00000000 1c84b550 00000000
7ee0: 00000002 00000000 00000000 00000000 1c84b550 00000000 1c84b560 00000000
7f00: 80808080 00808080 fefeff62 fefefefe 00000040 00000000 fefefeff fefefefe
7f20: 7f7f7f7f 7f7f7f7f 01010101 01010101 00000008 00000000 00000400 00000000
7f40: 00502b5c 00000050 00001000 00000000 00000000 00000000 00602000 00000000
7f60: 005fd000 00000000 00000001 00000000 1c84b550 00000000 00000002 00000000
7f80: 00601000 00000000 1c84b550 00000000 00000020 00000000 00000000 00000000
7fa0: 00601000 00000000 005820f0 00000000 da4cdfba 0000ffff da4cd8c0 0000ffff
7fc0: 0044a504 00000000 da4cd180 0000ffff 00409398 00000000 60000000 00000000
7fe0: 00000001 00000000 00000040 00000000 00000000 00000000 00000000 00000000
Call trace:
[<ffff800000377cd4>] sysrq_handle_crash+0x14/0x1c
[<ffff800000378b00>] write_sysrq_trigger+0x50/0x64
[<ffff8000001f2f58>] proc_reg_write+0x54/0x84
[<ffff80000019664c>] vfs_write+0x98/0x1d8
[<ffff80000019709c>] SyS_write+0x40/0xa0
Code: 52800020 b903a420 d5033e9f d2800001 (39000020)
---[ end trace 2fd7253656805fb6 ]---
SP和FP(x29)寄存器中的值都是ffff800077807dd0，從後面信息可以進行堆棧回溯：
ffff8000 77807dd0 --- FP
ffff8000 77807e10 --- FP'(從FP寄存器地址處讀取)
ffff8000 77807e30 --- FP''(從FP'寄存器地址處讀取)
ffff8000 77807e50 --- FP'''(從FP''寄存器地址處讀取)
ffff8000 77807e90 --- FP''''(從FP'''寄存器地址處讀取)
0000ffff da4cd8c0 --- 該值已經不是內核地址了，所以已經超出了內核堆棧區域
我們找到了FP地址後，可以進一步找到LR寄存器中保存的返回地址：
ffff8000 00378b04 --- FP' + 8
ffff8000 001f2f5c --- FP'' + 8
ffff8000 00196650 --- FP''' + 8
ffff8000 001970a0 --- FP'''' + 8
接下來看是一個一個查看對應的堆棧調用關係，從第一個LR地址開始查看，進入gdb調試vmlinux：
aarch64-linux-gnu-gdb vmlinux
跳轉關係1
使用反彙編指令：
(gdb) disassemble 0xffff800000378b04
Dump of assembler code for function write_sysrq_trigger:
0xffff800000378ab0 <+0>: stp x29, x30, [sp,#-32]!
0xffff800000378ab4 <+4>: mov x29, sp
0xffff800000378ab8 <+8>: str x19, [sp,#16]
0xffff800000378abc <+12>: mov x19, x2
0xffff800000378ac0 <+16>: cbz x2, 0xffff800000378b04 <write_sysrq_trigger+84>
0xffff800000378ac4 <+20>: mov x0, sp
0xffff800000378ac8 <+24>: mov x2, x1
0xffff800000378acc <+28>: and x3, x0, #0xffffffffffffc000
0xffff800000378ad0 <+32>: mov x0, #0xfffffffffffffff2 // #-14
0xffff800000378ad4 <+36>: ldr x3, [x3,#8]
0xffff800000378ad8 <+40>: adds x2, x2, #0x1
0xffff800000378adc <+44>: ccmp x2, x3, #0x2, cc
0xffff800000378ae0 <+48>: cset x4, ls
0xffff800000378ae4 <+52>: cbz x4, 0xffff800000378b08 <write_sysrq_trigger+88>
0xffff800000378ae8 <+56>: mov w2, #0x0 // #0
0xffff800000378aec <+60>: ldrb w3, [x1]
0xffff800000378af0 <+64>: uxtb w3, w3
0xffff800000378af4 <+68>: cbnz w2, 0xffff800000378b08 <write_sysrq_trigger+88>
0xffff800000378af8 <+72>: mov w1, #0x0 // #0
0xffff800000378afc <+76>: mov w0, w3
0xffff800000378b00 <+80>: bl 0xffff800000378564 <__handle_sysrq>
0xffff800000378b04 <+84>: mov x0, x19
0xffff800000378b08 <+88>: ldr x19, [sp,#16]
0xffff800000378b0c <+92>: ldp x29, x30, [sp],#32
0xffff800000378b10 <+96>: ret
關鍵的在如下的位置：
0xffff800000378b00 <+80>: bl 0xffff800000378564 <__handle_sysrq>
0xffff800000378b04 <+84>: mov x0, x19
我們對應的LR地址爲0xffff800000378b04，該值-4就應該是跳轉指令：所以上一級函數是write_sysrq_trigger，跳轉進入的函數symbol爲__handle_sysrq。
跳轉關係2
使用反彙編指令：
(gdb) disassemble 0xffff8000001f2f5c
Dump of assembler code for function proc_reg_write:
0xffff8000001f2f04 <+0>: stp x29, x30, [sp,#-32]!
0xffff8000001f2f08 <+4>: mov w4, #0x0 // #0
0xffff8000001f2f0c <+8>: mov x29, sp
0xffff8000001f2f10 <+12>: stp x19, x20, [sp,#16]
0xffff8000001f2f14 <+16>: ldr x5, [x0,#32]
0xffff8000001f2f18 <+20>: ldur x19, [x5,#-32]
0xffff8000001f2f1c <+24>: add x5, x19, #0x64
0xffff8000001f2f20 <+28>: add w6, w4, #0x1
0xffff8000001f2f24 <+32>: dmb ish
0xffff8000001f2f28 <+36>: ldxr w7, [x5]
0xffff8000001f2f2c <+40>: cmp w7, w4
0xffff8000001f2f30 <+44>: b.ne 0xffff8000001f2f3c <proc_reg_write+56>
0xffff8000001f2f34 <+48>: stxr w8, w6, [x5]
0xffff8000001f2f38 <+52>: cbnz w8, 0xffff8000001f2f28 <proc_reg_write+36>
0xffff8000001f2f3c <+56>: dmb ish
0xffff8000001f2f40 <+60>: cmp w4, w7
0xffff8000001f2f44 <+64>: b.ne 0xffff8000001f2f78 <proc_reg_write+116>
0xffff8000001f2f48 <+68>: ldr x4, [x19,#40]
0xffff8000001f2f4c <+72>: mov x20, #0xfffffffffffffffb // #-5
0xffff8000001f2f50 <+76>: ldr x4, [x4,#24]
0xffff8000001f2f54 <+80>: cbz x4, 0xffff8000001f2f60 <proc_reg_write+92>
0xffff8000001f2f58 <+84>: blr x4
0xffff8000001f2f5c <+88>: mov x20, x0
0xffff8000001f2f60 <+92>: mov x0, x19
0xffff8000001f2f64 <+96>: bl 0xffff8000001f2bf4 <unuse_pde>
0xffff8000001f2f68 <+100>: mov x0, x20
0xffff8000001f2f6c <+104>: ldp x19, x20, [sp,#16]
0xffff8000001f2f70 <+108>: ldp x29, x30, [sp],#32
0xffff8000001f2f74 <+112>: ret
0xffff8000001f2f78 <+116>: mov w4, w7
0xffff8000001f2f7c <+120>: tbz w7, #31, 0xffff8000001f2f20 <proc_reg_write+28>
0xffff8000001f2f80 <+124>: mov x20, #0xfffffffffffffffb // #-5
0xffff8000001f2f84 <+128>: b 0xffff8000001f2f68 <proc_reg_write+100>
End of assembler dump.
關鍵的在如下的位置：
0xffff8000001f2f58 <+84>: blr x4
0xffff8000001f2f5c <+88>: mov x20, x0
我們對應的LR地址爲0xffff8000001f2f5c，該值-4就應該是跳轉指令：blr x4 實際上是指跳轉到x4寄存器中的一個地址處。返回地址位於proc_reg_write函數中，因此該函數應該就是上一級調用的函數，至於這個x4寄存器地址對應的symbol，實際上就是write_sysrq_trigger，因爲是從它返回過來的。
跳轉關係3
使用反彙編指令：
(gdb) disassemble 0xffff800000196650
Dump of assembler code for function vfs_write:
0xffff8000001965b4 <+0>: stp x29, x30, [sp,#-64]!
0xffff8000001965b8 <+4>: mov x29, sp
0xffff8000001965bc <+8>: stp x19, x20, [sp,#16]
0xffff8000001965c0 <+12>: stp x21, x22, [sp,#32]
0xffff8000001965c4 <+16>: str x23, [sp,#48]
0xffff8000001965c8 <+20>: ldr w4, [x0,#68]
0xffff8000001965cc <+24>: tbz w4, #1, 0xffff80000019676c <vfs_write+440>
0xffff8000001965d0 <+28>: tbz w4, #18, 0xffff800000196774 <vfs_write+448>
0xffff8000001965d4 <+32>: mov x4, sp
0xffff8000001965d8 <+36>: and x5, x4, #0xffffffffffffc000
0xffff8000001965dc <+40>: mov x4, x1
0xffff8000001965e0 <+44>: ldr x5, [x5,#8]
0xffff8000001965e4 <+48>: adds x4, x4, x2
0xffff8000001965e8 <+52>: ccmp x4, x5, #0x2, cc
0xffff8000001965ec <+56>: cset x6, ls
0xffff8000001965f0 <+60>: cbz x6, 0xffff800000196750 <vfs_write+412>
0xffff8000001965f4 <+64>: mov x21, x3
0xffff8000001965f8 <+68>: mov x22, x1
0xffff8000001965fc <+72>: mov x3, x2
0xffff800000196600 <+76>: mov x1, x0
0xffff800000196604 <+80>: mov x19, x0
0xffff800000196608 <+84>: mov x2, x21
0xffff80000019660c <+88>: mov w0, #0x1 // #1
0xffff800000196610 <+92>: bl 0xffff8000001964c4 <rw_verify_area>
0xffff800000196614 <+96>: sxtw x20, w0
0xffff800000196618 <+100>: tbnz x20, #63, 0xffff8000001966c0 <vfs_write+268>
0xffff80000019661c <+104>: ldr x1, [x19,#32]
0xffff800000196620 <+108>: ldrh w0, [x1]
0xffff800000196624 <+112>: and w0, w0, #0xf000
0xffff800000196628 <+116>: cmp w0, #0x8, lsl #12
0xffff80000019662c <+120>: b.eq 0xffff800000196758 <vfs_write+420>
0xffff800000196630 <+124>: ldr x0, [x19,#40]
0xffff800000196634 <+128>: ldr x4, [x0,#24]
0xffff800000196638 <+132>: cbz x4, 0xffff8000001966d8 <vfs_write+292>
0xffff80000019663c <+136>: mov x2, x20
0xffff800000196640 <+140>: mov x3, x21
0xffff800000196644 <+144>: mov x1, x22
0xffff800000196648 <+148>: mov x0, x19
0xffff80000019664c <+152>: blr x4
0xffff800000196650 <+156>: mov x20, x0
關鍵的在如下的位置：
0xffff80000019664c <+152>: blr x4
0xffff800000196650 <+156>: mov x20, x0
我們對應的LR地址爲0xffff800000196650，該值-4就應該是跳轉指令：blr x4 ，至於這個x4寄存器地址對應的symbol，實際上就是proc_reg_write，該函數運行結束後會返回到vfs_write中，因此上一級的函數是vfs_write。
跳轉關係4
使用反彙編指令：
(gdb) disassemble 0xffff8000001970a0
Dump of assembler code for function SyS_write:
0xffff80000019705c <+0>: stp x29, x30, [sp,#-64]!
0xffff800000197060 <+4>: mov x29, sp
0xffff800000197064 <+8>: stp x19, x20, [sp,#16]
0xffff800000197068 <+12>: stp x21, x22, [sp,#32]
0xffff80000019706c <+16>: mov x21, x1
0xffff800000197070 <+20>: mov x22, x2
0xffff800000197074 <+24>: bl 0xffff8000001b2f14 <__fdget_pos>
0xffff800000197078 <+28>: ands x20, x0, #0xfffffffffffffffc
0xffff80000019707c <+32>: mov x19, x0
0xffff800000197080 <+36>: b.eq 0xffff8000001970f4 <SyS_write+152>
0xffff800000197084 <+40>: add x3, x29, #0x40
0xffff800000197088 <+44>: ldr x4, [x20,#112]
0xffff80000019708c <+48>: mov x1, x21
0xffff800000197090 <+52>: mov x2, x22
0xffff800000197094 <+56>: mov x0, x20
0xffff800000197098 <+60>: str x4, [x3,#-8]!
0xffff80000019709c <+64>: bl 0xffff8000001965b4 <vfs_write>
0xffff8000001970a0 <+68>: mov x21, x0
0xffff8000001970a4 <+72>: tbnz x21, #63, 0xffff8000001970b0 <SyS_write+84>
0xffff8000001970a8 <+76>: ldr x0, [x29,#56]
0xffff8000001970ac <+80>: str x0, [x20,#112]
0xffff8000001970b0 <+84>: tbnz w19, #1, 0xffff8000001970cc <SyS_write+112>
0xffff8000001970b4 <+88>: tbnz w19, #0, 0xffff8000001970d8 <SyS_write+124>
0xffff8000001970b8 <+92>: mov x0, x21
0xffff8000001970bc <+96>: ldp x19, x20, [sp,#16]
0xffff8000001970c0 <+100>: ldp x21, x22, [sp,#32]
0xffff8000001970c4 <+104>: ldp x29, x30, [sp],#64
0xffff8000001970c8 <+108>: ret
0xffff8000001970cc <+112>: add x0, x20, #0x48
0xffff8000001970d0 <+116>: bl 0xffff8000005611bc <mutex_unlock>
0xffff8000001970d4 <+120>: tbz w19, #0, 0xffff8000001970b8 <SyS_write+92>
0xffff8000001970d8 <+124>: mov x0, x20
0xffff8000001970dc <+128>: bl 0xffff800000198128 <fput>
0xffff8000001970e0 <+132>: mov x0, x21
0xffff8000001970e4 <+136>: ldp x19, x20, [sp,#16]
0xffff8000001970e8 <+140>: ldp x21, x22, [sp,#32]
0xffff8000001970ec <+144>: ldp x29, x30, [sp],#64
0xffff8000001970f0 <+148>: ret
0xffff8000001970f4 <+152>: mov x21, #0xfffffffffffffff7 // #-9
0xffff8000001970f8 <+156>: b 0xffff8000001970b8 <SyS_write+92>
End of assembler dump.
關鍵的在如下的位置：
0xffff80000019709c <+64>: bl 0xffff8000001965b4 <vfs_write>
0xffff8000001970a0 <+68>: mov x21, x0
我們對應的LR地址爲0xffff8000001970a0，該值-4就應該是跳轉指令：bl 0xffff8000001965b4 <vfs_write> ，這裏看起來就很清晰了，從SyS_write函數跳轉到vfs_write函數，完成之後又返回到了SyS_write函數，因此上一級的函數是SyS_write。
總結
從上面的分析過程，我們可以梳理出函數的調用關係如下：
SyS_write --> vfs_write --> proc_reg_write --> write_sysrq_trigger --> __handle_sysrq --> sysrq_handle_crash
看看這個結果是不是與dumpstack中的一致，說明整個分析過程是正確的。哈哈~