Kprobe

linux内核源码Documentation目录下存在kprobe介绍文档如下

Kprobes allows multiple probes at the same address.  Currently,

however, there cannot be multiple jprobes on the same function at

the same time.

If you install a probe in an inline-able function, Kprobes makes

no attempt to chase down all inline instances of the function and

install probes there.  gcc may inline a function without being asked,

so keep this in mind if you're not seeing the probe hits you expect.

A probe handler can modify the environment of the probed function

-- e.g., by modifying kernel data structures, or by modifying the

contents of the pt_regs struct (which are restored to the registers

upon return from the breakpoint).  So Kprobes can be used, for example,

to install a bug fix or to inject faults for testing.  Kprobes, of

course, has no way to distinguish the deliberately injected faults

from the accidental ones.  Don't drink and probe.

Kprobes makes no attempt to prevent probe handlers from stepping on

each other -- e.g., probing printk() and then calling printk() from a

probe handler.  If a probe handler hits a probe, that second probe's

handlers won't be run in that instance, and the kprobe.nmissed member

of the second probe will be incremented.

Kprobes does not use mutexes or allocate memory except during

registration and unregistration.

/sys/kernel/debug/kprobes

1. crash> gdb disass /r  tcp_v4_rcv   #加载了tcp_debug模块 jprobe
2. Dump of assembler code forfunction tcp_v4_rcv:
3.   0xffffffff81782980<+0>:     e8 7b c6 893e  callq  0xffffffffc001f000
4.   0xffffffff81782985<+5>:     55      push   %rbp
5.   0xffffffff81782986<+6>:     4889 e5        mov    %rsp,%rbp
6.   0xffffffff81782989<+9>:     4157   push   %r15
7.   0xffffffff8178298b<+11>:    4156   push   %r14
8.   0xffffffff8178298d<+13>:    4155   push   %r13
9.   0xffffffff8178298f<+15>:    4154   push   %r12
10.   0xffffffff81782991<+17>:    53      push   %rbx
11.   0xffffffff81782992<+18>:    4889 fb        mov    %rdi,%rbx
12.   0xffffffff81782995<+21>:    4883 ec 60     sub    $0x60,%rsp
13. crash>
14. crash> gdb disass /r  tcp_v4_rcv    #卸载tcp_debug模块 jprobe
15. Dump of assembler code forfunction tcp_v4_rcv:
16.   0xffffffff81782980<+0>:     6666666690  data32 data32 data32 xchg %ax,%ax
17.   0xffffffff81782985<+5>:     55      push   %rbp
18.   0xffffffff81782986<+6>:     4889 e5        mov    %rsp,%rbp
19.   0xffffffff81782989<+9>:     4157   push   %r15
20.   0xffffffff8178298b<+11>:    4156   push   %r14
21.   0xffffffff8178298d<+13>:    4155   push   %r13
22.   0xffffffff8178298f<+15>:    4154   push   %r12
23.   0xffffffff81782991<+17>:    53      push   %rbx
24.   0xffffffff81782992<+18>:    4889 fb        mov    %rdi,%rbx
25.   0xffffffff81782995<+21>:    4883 ec 60     sub    $0x60,%rsp
26. crash>

1. (gdb) disass /r  tcp_v4_rcv    #vmlinux中原始文件反汇编
2. Dump of assembler code forfunction tcp_v4_rcv:
3.   0xffffffff81782980<+0>: e8 ab 9f0a00 callq  0xffffffff8182c930<__fentry__>
4.   0xffffffff81782985<+5>:55 push   %rbp
5.   0xffffffff81782986<+6>:4889 e5 mov    %rsp,%rbp
6.   0xffffffff81782989<+9>:4157 push   %r15
7.   0xffffffff8178298b<+11>:4156 push   %r14
8.   0xffffffff8178298d<+13>:4155 push   %r13
9.   0xffffffff8178298f<+15>:4154 push   %r12
10.   0xffffffff81782991<+17>:53 push   %rbx
11.   0xffffffff81782992<+18>:4889 fb mov    %rdi,%rbx
12.   0xffffffff81782995<+21>:4883 ec 60 sub    $0x60,%rsp
13.   0xffffffff81782999<+25>: f6 879000000007 testb  $0x7,0x90(%rdi)
14.   0xffffffff817829a0<+32>:7556 jne    0xffffffff817829f8<tcp_v4_rcv+120>

1. crash> gdb disass /r  jprobe_return
2. Dump of assembler code forfunction jprobe_return:
3.   0xffffffff8105dcc0<+0>:     6666666690  data32 data32 data32 xchg %ax,%ax
4.   0xffffffff8105dcc5<+5>:     55      push   %rbp
5.   0xffffffff8105dcc6<+6>:     48 c7 c0 a0 d7 0000    mov    $0xd7a0,%rax
6.   0xffffffff8105dccd<+13>:    4889 e5        mov    %rsp,%rbp
7.   0xffffffff8105dcd0<+16>:    53      push   %rbx
8.   0xffffffff8105dcd1<+17>:    65480305 af c4 fa 7e add    %gs:0x7efac4af(%rip),%rax        # 0xa188
9.   0xffffffff8105dcd9<+25>:    488b5818     mov    0x18(%rax),%rbx
10.   0xffffffff8105dcdd<+29>:    4887 dc        xchg   %rbx,%rsp
11.   0xffffffff8105dce0<+32>:    cc      int3  
12.   0xffffffff8105dce1<+33>:    90      nop
13.   0xffffffff8105dce2<+34>:    5b      pop    %rbx
14.   0xffffffff8105dce3<+35>:    5d      pop    %rbp
15.   0xffffffff8105dce4<+36>:    c3      retq  
16. End of assembler dump.

对tcp_ack中0xffffffff817748bf位置进行kprobe前后对比

通过/proc/kallsyms查看的文件类型对应如下（./scripts/mksysmap）：

#   The second row specify the type of the symbol:

#   A = Absolute

#   B = Uninitialised data (.bss)

#   C = Common symbol

#   D = Initialised data

#   G = Initialised data for small objects

#   I = Indirect reference to another symbol

#   N = Debugging symbol

#   R = Read only

#   S = Uninitialised data for small objects

#   T = Text code symbol

#   U = Undefined symbol

#   V = Weak symbol

#   W = Weak symbol

#   Corresponding small letters are local symbols

# For System.map filter away:

#   a - local absolute symbols

#   U - undefined global symbols

#   N - debugging symbols

#   w - local weak symbols

__kstrtab节（保存符号名）、__ksymtab节（所有模块可使用的符号地址）和__ksymtab_gpl节（GPL兼容许可证下发布的模块可以使用的符号地址，其他的未找到

1. cat /proc/kallsyms | cut -d " "-f 2| sort -u    //查看文件类型
2. cat /proc/kallsyms | awk '$2=="a" {print $2 "\t" $3}'    //查看某个文件类型对应的符号

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