variables - 内核全局变量存储在哪里？
问题描述
据我所知，全局变量（在用户空间程序中）存储在对应的 DATA（和 BSS）段中。但是在内核代码的情况下，它们是如何以及在哪里存储的呢？
解决方案
例如，让我们看一下 Linux 的全局变量之一。基本上，Linux 是使用 gzip 或其他压缩工具压缩的 ELF 映像。如维基百科（https://en.wikipedia.org/wiki/Vmlinux）所述：
传统上，在创建可引导内核映像时，内核也使用 gzip 进行压缩，或者，从 Linux 2.6.30 开始，[3] 使用 LZMA 或 bzip2，这需要在生成的映像中包含一个非常小的解压缩存根。存根解压缩内核代码，在某些系统上将点打印到控制台以指示进度，然后继续引导过程。稍后添加了对 LZO、[4] xz[5] 和 LZ4[6] 压缩的支持。
extract-vmlinux在 Ubuntu 上，您的 Linux 内核版本附带了一个名为的特殊脚本（ https://blog.packagecloud.io/eng/2016/03/08/how-to-extract-and-disassmble-a-linux -kernel-image-vmlinuz/）。Linux ELF 文件是 /boot/vmlinuz。要解压它，您必须使用可安装的 extract-vmlinux 脚本
sudo apt-get install linux-headers-$(uname -r)
该脚本可以安全使用，因为它是一个标准脚本，可以在您的分发包的官方存储库中找到（那里的大多数包都是开源且安全的）。现在使用
sudo /usr/src/kernels/$(uname -r)/scripts/extract-vmlinux /boot/vmlinuz-$(uname -r) > vmlinux
从压缩的 vmlinux 映像中提取 Linux 内核的 ELF 文件到您的主存储库。现在我们可以查看Linux内核的ELF文件。用于readelf -a vmlinux对内容进行摘要。我的输出是这样的：
user@user-System-Product-Name:~$ readelf -a vmlinux
ELF Header:
Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
Class: ELF64
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: EXEC (Executable file)
Machine: Advanced Micro Devices X86-64
Version: 0x1
Entry point address: 0x1000000
Start of program headers: 64 (bytes into file)
Start of section headers: 46137728 (bytes into file)
Flags: 0x0
Size of this header: 64 (bytes)
Size of program headers: 56 (bytes)
Number of program headers: 5
Size of section headers: 64 (bytes)
Number of section headers: 35
Section header string table index: 34
Section Headers:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
[ 0] NULL 0000000000000000 00000000
0000000000000000 0000000000000000 0 0 0
[ 1] .text PROGBITS ffffffff81000000 00200000
0000000000e025f7 0000000000000000 AX 0 0 4096
[ 2] .rodata PROGBITS ffffffff82000000 01200000
00000000004e46f2 0000000000000000 WA 0 0 4096
[ 3] .pci_fixup PROGBITS ffffffff824e4700 016e4700
00000000000032b0 0000000000000000 A 0 0 16
[ 4] .tracedata PROGBITS ffffffff824e79b0 016e79b0
0000000000000078 0000000000000000 A 0 0 1
[ 5] __ksymtab PROGBITS ffffffff824e7a28 016e7a28
0000000000010a34 0000000000000000 A 0 0 4
[ 6] __ksymtab_gpl PROGBITS ffffffff824f845c 016f845c
00000000000121ec 0000000000000000 A 0 0 4
[ 7] __kcrctab PROGBITS ffffffff8250a648 0170a648
00000000000058bc 0000000000000000 A 0 0 4
[ 8] __kcrctab_gpl PROGBITS ffffffff8250ff04 0170ff04
00000000000060a4 0000000000000000 A 0 0 4
[ 9] __ksymtab_strings PROGBITS ffffffff82515fa8 01715fa8
0000000000037bea 0000000000000001 AMS 0 0 1
[10] __init_rodata PROGBITS ffffffff8254dba0 0174dba0
00000000000002a8 0000000000000000 A 0 0 32
[11] __param PROGBITS ffffffff8254de48 0174de48
00000000000038e0 0000000000000000 A 0 0 8
[12] __modver PROGBITS ffffffff82551728 01751728
00000000000005a0 0000000000000000 WA 0 0 8
[13] __ex_table PROGBITS ffffffff82551cd0 01751cd0
0000000000001af4 0000000000000000 A 0 0 4
[14] .notes NOTE ffffffff825537c4 017537c4
00000000000001ec 0000000000000000 A 0 0 4
[15] .data PROGBITS ffffffff82600000 01800000
00000000003657c0 0000000000000000 WA 0 0 8192
[16] __bug_table PROGBITS ffffffff829657c0 01b657c0
00000000000178d4 0000000000000000 WA 0 0 1
[17] .vvar PROGBITS ffffffff8297e000 01b7e000
0000000000001000 0000000000000000 WA 0 0 16
[18] .data..percpu PROGBITS 0000000000000000 01c00000
000000000002f000 0000000000000000 WA 0 0 4096
[19] .init.text PROGBITS ffffffff829ae000 01dae000
00000000000772e5 0000000000000000 AX 0 0 16
[20] .altinstr_aux PROGBITS ffffffff82a252e5 01e252e5
0000000000002d9b 0000000000000000 AX 0 0 1
[21] .init.data PROGBITS ffffffff82a2a000 01e2a000
00000000001c74f0 0000000000000000 WA 0 0 8192
[22] .x86_cpu_dev.init PROGBITS ffffffff82bf14f0 01ff14f0
0000000000000028 0000000000000000 A 0 0 8
[23] .parainstructions PROGBITS ffffffff82bf1518 01ff1518
0000000000022b7c 0000000000000000 A 0 0 8
[24] .altinstructions PROGBITS ffffffff82c14098 02014098
0000000000006660 0000000000000000 A 0 0 1
[25] .altinstr_replace PROGBITS ffffffff82c1a6f8 0201a6f8
0000000000001789 0000000000000000 AX 0 0 1
[26] .iommu_table PROGBITS ffffffff82c1be88 0201be88
00000000000000f0 0000000000000000 A 0 0 8
[27] .apicdrivers PROGBITS ffffffff82c1bf78 0201bf78
0000000000000040 0000000000000000 WA 0 0 8
[28] .exit.text PROGBITS ffffffff82c1bfb8 0201bfb8
0000000000001f4b 0000000000000000 AX 0 0 1
[29] .smp_locks PROGBITS ffffffff82c1e000 0201e000
000000000000a000 0000000000000000 A 0 0 4
[30] .data_nosave PROGBITS ffffffff82c28000 02028000
0000000000001000 0000000000000000 WA 0 0 4
[31] .bss NOBITS ffffffff82c29000 02029000
00000000005d7000 0000000000000000 WA 0 0 4096
[32] .brk NOBITS ffffffff83200000 02029000
000000000002c000 0000000000000000 WA 0 0 1
[33] .init.scratch PROGBITS ffffffff83400000 02800000
0000000000400000 0000000000000000 WA 0 0 32
[34] .shstrtab STRTAB 0000000000000000 02c00000
0000000000000180 0000000000000000 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
L (link order), O (extra OS processing required), G (group), T (TLS),
C (compressed), x (unknown), o (OS specific), E (exclude),
l (large), p (processor specific)
There are no section groups in this file.
Program Headers:
Type Offset VirtAddr PhysAddr
FileSiz MemSiz Flags Align
LOAD 0x0000000000200000 0xffffffff81000000 0x0000000001000000
0x00000000015539b0 0x00000000015539b0 R E 0x200000
LOAD 0x0000000001800000 0xffffffff82600000 0x0000000002600000
0x000000000037f000 0x000000000037f000 RW 0x200000
LOAD 0x0000000001c00000 0x0000000000000000 0x000000000297f000
0x000000000002f000 0x000000000002f000 RW 0x200000
LOAD 0x0000000001dae000 0xffffffff829ae000 0x00000000029ae000
0x0000000000e52000 0x0000000000e52000 RWE 0x200000
NOTE 0x00000000017537c4 0xffffffff825537c4 0x00000000025537c4
0x00000000000001ec 0x00000000000001ec 0x4
Section to Segment mapping:
Segment Sections...
00 .text .rodata .pci_fixup .tracedata __ksymtab __ksymtab_gpl __kcrctab __kcrctab_gpl __ksymtab_strings __init_rodata __param __modver __ex_table .notes
01 .data __bug_table .vvar
02 .data..percpu
03 .init.text .altinstr_aux .init.data .x86_cpu_dev.init .parainstructions .altinstructions .altinstr_replacement .iommu_table .apicdrivers .exit.text .smp_locks .data_nosave .bss .brk .init.scratch
04 .notes
There is no dynamic section in this file.
There are no relocations in this file.
The decoding of unwind sections for machine type Advanced Micro Devices X86-64 is not currently supported.
No version information found in this file.
Displaying notes found in: .notes
Owner Data size Description
Xen 0x00000006 Unknown note type: (0x00000006)
description data: 6c 69 6e 75 78 00
Xen 0x00000004 Unknown note type: (0x00000007)
description data: 32 2e 36 00
Xen 0x00000008 Unknown note type: (0x00000005)
description data: 78 65 6e 2d 33 2e 30 00
Xen 0x00000008 Unknown note type: (0x00000003)
description data: 00 00 00 80 ff ff ff ff
Xen 0x00000008 Unknown note type: (0x0000000f)
description data: 00 00 00 00 80 00 00 00
Xen 0x00000008 NT_VERSION (version)
description data: c0 e1 9a 82 ff ff ff ff
Xen 0x00000008 NT_ARCH (architecture)
description data: 00 20 00 81 ff ff ff ff
Xen 0x00000029 Unknown note type: (0x0000000a)
description data: 21 77 72 69 74 61 62 6c 65 5f 70 61 67 65 5f 74 61 62 6c 65 73 7c 70 61 65 5f 70 67 64 69 72 5f 61 62 6f 76 65 5f 34 67 62
Xen 0x00000004 Unknown note type: (0x00000011)
description data: 01 88 00 00
Xen 0x00000004 Unknown note type: (0x00000009)
description data: 79 65 73 00
Xen 0x00000008 Unknown note type: (0x00000008)
description data: 67 65 6e 65 72 69 63 00
Xen 0x00000010 Unknown note type: (0x0000000d)
description data: 01 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00
Xen 0x00000004 Unknown note type: (0x0000000e)
description data: 01 00 00 00
Xen 0x00000004 Unknown note type: (0x00000010)
description data: 01 00 00 00
Xen 0x00000008 Unknown note type: (0x0000000c)
description data: 00 00 00 00 00 80 ff ff
Xen 0x00000008 Unknown note type: (0x00000004)
description data: 00 00 00 00 00 00 00 00
GNU 0x00000014 NT_GNU_BUILD_ID (unique build ID bitstring)
Build ID: c0263a3075bc0a9388365ddf35ab5422da3356a9
Linux 0x00000001 OPEN
description data: 00
Xen 0x00000008 Unknown note type: (0x00000012)
description data: 40 07 00 01 00 00 00 00
该问题的重要部分是以下Program headers部分：
Program Headers:
Type Offset VirtAddr PhysAddr
FileSiz MemSiz Flags Align
LOAD 0x0000000000200000 0xffffffff81000000 0x0000000001000000
0x00000000015539b0 0x00000000015539b0 R E 0x200000
LOAD 0x0000000001800000 0xffffffff82600000 0x0000000002600000
0x000000000037f000 0x000000000037f000 RW 0x200000
LOAD 0x0000000001c00000 0x0000000000000000 0x000000000297f000
0x000000000002f000 0x000000000002f000 RW 0x200000
LOAD 0x0000000001dae000 0xffffffff829ae000 0x00000000029ae000
0x0000000000e52000 0x0000000000e52000 RWE 0x200000
NOTE 0x00000000017537c4 0xffffffff825537c4 0x00000000025537c4
0x00000000000001ec 0x00000000000001ec 0x4
我猜 RW 部分是静态和独立内核映像的数据/bss 段。此内核映像中没有太多 bss 数据（未初始化的数据）。您可以通过查看不同段的文件大小与内存大小来看到这一点。在 bss 数据中，只有大小写入可执行文件。然后加载程序将零分配给这些段。由于我们只有与文件大小相等的 memsize，所以我们这里没有太多 bss。
此外，我们还可以查看内核在解压后加载自身的不同虚拟地址。这对于查找某个全局变量所在的段很有用。
现在，要确定全局变量的加载位置，可以查看 /boot 目录中的 System.map 文件。例如，我们可以在 kernel/sched/core.c 中调用一个随机全局变量，sysctl_sched_rt_period并使用
user@user-System-Product-Name:~$ sudo grep "sysctl_sched_rt_period" /boot/System.map-$(uname -r)
ffffffff8266eb24 D sysctl_sched_rt_period
地址是这样的ffffffff8266eb24。该地址位于 ffffffff82600000 加载的 RW 段中，因为该段的大小为 37f000 并且因为ffffffff82600000 + 37f000 > ffffffff8266eb24. 可以清楚的看到这个全局变量存放在内核可执行文件的数据段中。
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