Linux -- Getting started
It is so hard to compile Linux kernel on my Macbook M1. Since I only need the compilation database, I decided to build it in a Linux box, and then copy the the file compile_commands.json out.
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cd ~/code/linux
# install dependencies
sudo apt-get install flex libelf-dev bc bear liblz4-tool
# Disable certificates
scripts/config --disable SYSTEM_TRUSTED_KEYS
scripts/config --disable SYSTEM_REVOCATION_KEYS
# enable cgroup
scripts/config --enable CONFIG_CGROUPS
scripts/config --enable CONFIG_RESOURCE_COUNTERS
scripts/config --enable CONFIG_MEMCG
scripts/config --enable CONFIG_MEMCG_SWAP
scripts/config --enable CONFIG_MEMCG_KMEM
# unselect all options
make menuconfig
# build
bear -- make -j4
# copy it out
scp ...
# fix the commands
sed -i 's;home/admin;Users/xiongding;g' compile_commands.json
It only took a few minutes because I disabled all options at the menu config step. Now, clangd jumps perfectly!
To avoid compilation again in the future, I uploaded the compilation database file to gist.
ELF
Quote from ChatGPT:
An ELF (Executable and Linkable Format) file is a binary file format used by most Unix-based operating systems for executable files, object code, shared libraries, and core dumps. It provides a standard way of organizing executable code and data, as well as facilitating the loading and execution of programs.
Sections
Example sections:
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# readelf -S /bin/ls -W
There are 30 section headers, starting at offset 0x23768:
Section Headers:
[Nr] Name Type Address Off Size ES Flg Lk Inf Al
[ 0] NULL 0000000000000000 000000 000000 00 0 0 0
[ 1] .interp PROGBITS 00000000000002a8 0002a8 00001c 00 A 0 0 1
[ 2] .note.gnu.build-id NOTE 00000000000002c4 0002c4 000024 00 A 0 0 4
[ 3] .note.ABI-tag NOTE 00000000000002e8 0002e8 000020 00 A 0 0 4
[ 4] .gnu.hash GNU_HASH 0000000000000308 000308 0000ac 00 A 5 0 8
[ 5] .dynsym DYNSYM 00000000000003b8 0003b8 000c00 18 A 6 1 8
[ 6] .dynstr STRTAB 0000000000000fb8 000fb8 0005c5 00 A 0 0 1
[ 7] .gnu.version VERSYM 000000000000157e 00157e 000100 02 A 5 0 2
[ 8] .gnu.version_r VERNEED 0000000000001680 001680 0000a0 00 A 6 2 8
[ 9] .rela.dyn RELA 0000000000001720 001720 001440 18 A 5 0 8
[10] .rela.plt RELA 0000000000002b60 002b60 0009d8 18 AI 5 24 8
[11] .init PROGBITS 0000000000004000 004000 000017 00 AX 0 0 4
[12] .plt PROGBITS 0000000000004020 004020 0006a0 10 AX 0 0 16
[13] .plt.got PROGBITS 00000000000046c0 0046c0 000018 08 AX 0 0 8
[14] .text PROGBITS 00000000000046e0 0046e0 013abe 00 AX 0 0 16
[15] .fini PROGBITS 00000000000181a0 0181a0 000009 00 AX 0 0 4
[16] .rodata PROGBITS 0000000000019000 019000 005321 00 A 0 0 32
[17] .eh_frame_hdr PROGBITS 000000000001e324 01e324 000944 00 A 0 0 4
[18] .eh_frame PROGBITS 000000000001ec68 01ec68 0032a0 00 A 0 0 8
[19] .init_array INIT_ARRAY 0000000000023350 022350 000008 08 WA 0 0 8
[20] .fini_array FINI_ARRAY 0000000000023358 022358 000008 08 WA 0 0 8
[21] .data.rel.ro PROGBITS 0000000000023360 022360 000a78 00 WA 0 0 32
[22] .dynamic DYNAMIC 0000000000023dd8 022dd8 0001f0 10 WA 6 0 8
[23] .got PROGBITS 0000000000023fc8 022fc8 000038 08 WA 0 0 8
[24] .got.plt PROGBITS 0000000000024000 023000 000360 08 WA 0 0 8
[25] .data PROGBITS 0000000000024360 023360 000268 00 WA 0 0 32
[26] .bss NOBITS 00000000000245e0 0235c8 0012d8 00 WA 0 0 32
[27] .gnu_debugaltlink PROGBITS 0000000000000000 0235c8 000049 00 0 0 1
[28] .gnu_debuglink PROGBITS 0000000000000000 023614 000034 00 0 0 4
[29] .shstrtab STRTAB 0000000000000000 023648 00011c 00 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)
The first section is always a dummy section.
.shstrtab: Section header names table.
.interp: Interp section stores the dynamic linker path.
.dynamic
.dynamic section is used in dynamic linking. Check out https://refspecs.linuxbase.org/elf/gabi4+/ch5.dynamic.html for the meaning of all these tags.
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root@xiong-test:~/mold/build# readelf -d mold
Dynamic section at offset 0x1320530 contains 33 entries:
Tag Type Name/Value
0x0000000000000001 (NEEDED) Shared library: [libz.so.1]
0x0000000000000001 (NEEDED) Shared library: [libcrypto.so.3]
0x0000000000000001 (NEEDED) Shared library: [libstdc++.so.6]
0x0000000000000001 (NEEDED) Shared library: [libm.so.6]
0x0000000000000001 (NEEDED) Shared library: [libgcc_s.so.1]
0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
0x0000000000000001 (NEEDED) Shared library: [ld-linux-x86-64.so.2]
0x000000000000000c (INIT) 0xf0000
0x000000000000000d (FINI) 0x1193864
0x0000000000000019 (INIT_ARRAY) 0x12c7e80
0x000000000000001b (INIT_ARRAYSZ) 2424 (bytes)
0x000000000000001a (FINI_ARRAY) 0x12c87f8
0x000000000000001c (FINI_ARRAYSZ) 8 (bytes)
0x000000006ffffef5 (GNU_HASH) 0x3d8
0x0000000000000005 (STRTAB) 0x2558
0x0000000000000006 (SYMTAB) 0x668
0x000000000000000a (STRSZ) 8077 (bytes)
0x000000000000000b (SYMENT) 24 (bytes)
0x0000000000000015 (DEBUG) 0x0
0x0000000000000003 (PLTGOT) 0x1321780
0x0000000000000002 (PLTRELSZ) 5232 (bytes)
0x0000000000000014 (PLTREL) RELA
0x0000000000000017 (JMPREL) 0xede80
0x0000000000000007 (RELA) 0x4a20
0x0000000000000008 (RELASZ) 955488 (bytes)
0x0000000000000009 (RELAENT) 24 (bytes)
0x000000000000001e (FLAGS) BIND_NOW
0x000000006ffffffb (FLAGS_1) Flags: NOW PIE
0x000000006ffffffe (VERNEED) 0x4780
0x000000006fffffff (VERNEEDNUM) 7
0x000000006ffffff0 (VERSYM) 0x44e6
0x000000006ffffff9 (RELACOUNT) 34038
0x0000000000000000 (NULL) 0x0
GOT (global offset table) and PLT (procedure linkage table)
This is the best post about position independent code (PIC) ever: https://eli.thegreenplace.net/2011/11/03/position-independent-code-pic-in-shared-libraries/.
I found this answer expresses the goal and motivation behind GOT and PLT best. Also, follow this post to see the assembly details.
DWARF
Introduction to the DWARF Debugging Format is a must read for beginners to learn about DWARF. Cited from this pager.
it (DWARF) is a compact representation of the relationship between the executable program and the source in a way that is reasonably efficient for a debugger to process.
The format is extensible in a uniform fashion, so that a debugger can recognize and ignore an extension, even if it might not understand its meaning. (This is much better than the situation with most other debugging formats where the debugger gets fatally confused attempting to read unrecognized data.)
Debugging Information Entry DIE: tag & attributes
Line number table
CFI: call frame information The CFI describes how to unwind the stack.
System Calls
brk and sbrk
brk stands for break. The program break is the first location after bss section. So it represents the end of data section of a process’s memory space. You can check out mm_strcut.