Various things

[an3l]

• Using groff's source mechanism to link 2 pages l1.
• asan and mariadb l2.
• full stack trace in mariadb l3.
• PS4='$LINENO: ' && bash -x ./mysqld_safe --defaults-file=~/.my.cnf
• cmake ../../10.5 && sudo cmake --build . --target install -- DESTDIR=../anel -j8 and cpack l4
• cmake ../../10.2 -DCMAKE_BUILD_TYPE=Debug -DCMAKE_CXX_COMPILER=g++-9 -DCMAKE_C_COMPILER=gcc-9 -DWITH_EMBEDDED_SERVER=OFF -DWITH_UNIT_TESTS=OFF -DCMAKE_BUILD_TYPE=Debug -DPLUGIN_{ARCHIVE,TOKUDB,MROONGA,OQGRAPH,ROCKSDB,PERFSCHEMA,SPIDER,SPHINX}=NO

[an3l]

• Cppcoreguidlenes [l1].

• cppref [l2]

• godbolt [l3]

• quick-bench [l4]

• Talk [l5]

  • empty program 8KB gcc -0s (optimize size)

  • objdump --no-show-raw-insn -dC c/empyt (-d: dissassembly, C: demangling (decode low level symbol names into user names man objdump), --no-show: no bytecode (instruction bytes) when dissassemblyng the instruction (--prefix-addresses) addresses about 200hex -> 512B

  • Dump everything readelf -a empty (elf -> executable and link format)

    • Header, program header table, sections, section header table
    • sections (blocks of code or data in executable or maybe not):
      • .text (code)
      • .rodata(read-only data - can be loaded somewhere else and attempt can to write can cause error)
      • .data(rw data)
      • .bss(block started by symbol -> zero-initialised data, global scope and set to zero, will be there)

  • Create a struct example with global object with behavior to run at construction time -> bss section global variable, without declared object it will be 0, and with 1.

  • Breakpoint on constructor to see when it gets incremented.

  • init_array section different from .text

• Linker collect .o files, resolves references between .o files, determining layout of exe, writes metadata. gcc -c -0s hello.c -o hello.o && gcc -c -Os message.c -message.o && gcc -Os hello.o message.o -o hello. To see the files file hello hello.o message.o

  $ file hello hello.o message.o 
  hello:     ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically 
  linked, interpreter /lib64/l, for GNU/Linux 3.2.0, 
  BuildID[sha1]=f5c45fe1311787809821b531a0e0de9701f998ea, not stripped
  hello.o:   ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped
  message.o: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped

  • Using objdump -d hello.o:

    
    Disassembly of section .text:

  0000000000000000 : 0: 48 83 ec 08 sub $0x8,%rsp 4: 31 c0 xor %eax,%eax 6: e8 00 00 00 00 callq b <greet+0xb> # this line is calling next instruction (0xb), e8 op code for call zero offset, don't know where to look for extern function, left to linker to look for. b: 48 89 c7 mov %rax,%rdi

  The same situation is for main

  0000000000000000

  : 0: 48 83 ec 08 sub $0x8,%rsp 4: 31 c0 xor %eax,%eax 6: e8 00 00 00 00 callq b <main+0xb> b: 31 c0 xor %eax,%eax

  - **Relocation**: Linker needs to find the symbol and when find it where to put it.
  - Zeros in opcode needs to be converted to something by linker ->  `objdump --reloc -d hello.o`, show content of relocation section, separate section in obj. files that describes all things needed to be done to link exe. 

  Disassembly of section .text:

  0000000000000000 : 0: 48 83 ec 08 sub $0x8,%rsp 4: 31 c0 xor %eax,%eax 6: e8 00 00 00 00 callq b <greet+0xb> 7: R_X86_64PLT32 getMessage-0x4 # saying linker to find address of getMessage - 4 and poke to R* at address 7. Type of patches PLT32 (procedural linkage table),GOT (global object table) b: 48 89 c7 mov %rax,%rdi e: 58 pop %rax f: e9 00 00 00 00 jmpq 14 <greet+0x14> 10: R_X86_64_PLT32 puts-0x4 # here is printf

  Disassembly of section .text.startup:

  0000000000000000

  : 0: 48 83 ec 08 sub $0x8,%rsp 4: 31 c0 xor %eax,%eax 6: e8 00 00 00 00 callq b <main+0xb> 7: R_X86_64_PC32 greet-0x4

  - Symbols: section in a file providing all needed symbols `objdump --syms  -C hello.o` or `objdump -t -C hello.o`, `l`-> local, `g` -> global, `F`- >defined in this file,`getMessage` undef.

  SYMBOL TABLE: 0000000000000000 l df ABS 0000000000000000 hello.c 0000000000000000 l d .text 0000000000000000 .text 0000000000000000 l d .data 0000000000000000 .data 0000000000000000 l d .bss 0000000000000000 .bss 0000000000000000 l d .text.startup 0000000000000000 .text.startup 0000000000000000 l d .note.GNU-stack 0000000000000000 .note.GNU-stack 0000000000000000 l d .eh_frame 0000000000000000 .eh_frame 0000000000000000 l d .comment 0000000000000000 .comment 0000000000000000 g F .text 0000000000000014 greet 0000000000000000 UND 0000000000000000 _GLOBAL_OFFSETTABLE 0000000000000000 UND 0000000000000000 getMessage 0000000000000000 UND 0000000000000000 puts 0000000000000000 g F .text.startup 000000000000000f main

  $ objdump -t message.o

message.o: file format elf64-x86-64

SYMBOL TABLE: 0000000000000000 l df ABS 0000000000000000 message.c 0000000000000000 l d .text 0000000000000000 .text 0000000000000000 l d .data 0000000000000000 .data 0000000000000000 l d .bss 0000000000000000 .bss 0000000000000000 l d .rodata.str1.1 0000000000000000 .rodata.str1.1 0000000000000000 l d .note.GNU-stack 0000000000000000 .note.GNU-stack 0000000000000000 l d .eh_frame 0000000000000000 .eh_frame 0000000000000000 l .rodata.str1.1 0000000000000000 .LC0 0000000000000000 l d .comment 0000000000000000 .comment 0000000000000000 g F .text 0000000000000008 getMessage

[l1]: http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines
[l2]: https://en.cppreference.com/w/
[l3]: https://godbolt.org/
[l4]: http://quick-bench.com/ 
[l5]: https://www.youtube.com/watch?v=dOfucXtyEsU

[an3l]

Using preprocessor definitions

In C you preprocess, then compile and lastly link your object files. -D passes a preprocessor definition which should be passed even before compilation.

gcc -DMY_FUN='XXX' -c main.c
gcc -DMY_FUN='XXX' -c foo.c
gcc main.o foo.o
./a.out

First and second lines preprocess and compile. Third line links object files and produce actual executable with default name a.out

$ cat main.c 
#include "stdio.h"
extern const char *MY_FUN();
int main()
{
  printf("Hi !\n");
  printf("%s\n", MY_FUN());
  return 0;
}
$ cat foo.c 
const char *MY_FUN()
{
  const char *r= __func__;
  return r;
}

See preprocesor defaults declaration header search paths 1:

$ cpp -v /dev/null -o /dev/null
Using built-in specs.
COLLECT_GCC=cpp
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 7.5.0-3ubuntu1~18.04' --with-bugurl=file:///usr/share/doc/gcc-7/README.Bugs --enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++ --prefix=/usr --with-gcc-major-version-only --program-suffix=-7 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-bootstrap --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-libmpx --enable-plugin --enable-default-pie --with-system-zlib --with-target-system-zlib --enable-objc-gc=auto --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 7.5.0 (Ubuntu 7.5.0-3ubuntu1~18.04) 
COLLECT_GCC_OPTIONS='-E' '-v' '-o' '/dev/null' '-mtune=generic' '-march=x86-64'
 /usr/lib/gcc/x86_64-linux-gnu/7/cc1 -E -quiet -v -imultiarch x86_64-linux-gnu /dev/null -o /dev/null -mtune=generic -march=x86-64 -fstack-protector-strong -Wformat -Wformat-security
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/7/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
**#include <...> search starts here:**
 /usr/lib/gcc/x86_64-linux-gnu/7/include
 /usr/local/include
 /usr/lib/gcc/x86_64-linux-gnu/7/include-fixed
 /usr/include/x86_64-linux-gnu
 **/usr/include**
End of search list.
COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/7/:/usr/lib/gcc/x86_64-linux-gnu/7/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/7/:/usr/lib/gcc/x86_64-linux-gnu/
**LIBRARY_PATH**=/usr/lib/gcc/x86_64-linux-gnu/7/:/usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/7/../../../../lib/:/lib/x86_64-linux-gnu/:/lib/../lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/7/../../../:/lib/:**/usr/lib/**
COLLECT_GCC_OPTIONS='-E' '-v' '-o' '/dev/null' '-mtune=generic' '-march=x86-64'

Definitions in /usr/lib/. find . -name 'libm.a -> /lib/x86_64-linux-gnu/libm.a' gcc /abspath/libm.a CPATH same as gcc -Icustomdir 2. Change #include "some.h" from current file to something else iqoute header1 -> test cpp -iqoute header1 -v, use Makefile.

[an3l]

Multithreading

• Inspection of threads with thread apply all bt or coredump gdb <binary> <coredump> -ex "thread apply all bt" -ex "quit" > output.log

[an3l]

GDB things

Set breakpoint to all function in class

rbreak slave_connection_state::.

Can be used for all functions in file also

rbreak file:.

also if you want a lot of breakpoints, you can mistype thr a a br instead of thr a a bt

does rbreak create one breakpoint or many? I mean, in i b output