diff --git a/Misc/linkers.md b/Misc/linkers.md index 7ede80c..1d527ac 100644 --- a/Misc/linkers.md +++ b/Misc/linkers.md @@ -492,25 +492,29 @@ With the linker language we can control: Commands written in the linker control language are usually placed in a file called linker script. We can pass it to `ld` with the `-T` command line option. The main command in a linker script is the `SECTIONS` command. Each linker script must contain this command and it determines the `map` of the output file. The special variable `.` contains current position of the output. Let's write a simple assembly program and we will look at how we can use a linker script to control linking of this program. We will take a hello world program for this example: ```assembly -section .data - msg db "hello, world!",`\n` -section .text - global _start +.data + msg: .ascii "hello, world!\n" + +.text + +.global _start + _start: - mov rax, 1 - mov rdi, 1 - mov rsi, msg - mov rdx, 14 - syscall - mov rax, 60 - mov rdi, 0 - syscall + mov $1,%rax + mov $1,%rdi + mov $msg,%rsi + mov $14,%rdx + syscall + + mov $60,%rax + mov $0,%rdi + syscall ``` We can compile and link it with the following commands: ``` -$ nasm -f elf64 -o hello.o hello.asm +$ as -o hello.o hello.asm $ ld -o hello hello.o ``` @@ -540,14 +544,14 @@ SECTIONS On the first three lines you can see a comment written in `C` style. After it the `OUTPUT` and the `OUTPUT_FORMAT` commands specify the name of our executable file and its format. The next command, `INPUT`, specifies the input file to the `ld` linker. Then, we can see the main `SECTIONS` command, which, as I already wrote, must be present in every linker script. The `SECTIONS` command represents the set and order of the sections which will be in the output file. At the beginning of the `SECTIONS` command we can see following line `. = 0x200000`. I already wrote above that `.` command points to the current position of the output. This line says that the code should be loaded at address `0x200000` and the line `. = 0x400000` says that data section should be loaded at address `0x400000`. The second line after the `. = 0x200000` defines `.text` as an output section. We can see `*(.text)` expression inside it. The `*` symbol is wildcard that matches any file name. In other words, the `*(.text)` expression says all `.text` input sections in all input files. We can rewrite it as `hello.o(.text)` for our example. After the following location counter `. = 0x400000`, we can see definition of the data section. -We can compile and link it with the: +We can compile and link it with the following command: ``` -$ nasm -f elf64 -o hello.o hello.S && ld -T linker.script && ./hello +$ as -o hello.o hello.S && ld -T linker.script && ./hello hello, world! ``` -If we will look inside it with the `objdump` util, we can see that `.text` section starts from the address `0x200000` and the `.data` sections starts from the address `0x400000`: +If we look inside it with the `objdump` util, we can see that `.text` section starts from the address `0x200000` and the `.data` sections starts from the address `0x400000`: ``` $ objdump -D hello @@ -555,7 +559,7 @@ $ objdump -D hello Disassembly of section .text: 0000000000200000 <_start>: - 200000: b8 01 00 00 00 mov $0x1,%eax + 200000: 48 c7 c0 01 00 00 00 mov $0x1,%rax ... Disassembly of section .data: