Comments (4)
sayon
commented on August 22, 2024
Thank you for this issue! This is indeed an error.
The right sequence should be:
$ nasm -f elf64 -o main.o main.asm
$ nasm -f elf64 -o libso.o libso.asm
$ ld -shared -o libso.so libso.o
$ ld -o main main.o -d libso.so --dynamic-linker=/lib64/ld-linux-x86-64.so.2
The --dynamic-linker should be provided on the final stage when the executable file is produced, not when creating the shared library itself.
If you compare readelf output on the object file you've created with the new one you will see a difference:
$ # Without dynamic linker
$ ld -o main main.o -d libso.so $ ./main bash: no such file or directory: ./main
$ readelf -l main
Elf file type is EXEC (Executable file)
Entry point 0x400270
There are 5 program headers, starting at offset 64
Program Headers:
Type Offset VirtAddr PhysAddr
FileSiz MemSiz Flags Align
PHDR 0x0000000000000040 0x0000000000400040 0x0000000000400040
0x0000000000000118 0x0000000000000118 R E 8
INTERP 0x0000000000000158 0x0000000000400158 0x0000000000400158
0x000000000000000f 0x000000000000000f R 1
[Requesting program interpreter: /lib/ld64.so.1]
LOAD 0x0000000000000000 0x0000000000400000 0x0000000000400000
0x0000000000000288 0x0000000000000288 R E 200000
LOAD 0x0000000000000288 0x0000000000600288 0x0000000000600288
0x0000000000000130 0x0000000000000130 RW 200000
DYNAMIC 0x0000000000000288 0x0000000000600288 0x0000000000600288
0x0000000000000110 0x0000000000000110 RW 8
Section to Segment mapping:
Segment Sections...
00
01 .interp
02 .interp .hash .dynsym .dynstr .rela.plt .plt .text
03 .dynamic .got.plt
04 .dynamic
$ # Now with dynamic linker
$ ld -o main main.o -d libso.so --dynamic-linker=/lib64/ld-linux-x86-64.so.2
$ ./main
Shared object
$ readelf -l main sayon@antheus
Elf file type is EXEC (Executable file)
Entry point 0x400280
There are 5 program headers, starting at offset 64
Program Headers:
Type Offset VirtAddr PhysAddr
FileSiz MemSiz Flags Align
PHDR 0x0000000000000040 0x0000000000400040 0x0000000000400040
0x0000000000000118 0x0000000000000118 R E 8
INTERP 0x0000000000000158 0x0000000000400158 0x0000000000400158
0x000000000000001c 0x000000000000001c R 1
[Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
LOAD 0x0000000000000000 0x0000000000400000 0x0000000000400000
0x0000000000000298 0x0000000000000298 R E 200000
LOAD 0x0000000000000298 0x0000000000600298 0x0000000000600298
0x0000000000000130 0x0000000000000130 RW 200000
DYNAMIC 0x0000000000000298 0x0000000000600298 0x0000000000600298
0x0000000000000110 0x0000000000000110 RW 8
Section to Segment mapping:
Segment Sections...
00
01 .interp
02 .interp .hash .dynsym .dynstr .rela.plt .plt .text
03 .dynamic .got.plt
04 .dynamic
Without -dynamic-linker you are going to silently get a "default" dynamic loader, whose path is hardcoded in ld:
[Requesting program interpreter: /lib/ld64.so.1]
Most of us do not have a dynamic loader stored with such a name in /lib. That is why the explicit -dynamic-linker option is mandatory in order to obtain a working executable. It changes the relevant field in the resulting ELF program header so that it stores a correct path to the dynamic loader:
[Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
GCC resolves the loader path for you before calling linker internally.
You will read more on that in Chapter 15 which is entirely dedicated to writing dynamic libraries in C and assembly.
Again thank you and I am going to correct that in my next commit.
from low-level-programming.
sayon
commented on August 22, 2024
Fixed in #13
from low-level-programming.
yeah-boi
commented on August 22, 2024
Thanks for the solusion and explanation.
The same correction needs to be done for the make files also. I'll create a pull request for that.
from low-level-programming.
yeah-boi
commented on August 22, 2024
Makefiles fixed in pull request #14.
from low-level-programming.
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