为什么动态 ELF 文件中没有.rel.dyn/.got.plt 部分？

Question

I have code like this

// test_printf.c
#include <stdio.h>

int f(){
    printf("aaa %d\n", 1);
}

And I compile it with the following code

gcc -shared -fPIC -m32 -g -c -o test_printf.so test_printf.c

I think if run readelf -S test_printf.so , I will see .rel.dyn and .rel.plt . This is because both of these sections act like what .rel.data and .rel.text in static linked programs do.

For example, in my program, since printf is an external symbol, and is referred by my test_printf.so . So when I look into test_printf.so 's relocation table, there should be one entry names printf . I check that, and the entry exists.

Then I think, since printf is an external symbol, its location should be determined at runtime. However, we must allocate a .got.plt section for this printf function, and the section should be in both a dynamicly linked executive and a dynamic library(test_printf.so).

However, when I run readelf -S , there is no .got.plt section, and I am confused about this. Is this section not necessary in a dynamic library(test_printf.so)? I don't think it is possible. Suppose test_printf.so is finally linked with executive program a , then how can a know where is the .got.plt section for printf ? Does this .got.plt finally generated in a ?

Meanwhile, I have a question 2. Are .rel.dyn and .rel.plt always present, if there are .got and .got.plt ?

Answer 1

.rel.dyn and .got.plt can be present in a shared library .elf it depends on the structure of the functions in the library, this is in https://www.technovelty.org/linux/plt-and-got-the-key-to-code-sharing-and-dynamic-libraries.html

.rela.dyn is present if on extern function is included:

$ cat test.c
extern int foo;

int function(void) {
    return foo;
}
$ gcc -shared -fPIC -o libtest.so test.c

Also see https://eli.thegreenplace.net/2011/08/25/load-time-relocation-of-shared-libraries/ and https://eli.thegreenplace.net/2011/11/03/position-independent-code-pic-in-shared-libraries

https://blog.ramdoot.in/how-can-i-link-a-static-library-to-a-dynamic-library-e1f25c8095ef (linking static libraries into a dynamic library)