How do you get the start and end addresses of a custom ELF section?

Question

I\'m working in C on Linux. I\'ve seen the usage of of the gcc __section__ attribute (especially in the Linux kernel) to collect data (usually function pointers) into custom

Answer 1

As long as the section name results in a valid C variable name, gcc (ld, rather) generates two magic variables: __start_SECTION and __stop_SECTION. Those can be used to retrieve the start and end addresses of a section, like so:

/**
 * Assuming you've tagged some stuff earlier with:
 * __attribute((__section__("my_custom_section")))
 */

struct thing *iter = &__start_my_custom_section;

for ( ; iter < &__stop_my_custom_section; ++iter) {
    /* do something with *iter */
}

I couldn’t find any formal documentation for this feature, only a few obscure mailing list references. If you know where the docs are, drop a comment!

If you're using your own linker script (as the Linux kernel does) you'll have to add the magic variables yourself (see vmlinux.lds.[Sh] and this SO answer).

See here for another example of using custom ELF sections.

Answer 2

So the answer above, __start_SECTION and __stop_SECTION will work, however for the program to be able to use the information from the linker you to need to declare those variables as extern char* __start_SECTION. Enjoy!

extern char * __start_blobby;

...
printf("This section starts at %p\n", (unsigned int)&__start_blobby);
...

Answer 3

Linker can use the symbols defined in the code, and can assign their initial values if you use the exact name in the linker script:

_smysection = .;
*(.mysection)
*(.mysection*)
_emysection = .;

Just define a variable in C code:

const void * _smysection;

And then you can access that as a regular variable.

u32 someVar = (u32)&_smysection;

Answer 4

HI: like this.

extern const struct pseudo_ta_head __start_ta_head_section;
extern const struct pseudo_ta_head __stop_ta_head_section;    

const struct pseudo_ta_head *start = &__start_ta_head_section;
const struct pseudo_ta_head *end = &__stop_ta_head_section;

Answer 5

Collecting the information together from various answers, here is a working example of how to collect information into a custom linker section and then read the information from that section using the magic variables __start_SECTION and __stop_SECTION in your C program, where SECTION is the name of the section in the link map.

The __start_SECTION and __stop_SECTION variables are made available by the linker so explicit extern references need to be created for these variables when they are used from C code.

There are also some problems if the alignment used by the compiler for calculating pointer/array offsets is different than the alignment of the objects packed in each section by the linker. One solution (used in this example) is to store only a pointer to the data in the linker section.

#include <stdio.h>

struct thing {
    int val;
    const char* str;
    int another_val;
};
struct thing data1 = {1, "one"};
struct thing data2 = {2, "two"};

/* The following two pointers will be placed in "my_custom_section".
 * Store pointers (instead of structs) in "my_custom_section" to ensure
 * matching alignment when accessed using iterator in main(). */
struct thing *p_one __attribute__((section("my_custom_section"))) = &data1; 
struct thing *p_two __attribute__((section("my_custom_section"))) = &data2;

/* The linker automatically creates these symbols for "my_custom_section". */
extern struct thing *__start_my_custom_section;
extern struct thing *__stop_my_custom_section;

int main(void) {
    struct thing **iter = &__start_my_custom_section;
    for ( ; iter < &__stop_my_custom_section; ++iter) {
        printf("Have thing %d: '%s'\n", (*iter)->val, (*iter)->str);
    }
    return 0;
}