How to get the first address of initialized data segment - c

Question

my program is working on linux using gcc. Through the manual page, I find edata, which represent the first address past the end of the initialized data segment.
But I want know the first address of initialized data segment
How can I get it?
I have tried treating etext as the first address of initialized data segment. Then I got a segment fault when I increase the address and access the variable stored in it. I think some address space between etext and edata was not mapped into virtual memory. Is that right?

Answer 1

That depends on your linker scripts. For example on some platforms you have the symbol __bss_start at the beginning of BSS. It's a symbol without any data associated with it, you can get a pointer to it by extern declaring a variable with that name (only for the sake of taking the address of that variable). For example:
#include <stdio.h>
extern char __bss_start;
int main()
{
printf("%p\n", &__bss_start);
return 0;
}
You find this by looking in the linker script, for example in /usr/lib/ldscripts/elf_x64_64.x:
.data :
{
*(.data .data.* .gnu.linkonce.d.*)
SORT(CONSTRUCTORS)
}
.data1 : { *(.data1) }
_edata = .; PROVIDE (edata = .);
__bss_start = .; /* <<<<< this is what you're looking for /*
.bss :
{
*(.dynbss)
*(.bss .bss.* .gnu.linkonce.b.*)
*(COMMON)
/* Align here to ensure that the .bss section occupies space up to
_end. Align after .bss to ensure correct alignment even if the
.bss section disappears because there are no input sections.
FIXME: Why do we need it? When there is no .bss section, we don't
pad the .data section. */
. = ALIGN(. != 0 ? 64 / 8 : 1);
}
You can also see the edata you mentioned, but as edata is not reserved for the implementation (the PROVIDE means only to create this symbol if it otherwise isn't used) you should probably use _edata instead.
If you want the address to the start of the data section you could modify the linker script:
__data_start = . ;
.data :
{
*(.data .data.* .gnu.linkonce.d.*)
SORT(CONSTRUCTORS)
}
.data1 : { *(.data1) }
_edata = .; PROVIDE (edata = .);
__bss_start = .; /* <<<<< this is what you're looking for /*
.bss :
{
*(.dynbss)
*(.bss .bss.* .gnu.linkonce.b.*)
*(COMMON)
/* Align here to ensure that the .bss section occupies space up to
_end. Align after .bss to ensure correct alignment even if the
.bss section disappears because there are no input sections.
FIXME: Why do we need it? When there is no .bss section, we don't
pad the .data section. */
. = ALIGN(. != 0 ? 64 / 8 : 1);
}
You probably want to make a copy of the linker script (look for the right one in /usr/lib/ldscripts, they are different depending on what kind of output you're targeting) and supply it when you compile:
gcc -o execfile source.c -Wl,-T ldscript
Another option if you don't want to modify the linker script could be to use the __executable_start and parse the ELF headers (hoping that the executable is sufficiently linearly mapped)
As for _etext, it is the end of the text section (you can read that in the linker script as well, but I didn't include it in the excerpt), but the text section is followed by rodata, trying to write there is probably going to segfault.

Answer 2

You can use the linux tool size (binutils package in Debian/Ubuntu).
Example
size -A /usr/bin/gcc
results in
/usr/bin/gcc :
section size addr
.interp 28 4194928
.note.ABI-tag 32 4194956
.note.gnu.build-id 36 4194988
.gnu.hash 240 4195024
.dynsym 4008 4195264
.dynstr 2093 4199272
.gnu.version 334 4201366
.gnu.version_r 160 4201704
.rela.dyn 720 4201864
.rela.plt 3240 4202584
.init 14 4205824
.plt 2176 4205840
.text 384124 4208016
.fini 9 4592140
.rodata 303556 4592160
.eh_frame_hdr 8540 4895716
.eh_frame 50388 4904256
.gcc_except_table 264 4954644
.tbss 16 7052632
.init_array 16 7052632
.fini_array 8 7052648
.jcr 8 7052656
.data.rel.ro 3992 7052672
.dynamic 480 7056664
.got 216 7057144
.got.plt 1104 7057384
.data 2520 7058496
.bss 80976 7061024
.gnu_debuglink 12 0
Total 849310