Linker script problem: extraneous alignment
Posted: Tue Jul 16, 2013 9:46 am
I’ve just built a cross-compiler, and before I start actually writing code in C, I want to figure out how to link object files the way I need. So, I’ve got test.o, which contains three small sections with random data (.text, .data, and .rodata). Then I’ve got this linker script, linker.ld:
I run the linker using a slightly modified version of the command provided in the Bare Bones tutorial (without actually understanding most of the options 
):
And it runs fine, and produces a binary. Problem is, it’s huge: test.o is only about 1.3 KB, while result is over 2 MB!
readelf tells me this:
You can see the problem: .text starts at 200000h! I beilieve (am I right here?) it’s because the linker tries to make sure that the offset of the section in the file and the address where the section is to be loaded are congruent modulo the page size. Which is fine; that’s the behavior I want; with one exception: the page size I use is 4 KiB, not 2 MiB.
So, the question is, how do I tell the linker what page size I want (assuming that it is indeed the cause of the problem)?
Code: Select all
OUTPUT_FORMAT(elf64-x86-64)
PHDRS {
headers PT_PHDR PHDRS;
text PT_LOAD;
data PT_LOAD;
rodata PT_LOAD;
}
SECTIONS {
.text 0x4000000 /* random address here */ : {
*(.text)
} :text
.data ALIGN(4K) : {
*(.data)
} :data
.rodata ALIGN(4K) : {
*(.rodata)
} :rodata
}Code: Select all
x86_64-elf-gcc -T linker.ld -o result -ffreestanding -O2 -nostdlib test.o -lgccreadelf tells me this:
Code: Select all
Section Headers:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
[ 0] NULL 0000000000000000 00000000
0000000000000000 0000000000000000 0 0 0
[ 1] .text PROGBITS 0000000004000000 00200000
0000000000000001 0000000000000000 AX 0 0 16
[ 2] .data PROGBITS 0000000004001000 00201000
0000000000000200 0000000000000000 WA 0 0 4
[ 3] .rodata PROGBITS 0000000004002000 00202000
0000000000000014 0000000000000000 A 0 0 4
[ 4] .shstrtab STRTAB 0000000000000000 00202014
000000000000002f 0000000000000000 0 0 1
[ 5] .symtab SYMTAB 0000000000000000 00202208
00000000000000a8 0000000000000018 6 7 8
[ 6] .strtab STRTAB 0000000000000000 002022b0
0000000000000016 0000000000000000 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), l (large)
I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
O (extra OS processing required) o (OS specific), p (processor specific)So, the question is, how do I tell the linker what page size I want (assuming that it is indeed the cause of the problem)?