Crafting an ELF file using linker scripts without zero-initialized blocks between sections - linker

Question

I am trying to craft a linker-command script to be bootable by legacy grub (using multiboot). I am having difficulty getting the multiboot header in the required location (within the first 8192 bytes). My script looks something like:
SECTIONS
{
.multiboot :
{
__multiboot_header = .;
*(.multiboot)
}
.text 0x00100000 :
{
*(.text*)
*(.rodata)
}
/* ... remainder of script ... */
}
Overall, my objective is to have my custom executable loaded by the bootloader after the first 1MiB of physical memory; the address as part of the .text section declaration seems to have done this as I expected. Reading the ELF header gives the entry point as:
$ readelf -h kernel.elf | grep Entry
Entry point address: 0x100000
However, in doing so I seem to have also increased the file by this much.
$ ls -l file.elf
-rwxr-xr-x 1 user user 1049960 May 13 02:20 file.elf
The area between the ELF header and the .text section is initialized to zeros.
$ hexdump -C file.elf
00000000 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00 |.ELF............|
00000010 02 00 03 00 01 00 00 00 00 00 10 00 34 00 00 00 |............4...|
00000020 00 04 10 00 00 00 00 00 34 00 20 00 02 00 28 00 |........4. ...(.|
00000030 09 00 08 00 01 00 00 00 00 00 00 00 00 00 00 00 |................|
00000040 00 00 00 00 f8 00 10 00 f8 f0 4e 00 07 00 00 00 |..........N.....|
00000050 00 00 20 00 51 e5 74 64 00 00 00 00 00 00 00 00 |.. .Q.td........|
00000060 00 00 00 00 00 00 00 00 00 00 00 00 07 00 00 00 |................|
00000070 10 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000080 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00100000 8b 25 f8 f0 4e 00 50 53 e8 66 00 00 00 fa f4 eb |.%..N.PS.f......|
00100010 fc 55 89 e5 53 83 ec 10 c7 45 f8 00 00 00 00 eb |.U..S....E......|
00100020 38 a1 f4 00 10 00 8b 55 f8 01 d2 01 d0 8b 15 f4 |8......U........|
Also, although readelf -s reports that __multiboot_header has a value of 0x0 (which should be the address of the structure since it was defined at the same point it was mentioned in the linker file, right?):
$ readelf -s kernel.elf | grep multiboot
22: 00000000 0 NOTYPE GLOBAL DEFAULT 1 __multiboot_header
The output of readelf -S seems to conflict:
Section Headers:
[Nr] Name Type Addr Off Size ES Flg Lk Inf Al
[ 1] .multiboot PROGBITS 00000000 1000f8 00000c 00 0 0 1
[ 2] .text PROGBITS 00100000 100000 000096 00 AX 0 0 1
Which implies that the .multiboot section is actually inside the .text section.
If I offset into the file by 0x1000f8 then I can find the structure, however, I am not sure where the offset came from.
tl;dr
1) How can I ensure a specific data structure is within the first 8192 bytes of the output file?
2) How can I specify the load address without inflating the output binary with large gaps of zero-initialized blocks?

Answer 1

The elf file format is generated by its own rules, how it stores the information is not directly influenced by the linker file (e.g. at what offsets the sections are stored must not correlate with their location at lifetime in memory). It's the memory layout which is described by the SECTIONS command in your linker file, and the elf file format describes this layout... You need a elf capable loader to load the individual sections into the target locations.
To get a flat binary file which can be loaded 1:1 into memory, use objcopy (e.g. something like objcopy -O binary myfile.elf myfile.bin). The layout of that file is directly influenced by your linker script, and the content of the .multiboot section should be really at offset 0.