Extra data appearing in binary after objcopy... Where is it coming from? - arm

Question

I have an STM32 project involving a linkerscript which is designed to cap off the end of the image with a ".firmware_header" section. The idea is that this header contains the crc of the image, and there should be no content after this point. However, when I do
arm-none-eabi-objcopy firmware.elf -O binary firmware.bin
there exists some data after the last section which for some reason is required for the image to boot. If I zero this mystery block, the image fails to boot. The first question is, from the elf sections I see, there should be no sections after .firmware_header, where is the data coming from? The second question is, why is this data required at boot? My linker script, readelf output, and a screenshot of the mystery data is below.
ENTRY(Reset_Handler)
MEMORY
{
FLASH (RX) : ORIGIN = 0x08020000, LENGTH = 896K
SRAM (RWX) : ORIGIN = 0x20000000, LENGTH = 512K
BKPSRAM (RW) : ORIGIN = 0x40024000, LENGTH = 4K
}
_estack = 0x20080000;
SECTIONS
{
.isr_vector :
{
. = ALIGN(4);
_isr_vector = .;
KEEP(*(.isr_vector))
. = ALIGN(4);
} > FLASH
.firmware_header_vector :
{
. = ALIGN(4);
KEEP(*(.firmware_header_vector))
. = ALIGN(4);
} > FLASH
.text :
{
. = ALIGN(4);
_stext = .;
*(.Reset_Handler)
*(.text)
*(.text*)
*(.rodata)
*(.rodata*)
*(.glue_7)
*(.glue_7t)
KEEP(*(.init))
KEEP(*(.fini))
. = ALIGN(4);
_etext = .;
} > FLASH
.ARM.extab :
{
. = ALIGN(4);
*(.ARM.extab)
*(.gnu.linkonce.armextab.*)
. = ALIGN(4);
} > FLASH
.exidx :
{
. = ALIGN(4);
PROVIDE(__exidx_start = .);
*(.ARM.exidx*)
. = ALIGN(4);
PROVIDE(__exidx_end = .);
} > FLASH
.preinit_array :
{
PROVIDE(__preinit_array_start = .);
KEEP(*(.preinit_array*))
PROVIDE(__preinit_array_end = .);
} > FLASH
.init_array :
{
PROVIDE(__init_array_start = .);
KEEP(*(SORT(.init_array.*)))
KEEP(*(.init_array*))
PROVIDE(__init_array_end = .);
} > FLASH
.fini_array :
{
PROVIDE(__fini_array_start = .);
KEEP(*(.fini_array*))
KEEP(*(SORT(.fini_array.*)))
PROVIDE(__fini_array_end = .);
} > FLASH
.firmware_header :
{
. = ALIGN(4);
KEEP(*(.firmware_header))
. = ALIGN(4);
} > FLASH
. = ALIGN(4);
_sidata = .;
.data : AT(_sidata)
{
. = ALIGN(4);
_sdata = .;
PROVIDE(__data_start__ = _sdata);
*(.data)
*(.data*)
. = ALIGN(4);
_edata = .;
PROVIDE(__data_end__ = _edata);
} > SRAM
.bss :
{
. = ALIGN(4);
_sbss = .;
PROVIDE(__bss_start__ = _sbss);
*(.bss)
*(.bss*)
*(COMMON)
. = ALIGN(4);
_ebss = .;
PROVIDE(__bss_end__ = _ebss);
} > SRAM
PROVIDE(end = .);
.heap (NOLOAD) :
{
. = ALIGN(4);
PROVIDE(__heap_start__ = .);
KEEP(*(.heap))
. = ALIGN(4);
PROVIDE(__heap_end__ = .);
} > SRAM
.reserved_for_stack (NOLOAD) :
{
. = ALIGN(4);
PROVIDE(__reserved_for_stack_start__ = .);
KEEP(*(.reserved_for_stack))
. = ALIGN(4);
PROVIDE(__reserved_for_stack_end__ = .);
} > SRAM
.battery_backed_sram (NOLOAD) :
{
. = ALIGN(4);
KEEP(*(.battery_backed_sram))
. = ALIGN(4);
} > BKPSRAM
/DISCARD/ :
{
*(.ARM.attributes)
}
}
arm-none-eabi-readelf -a ./fw.elf:
ELF Header:
Magic: 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00
Class: ELF32
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: EXEC (Executable file)
Machine: ARM
Version: 0x1
Entry point address: 0x8027c11
Start of program headers: 52 (bytes into file)
Start of section headers: 339928 (bytes into file)
Flags: 0x5000400, Version5 EABI, hard-float ABI
Size of this header: 52 (bytes)
Size of program headers: 32 (bytes)
Number of program headers: 3
Size of section headers: 40 (bytes)
Number of section headers: 25
Section header string table index: 24
Section Headers:
[Nr] Name Type Addr Off Size ES Flg Lk Inf Al
[ 0] NULL 00000000 000000 000000 00 0 0 0
[ 1] .isr_vector PROGBITS 08020000 010000 0001f8 00 WA 0 0 4
[ 2] .firmware_header_ PROGBITS 080201f8 0101f8 000004 00 WA 0 0 4
[ 3] .text PROGBITS 08020200 010200 021b44 00 AX 0 0 64
[ 4] .ARM.extab PROGBITS 08041d44 043a68 000000 00 W 0 0 1
[ 5] .exidx ARM_EXIDX 08041d44 031d44 000008 00 AL 3 0 4
[ 6] .init_array INIT_ARRAY 08041d4c 031d4c 000008 04 WA 0 0 4
[ 7] .fini_array FINI_ARRAY 08041d54 031d54 000004 04 WA 0 0 4
[ 8] .firmware_header PROGBITS 08041d58 031d58 000008 00 WA 0 0 4
[ 9] .data PROGBITS 20000000 040000 0009c8 00 WA 0 0 8
[10] .RxDecripSection PROGBITS 200009c8 0409c8 000080 00 WA 0 0 4
[11] .RxarraySection PROGBITS 20000a48 040a48 0017d0 00 WA 0 0 4
[12] .TxDescripSection PROGBITS 20002218 042218 000080 00 WA 0 0 4
[13] .TxarraySection PROGBITS 20002298 042298 0017d0 00 WA 0 0 4
[14] .bss NOBITS 20003a68 043a68 045da4 00 WA 0 0 4
[15] .heap PROGBITS 2004980c 043a68 000000 00 W 0 0 1
[16] .reserved_for_sta PROGBITS 2004980c 043a68 000000 00 W 0 0 1
[17] .battery_backed_s NOBITS 40024000 044000 00000c 00 WA 0 0 4
[18] .comment PROGBITS 00000000 043a68 000075 01 MS 0 0 1
[19] .debug_frame PROGBITS 00000000 043ae0 00144c 00 0 0 4
[20] .stab PROGBITS 00000000 044f2c 000084 0c 21 0 4
[21] .stabstr STRTAB 00000000 044fb0 000117 00 0 0 1
[22] .symtab SYMTAB 00000000 0450c8 009b30 10 23 1790 4
[23] .strtab STRTAB 00000000 04ebf8 0042bb 00 0 0 1
[24] .shstrtab STRTAB 00000000 052eb3 000122 00 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
L (link order), O (extra OS processing required), G (group), T (TLS),
C (compressed), x (unknown), o (OS specific), E (exclude),
y (purecode), p (processor specific)
There are no section groups in this file.
Program Headers:
Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
LOAD 0x010000 0x08020000 0x08020000 0x21d60 0x21d60 RWE 0x10000
LOAD 0x040000 0x20000000 0x08041d60 0x03a68 0x4980c RW 0x10000
LOAD 0x044000 0x40024000 0x40024000 0x00000 0x0000c RW 0x10000

Answer 1

Turns out this is an issue with the definition of the .data section. It's getting loaded after .firmware_header. Interesting that this wasn't made more clear in readefl, etc output. Solved!