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ELF File Analysis

ELF File

• ELF stands for Executable and Linkable Format.

• It is important to be able to analyze the final ELF file to understand various resource allocations.

• ELF file is essentially a collection of all necessary sections; .text, .data, .bss, etc.

• When doing bare-metal programming, you need to check if all the sections are placed at appropriate absolute addresses.

Analyzing ELF File

• There are many ways in which you can analyze an ELF file such as using tools like objdump, readelf, etc.

• You can also instruct the linker to create a special file called the memory map file (a.k.a. map file). A map file is useful in analyzing various resource allocations and placements in memory.

• To create a map file, you need to add the linker flag -Map=<filename>.map when running arm-none-eabi-gcc or arm-none-eabi-ld.

  Sometimes, linker specific flags may not be recognized when running arm-none-eabi-gcc. For such case, prepend -Wl, to -Map=<filename>.map. (i.e., -Wl,-Map=<filename>.map)

• Map file example:

  xxxxxxxxxx
  63
  1
  
  2
  Memory Configuration
  3
  
  4
  Name             Origin             Length             Attributes
  5
  FLASH            0x0000000008000000 0x0000000000100000 xr
  6
  SRAM             0x0000000020000000 0x0000000000020000 xrw 
  7
  *default*        0x0000000000000000 0xffffffffffffffff
  8
  
  9
  Linker script and memory map 
  10
  
  11
  LOAD main.o
  12
  LOAD led.o
  13
  LOAD stm32_startup.o
  14
  
  15
  .text           0x0000000008000000      0x766
  16
   *(.isr_vector)
  17
   .isr_vector    0x0000000008000000      0x188 stm32_startup.o
  18
                  0x0000000008000000                vectors
  19
   *(.text)
  20
   .text          0x0000000008000188      0x4d0 main.o
  21
                  0x0000000008000188                main
  22
                  0x00000000080001b4                idle_task
  23
                  0x00000000080001ba                task1_handler
  24
                  0x00000000080001dc                task2_handler
  25
                  0x00000000080001fe                task3_handler
  26
                  0x000000000800021c                task4_handler
  27
                  0x000000000800023a                init_systick_timer
  28
                  0x00000000080002a8                init_scheduler_stack
  29
                  0x00000000080002b2                init_tasks_stack
  30
                  0x00000000080003cc                update_next_task
  31
                  0x0000000008000454                get_psp_value
  32
                  0x0000000008000474                save_psp_value
  33
                  0x000000000800049c                switch_sp_to_psp
  34
                  0x00000000080004b6                enable_processor_faults
  35
                  0x00000000080004f4                schedule
  36
                  0x0000000008000518                task_delay
  37
                  0x0000000008000578                PendSV_Handler
  38
                  0x000000000800059e                update_global_tick_count
  39
                  0x00000000080005b8                unblock_tasks
  40
                  0x0000000008000618                SysTick_Handler
  41
                  0x0000000008000644                HardFault_Handler
  42
                  0x000000000800064a                MemManage_Handler
  43
                  0x0000000008000650                BusFault_Handler
  44
   .text          0x0000000008000658       0xfc led.o
  45
                  0x0000000008000658                delay
  46
                  0x0000000008000680                led_init_all
  47
                  0x00000000080006f4                led_on
  48
                  0x0000000008000724                led_off
  49
   .text          0x0000000008000754       0x12 stm32_startup.o
  50
                  0x0000000008000754                RTC_Alarm_IRQHandler
  51
                  0x0000000008000754                HASH_RNG_IRQHandler
  52
                  0x0000000008000754                EXTI2_IRQHandler
  53
                  0x0000000008000754                TIM8_CC_IRQHandler
  54
  
  55
  ...
  56
  
  57
                  0x0000000008000754                DMA2_Stream6_IRQHandler
  58
                  0x0000000008000754                DMA1_Stream3_IRQHandler
  59
                  0x000000000800075a                Reset_Handler
  60
   *(.rodata)
  61
                  0x0000000008000766                _etext = .
  62
  
  63
  
  

  • L15: .text section begins at 0x08000000 $\to$$\to$ correct!

  • L17: The first object placed in the .text section is the vector table and it comes from the file "stm32_startup.o". $\to$$\to$ correct!

  • L20: Next up is the .text section of the "main.o" file and its size is 0x4d0.

  • L44: Next up is the .text section of the "led.o"

  • L60: .rodata section is empty since we don't have any constant data in our project.

  • L61: End of .text section is the start of .data section. (i.e., _etext = _sdata)

  Padding will be introduced by the linker if necessary when the section start address is not word-aligned and will be marked with something like *fill*. However, the linker does not care about the word-alignment of the section ending. To make sure that we word-align the end of each section for the section that will come immediately after the current section, use ALIGN command.

  xxxxxxxxxx
  12
  1
  SECTIONS 
  2
  { 
  3
      .text : 
  4
      {
  5
          *(.isr_vector) 
  6
          *(.text) 
  7
          *(.rodata)
  8
          . = ALIGN(4); /* assigning 4-byte word-aligned address to the location counter */
  9
          _etext = .; 
  10
      }> FLASH 
  11
   ...
  12
   }
  

  ALIGN(4) forces 4-byte word boundary alignment.

References

Nayak, K. (2022). Embedded Systems Programming on ARM Cortex-M3/M4 Processor [Video file]. Retrieved from https://www.udemy.com/course/embedded-system-programming-on-arm-cortex-m3m4/