I have a Linux executable file (which is an ELF file) called a.out.

I use

readelf -hl a.out

and get output

ELF Header:
  Magic:   7f 45 4c 46 02 01 01 03 00 00 00 00 00 00 00 00 
  Class:                             ELF64
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - GNU
  ABI Version:                       0
  Type:                              EXEC (Executable file)
  Machine:                           Advanced Micro Devices X86-64
  Version:                           0x1
  Entry point address:               0x4008e0
  Start of program headers:          64 (bytes into file)
  Start of section headers:          910680 (bytes into file)
  Flags:                             0x0
  Size of this header:               64 (bytes)
  Size of program headers:           56 (bytes)
  Number of program headers:         6
  Size of section headers:           64 (bytes)
  Number of section headers:         33
  Section header string table index: 30

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  LOAD           0x0000000000000000 0x0000000000400000 0x0000000000400000
                 0x00000000000c9be6 0x00000000000c9be6  R E    200000
  LOAD           0x00000000000c9eb8 0x00000000006c9eb8 0x00000000006c9eb8
                 0x0000000000001c98 0x0000000000003550  RW     200000
  NOTE           0x0000000000000190 0x0000000000400190 0x0000000000400190
                 0x0000000000000044 0x0000000000000044  R      4
  TLS            0x00000000000c9eb8 0x00000000006c9eb8 0x00000000006c9eb8
                 0x0000000000000020 0x0000000000000050  R      8
  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000000000 0x0000000000000000  RW     10
  GNU_RELRO      0x00000000000c9eb8 0x00000000006c9eb8 0x00000000006c9eb8
                 0x0000000000000148 0x0000000000000148  R      1

 Section to Segment mapping:
  Segment Sections...
   00     .note.ABI-tag .note.gnu.build-id .rela.plt .init .plt .text __libc_freeres_fn __libc_thread_freeres_fn .fini .rodata __libc_subfreeres __libc_atexit .stapsdt.base __libc_thread_subfreeres .eh_frame .gcc_except_table 
   01     .tdata .init_array .fini_array .jcr .data.rel.ro .got .got.plt .data .bss __libc_freeres_ptrs 
   02     .note.ABI-tag .note.gnu.build-id 
   03     .tdata .tbss 
   04     
   05     .tdata .init_array .fini_array .jcr .data.rel.ro .got

According to the above output, this ELF file should have size

ELF header size : 64
  +
Section headers : 64*33 = 2112
  +
Program headers : 56*6 = 336
  +
Segments : 834090 = 0x00000000000c9be6 + 0x0000000000001c98 + 0x0000000000000044 + 0x0000000000000020 + 0x0000000000000148 

= 64 + 2112 + 336 + 834090 = 836602

However, /bin/ls reports file size 912792. where are the 912792 - 836602 = 76190 bytes? which parts did I forget to count?

UPDATE

according to @Jonathon Reinhart, I re count the size using section sizes instead of segment size by

echo "print 0x020 + 0x024 + 0x0f0 + 0x01a + 0x0a0 + 0x09eda4 + 0x02529 + 0x0de + 0x09 + 0x01d320 + 0x050 + 0x08 + 0x01 + 0x08 + 0x0b04c + 0x0b2 + 0x020 + 0x010 + 0x010 + 0x08 + 0x0e4 + 0x010 + 0x068 + 0x01ad0 + 0x023 + 0x0f18 + 0x0169 + 0x0b100 + 0x0685a" | /usr/bin/python 

The section size values used above are extracted from the output of readelf -S a.out and the size of section type of "NOBITS" were not counted.

The above code output 909459 which is bigger than segment size 834090, however, the total size is still not equal to the file size.

64 + 2112 + 336 + 909459 = 911971 != 912792

still missing 912792 - 911971 = 821 bytes.

update2 [solved]

according to the comment, I have test the offset. The result tells me that the adding pad does exist.