I have the following 'uppercaser.asm' assembly program in NASM which converts all lowercase letters input from user into uppercase:
section .bss
Buff resb 1
section .data
section .text
global _start
_start:
nop ; This no-op keeps the debugger happy
Read: mov eax,3 ; Specify sys_read call
mov ebx,0 ; Specify File Descriptor 0: Standard Input
mov ecx,Buff ; Pass offset of the buffer to read to
mov edx,1 ; Tell sys_read to read one char from stdin
int 80h ; Call sys_read
cmp eax,0 ; Look at sys_read's return value in EAX
je Exit ; Jump If Equal to 0 (0 means EOF) to Exit
; or fall through to test for lowercase
cmp byte [Buff],61h ; Test input char against lowercase 'a'
jb Write ; If below 'a' in ASCII chart, not lowercase
cmp byte [Buff],7Ah ; Test input char against lowercase 'z'
ja Write ; If above 'z' in ASCII chart, not lowercase
; At this point, we have a lowercase character
sub byte [Buff],20h ; Subtract 20h from lowercase to give uppercase...
; ...and then write out the char to stdout
Write: mov eax,4 ; Specify sys_write call
mov ebx,1 ; Specify File Descriptor 1: Standard output
mov ecx,Buff ; Pass address of the character to write
mov edx,1 ; Pass number of chars to write
int 80h ; Call sys_write...
jmp Read ; ...then go to the beginning to get another character
Exit: mov eax,1 ; Code for Exit Syscall
mov ebx,0 ; Return a code of zero to Linux
int 80H ; Make kernel call to exit program
The program is then assembled with the -g -F stabs option for the debugger and linked for 32-bit executables in ubuntu 18.04.
Running readelf --segments uppercaser for the segments and readelf -S uppercaser for the sections I see a difference in size of text segment and text section.
readelf --segments uppercaser
Elf file type is EXEC (Executable file)
Entry point 0x8048080
There are 2 program headers, starting at offset 52
Program Headers:
Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
LOAD 0x000000 0x08048000 0x08048000 0x000db 0x000db R E 0x1000
LOAD 0x0000dc 0x080490dc 0x080490dc 0x00000 0x00004 RW 0x1000
Section to Segment mapping:
Segment Sections...
00 .text
01 .bss
readelf -S uppercaser
Section Headers:
[Nr] Name Type Addr Off Size ES Flg Lk Inf Al
[ 0] NULL 00000000 000000 000000 00 0 0 0
[ 1] .text PROGBITS 08048080 000080 00005b 00 AX 0 0 16
[ 2] .bss NOBITS 080490dc 0000dc 000004 00 WA 0 0 4
[ 3] .stab PROGBITS 00000000 0000dc 000120 0c 4 0 4
[ 4] .stabstr STRTAB 00000000 0001fc 000011 00 0 0 1
[ 5] .comment PROGBITS 00000000 00020d 00001f 00 0 0 1
[ 6] .shstrtab STRTAB 00000000 00022c 00003e 00 0 0 1
[ 7] .symtab SYMTAB 00000000 0003d4 0000f0 10 8 11 4
[ 8] .strtab STRTAB 00000000 0004c4 000045 00 0 0 1
In the sections description one can see that the size of .text section is 5Bh=91 bytes (the same number one is getting with the size command) whereas in the segments description we see that the size is 0x000DB, a difference of 128 bytes. Why is that?
From the elf man pages for the Elf32_Phdr (program header) structure:
p_filesz This member holds the number of bytes in the file image of the segment. It may be zero.
p_memsz This member holds the number of bytes in the memory image of the segment. It may be zero.
Is the difference somehow related to the .bss section?
