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gcc segment usage

Etienne SOBOLE
Note: This was originally posted on 17th April 2011 at http://forums.arm.com

Hi!

With gcc, we have 4 segment:
.rodata : readonly data
.data : initialized global var
.bss : uninitialized global var
.text : assembler code source

To use ADR opcode, you have to put your data into .text segment.

For example


main:
    push {r7, lr}
    adr r0, .myString
    bl printf
    mov r0, #0
    pop {r7, pc}

.myString:
.asciz "Hello\n"


I'd like to know (to be sure in fact) if you can't write into .text segment.
Ie, you can't write into r0 after the ADR instruction !

Can anybody confirm me that .text segment is read only ?
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  • Note: This was originally posted on 17th April 2011 at http://forums.arm.com

    You have to use readelf (in linux) to check it:

    [14] .text    PROGBITS   00000000004022c0  000022c0
        0000000000010af8  0000000000000000  AX    0  0  16

    There is "AX" and no "W" so yes, it is locked to write.

    You could change these flags with mprotect or objcopy.
    Also you can check /proc/$pid/maps of running binary to see a lot of stuff about it including permissions.
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  • Note: This was originally posted on 17th April 2011 at http://forums.arm.com

    You have to use readelf (in linux) to check it:

    [14] .text    PROGBITS   00000000004022c0  000022c0
        0000000000010af8  0000000000000000  AX    0  0  16

    There is "AX" and no "W" so yes, it is locked to write.

    You could change these flags with mprotect or objcopy.
    Also you can check /proc/$pid/maps of running binary to see a lot of stuff about it including permissions.
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