Getting an address (offset) of a segment (PIE)

[kuraga]

Good day!

I'm trying to use PIE on ARM (STM32F4DISCOVERY, GCC ARM Embedded Toolchain).

Here is linker script:

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MEMORY {
    rom (rx):  ORIGIN = 0x08000000, LENGTH = 1M
    ram (rw):  ORIGIN = 0x20000000, LENGTH = 128k
}

_vector_table_size = 0x400;
_stack_size = 2048;

SECTIONS {
    .vec : {
        KEEP(startup.o(vector_table))
        . = ALIGN(4);
    } > rom

    .text : {
        *(.text*)
    } > rom

    .rodata : {
        *(.rodata*)
    } > rom

    .bss (NOLOAD) : {
        *(.bss*)
        . = ALIGN(4);
    } > ram

    .data : {
        *(.data*)
        . = ALIGN(4);
    } > ram AT > rom

    .stack (NOLOAD) : {
        _sstack = .;
        . = . + _stack_size;
        . = ALIGN(4);
        _estack = .;
    } > ram
}

Here is vector table initialization:

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__attribute__ (( noinline )) void start void() { }

__attribute__ (( section("vector_table") )) static uint32_t *vectors[] = {
    (uint32_t *) &_estack,
    (uint32_t *) start,
    <...>
};

Without PIE options, I have:

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(gdb) print *(uint32_t*)0x8000000
0x20005000

(gdb) print *(uint32_t*)0x8000004
0x8000d39

(gdb) print &_estack
0x20005000

With PIE options

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CFLAGS += -fpie -msingle-pic-base -mpic-register=r9 -mno-pic-data-is-text-relative
LDFLAGS += -pie

I have:

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(gdb) print $r9
0x8004400

(gdb) print *(uint32_t*)0x8000000
0x0

(gdb) print *(uint32_t*)0x8000004
0x8000d39

(gdb) print &_estack
0x20005000

Help me, please, to understand:

• Why element 0 (start) initialized with address, but element 1 (&_estack) with 0x0? What's the difference between (pointer to) uint32_t and (pointer to) function here?
• Why element 0 initialized by 0x0 but print &_estack prints a value?
• Is there some way to get it working? So get offset/etc. of .stack segment into vectors[0] at compile/linking time?
• Why (even with PIE) start initialized with absolute address (0x8004d39) but not offset by GOT/PLT? (btw, GOT or PLT?)

Thanks!