Permission of overlapping segments

Permission of overlapping segments

[Udo A. Steinberg]

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Hi all,

In the following example I've put .text and .rodata in the first program
header and .data and .bss in the second. Unless I'm page-aligning the
.data section, linking results in two segments that overlap page 0x400000
with different/conflicting permissions as follows:

Program Header:
    LOAD off    0x00000000 vaddr 0x00400000 paddr 0x00400000 align 2**12
         filesz 0x00000595 memsz 0x00000595 flags r-x
    LOAD off    0x00000598 vaddr 0x00400598 paddr 0x00400598 align 2**12
         filesz 0x00000008 memsz 0x00001000 flags rw-

What should an ELF loader do here - merge permissions such that 0x400000
is rwx or is such a segment layout illegal?

Cheers,

	- Udo

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Re: Permission of overlapping segments

[Alan Modra]

On Thu, Aug 09, 2007 at 08:20:29PM +0200, Udo A. Steinberg wrote:
>     LOAD off    0x00000000 vaddr 0x00400000 paddr 0x00400000 align 2**12
>          filesz 0x00000595 memsz 0x00000595 flags r-x
>     LOAD off    0x00000598 vaddr 0x00400598 paddr 0x00400598 align 2**12
>          filesz 0x00000008 memsz 0x00001000 flags rw-
> 
> What should an ELF loader do here - merge permissions such that 0x400000
> is rwx or is such a segment layout illegal?

You're asking for trouble.  The loader may refuse to load this file,
or it may mmap the pages occupied by the second PT_LOAD segment on top
of the first.

-- 
Alan Modra
Australia Development Lab, IBM