ppc64 sbrk returns executable heap in 32-bit emulation mode

[Alan Modra]

On Thu, May 12, 2016 at 03:41:09PM +0200, Florian Weimer wrote:
> We noticed that on ppc64, the sbrk system call in the 32-bit subsystem 
> returns executable memory.  I assume it is related to this, in 
> arch/powerpc/include/asm/page.h:
> 
> /*
>   * Unfortunately the PLT is in the BSS in the PPC32 ELF ABI,
>   * and needs to be executable.  This means the whole heap ends
>   * up being executable.
>   */
> #define VM_DATA_DEFAULT_FLAGS32 (VM_READ | VM_WRITE | VM_EXEC | \
>                                   VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC)
> 
> 
> What is the rationale for this?  This comment must be *really* old, 

I think the comment is just plain wrong.  ppc32 needs an executable
stack because it builds trampolines on the stack to support calling
nested functions.  I presume that's why the heap is executable.  (If
I'm wrong about heap+stack needing the same protection then I can't
think of any reason to require an executable heap.)

> because ld.so in glibc should make sure that the PLT is executable.  And 
> for current binaries, .bss is *not* executable, contrary to what the 
> comment suggests.
> 
> Is this comment about pre-ELF binaries?  If yes, would it possible to 
> change the default for ELF binaries?
> 
> Thanks,
> Florian

-- 
Alan Modra
Australia Development Lab, IBM