Re: ppc64 sbrk returns executable heap in 32-bit emulation mode

Alan Modra [off-list ref] writes:

On Thu, May 12, 2016 at 03:41:09PM +0200, Florian Weimer wrote:

quoted

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.

The stack and the heap are separate mappings.

 (If
I'm wrong about heap+stack needing the same protection then I can't
think of any reason to require an executable heap.)

The heap and the BSS initially share the same page.

Andreas.

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Andreas Schwab, schwab@linux-m68k.org
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