Re: RFC: Should we have all targets default to only creating an executable stack when explicitly requested ?

[Nick Clifton <nickc@redhat.com>]

Hi Michael,

> But a linker should not be limited to only one language.

Agrred.  Linkers must be language agnostic.

> There is Fortran
> for instance which _has_ nested functions that can be (sort-of)
> address-taken, and hence need trampolines somewhere.  There is Ada, there
> are other languages as well.  Requiring users using standard mandated
> functionality to add linker options just so that their programs work would
> be considered a bug in the toolchain by me, no matter if that "improves
> security".

But there is an already working method by which compilers can tell
the linker that an executable stack is needed - the .note-GNU_STACK
section.  So if compiler consistently use that feature, there is no
need for users to get involved at all.

What I am proposing for the BFD linker is to change a feature whereby
an object file which does not have a .note.GNU-STACK section will also
cause the creation of an executable stack - but only for certain
architectures.  On the AArch64 or PowerPC for example, the absence of
a .note-GNU_STACK section will not trigger the creation of an executable
stack.

So for an application to be affected by the change it would have to:

   * be restricted to architectures that default to an executable stack.

   * be compiled by a compiler that does not generate .note.GNU_STACK
     sections and/or contains assembler source code that does not include
     the provision of a .note.GNU_STACK section.

   * use a feature of the language(s) that actually requires an executable
     stack in order to work.

   * not add the "-z execstack" option to the linker command line when
     being built.

It is my hope that this will only be a small number of applications, but
even if it is not, proving a warning message about the proposed change in
behaviour for the next couple of releases of the binutils ought to be
sufficient to allow application builders to adapt.

Cheers
   Nick