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Re: Read-only data in ELF libraries may be remapped writable at runtime (upcoming NDSS'17 paper)
On 12/23/2016 07:32 PM, Mathias Payer wrote:
On December 23, 2016 7:18:29 PM GMT+01:00, Florian Weimer <email@example.com> wrote:
On 12/23/2016 07:04 PM, Mathias Payer wrote:
At the linker level you likely don't know if the referenced symbol is
or not (as it could come from any object) -- only the runtime loader
this knowledge and at that time it is too late as PIE uses relative
addressing inside the object.
The static linker necessarily sees a definition, otherwise it cannot
produce a copy relocation because copy relocations must refer to
of known size.
Agreed. The static linker knows about the symbol and size but not the permission. That information is discarded after compilation.
That's not quite true. At least in the vtable case, the vtable is
emitted in the .data.rel.ro section, and this information is definitely
available to the static linker.
For your class A, the compiler emits the following vtable:
.type _ZTV1A, @object
.size _ZTV1A, 24
And the static linker, when producing a DSO, puts that into the
.data.rel.ro section. (Otherwise, using -fPIC wouldn't server as a