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On 12/23/2016 07:32 PM, Mathias Payer wrote:
On December 23, 2016 7:18:29 PM GMT+01:00, Florian Weimer <fweimer@redhat.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 isROor not (as it could come from any object) -- only the runtime loaderhasthis 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 objects 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: .weak _ZTV1A .section .data.rel.ro._ZTV1A,"awG",@progbits,_ZTV1A,comdat .align 8 .type _ZTV1A, @object .size _ZTV1A, 24 _ZTV1A: .quad 0 .quad _ZTI1A .quad _ZN1A2a1EvAnd the static linker, when producing a DSO, puts that into the .data.rel.ro section. (Otherwise, using -fPIC wouldn't server as a workaround.)
Thanks, Florian
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