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Re: Preventing preemption of 'protected' symbols in GNU ld 2.26

> On Mar 28, 2016, at 3:38 PM, Cary Coutant <> wrote:
>>>> Did you look at what the costs were in startup time and dirty pages by using
>>>> copy relocations? What do you do if the size of the definition changes in a
>>>> new version of the library?
>>> There wouldn't be a measurable cost in dirty pages; the copied objects
>>> are simply allocated in bss in the executable.
>> Wouldn't references to the symbol from within the .so need to be relocated to reference the now-canonical copy in the executable?
> No, references from within the .so would have always used the GOT.
> Non-protected global symbols in a shared library are still
> pre-emptible, so they are always indirect, and there's always a
> dynamic relocation for the GOT entry. Whether the prevailing
> definition winds up in the executable or the shared library, the
> dynamic loader still has to bind the symbol and apply the relocation.

Ah, right, ELF symbols are preemptible by default, so you'd already be dirtying any pages containing data-to-data references to the symbol regardless.


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