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Re: Preventing preemption of 'protected' symbols in GNU ld 2.26
- From: Joe Groff <jgroff at apple dot com>
- To: Cary Coutant <ccoutant at gmail dot com>
- Cc: "H.J. Lu" <hjl dot tools at gmail dot com>, Binutils <binutils at sourceware dot org>
- Date: Mon, 28 Mar 2016 15:24:53 -0700
- Subject: Re: Preventing preemption of 'protected' symbols in GNU ld 2.26
- Authentication-results: sourceware.org; auth=none
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> On Mar 28, 2016, at 3:21 PM, Cary Coutant <ccoutant@gmail.com> 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?
-Joe
>
> Startup time wasn't a concern either, simply because COPY relocations
> weren't the real issue. We had to support an *occasional* COPY
> relocation in order to enable -fPIE to access globals directly rather
> than through the GOT. These would be the same COPY relocations that
> the apps would have already been using in non-PIE mode (things like
> stdin, stdout, and stderr, e.g.).
>
> The size of an object is baked into the ABI when a COPY relocation is
> used, so any change in size is an ABI change (often handled, as HJ
> pointed out, with versioning). For a lot of common COPY relocations,
> that's not really an issue, because both the size and the layout are
> baked in through macros like putc() and getc().
>
> -cary