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

[Joe Groff <jgroff at apple dot com>]

> On Mar 23, 2016, at 5:45 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> 
> On Wed, Mar 23, 2016 at 5:00 PM, Joe Groff <jgroff@apple.com> wrote:
>> Hi everyone. On the Swift project, we're getting bug reports that people are no longer able to link Swift programs using binutils 2.26:
>> 
>> https://bugs.swift.org/browse/SR-1023
>> 
>> This appears to be due to an intentional behavior change in commit https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=ca3fe95e469b9daec153caa2c90665f5daaec2b5, to allow protected symbols to be the targets of copy relocations. This breaks our intended use of "protected" in Swift, where we really want to be able to assume that the relative addresses of our symbols within a single executable or .so won't be preempted or moved. This lets us emit position-independent constant reflection metadata that doesn't need load-time relocations, saving us some startup time and dirty pages. I wanted to know if there's a supported way to express to the linker that "this symbol is protected, and also can never be copied". I'd like to avoid globally setting -Bsymbolic since I wouldn't want to impact non-Swift .o files in a mixed-language project. AIUI, -Bsymbolic also wouldn't prevent object files from trying to use copy relocations to our symbols and breaking our assumptions about their address; I'd like to know if it's possible to prevent that too. Thanks for any help you can give!
>> 
> 
> What you want is not protected symbol, which is expensive to lookup
> at run-time.  You need:
> 
>  -z nocopyreloc              Don't create copy relocs
> 
> But it requires that everything must be compiled with -fPIC, not even
> -fPIE, for both executable and shared library.  Otherwise, you will
> get dynamic relocations in text section.
> 
> Swift should do something like -fPIC-data, to access external data via
> PIC.  Then you can pass
> 
> -z nocopyreloc              Don't create copy relocs
> 
> to linker.

Thanks H.J. Is it possible to specify this in the object file, at the symbol level, or does "nocopyreloc" have to be specified by linker flags?

-Joe