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Copy relocations against protected symbols

Alan Modra amodra@gmail.com
Thu Dec 18 22:14:00 GMT 2014 
On Thu, Dec 18, 2014 at 11:02:59AM -0800, H.J. Lu wrote:
> Adding glibc.
> 
> On Thu, Dec 18, 2014 at 10:52 AM, Cary Coutant <ccoutant@google.com> wrote:
> >> Should we simply disallow creating DSO with protected data on targets
> >> with copy relocation?
> >
> > I don't think so. Protected symbols are useful, and their presence
> 
> As soon as they are used in executable, the program will misbehave.

Not if the references in the executable are in code compiled with
-fPIC.  I agree with Cary, disallowing protected visibility variables
in shared libraries, or disabling the compiler optimisation that
allows shared library code to access protected variables without a GOT
indirection, is too big a hammer.

> > doesn't mean that a copy relocation will be needed. It would be pretty
> > heavy-handed, since most targets do support copy relocations.
> >
> 
> When you create libfoo.so with normal data, bar, it will link and
> work fine with executable, x, which accesses bar.  But after
> you change bar in libfoo.so to protected without relinking x, x will
> misbehave and users may not have a clue what is going on.

Right, but this is just one of many ways to shoot yourself in the
foot.

-- 
Alan Modra
Australia Development Lab, IBM
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