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Re: PR 18167, Relax PR 15228 protected visibility restriction
- From: Pedro Alves <palves at redhat dot com>
- To: binutils at sourceware dot org
- Date: Fri, 27 Mar 2015 16:38:16 +0000
- Subject: Re: PR 18167, Relax PR 15228 protected visibility restriction
- Authentication-results: sourceware.org; auth=none
- References: <20150327054327 dot GB26234 at bubble dot grove dot modra dot org> <551531AA dot 1060006 at redhat dot com> <20150327122941 dot GC26234 at bubble dot grove dot modra dot org>
On 03/27/2015 12:29 PM, Alan Modra wrote:
> Modifying app:
> extern int protected_var;
> extern void library (int *);
> int main ()
> library (&protected_var);
> return 0;
> Now if the app is compiled -fPIE you shouldn't have the .dynbss copy
> and no abort, but if the app is non-PIC then yes, pointer comparisons
> won't be correct.
> HJ has gcc, glibc and ld patches to fix the mess by
> a) modifying gcc to emit the same code for protected visibility
> variables in shared libraries as default visibility, and
> b) modifying ld.so to enforce proper protected semantics, and
> c) disabling the linker checks.
> They look reasonable to me except there is no provision to deal with
> any mismatch in the toolchain components, which is a nasty problem.
> For example if you have a new gcc but older glibc then gcc will
> generate code that is quite wrong for protected variables. (It's also
> slower since access to a protected variable in a shared library uses a
> GOT indirection with HJ's patch.) Similarly with ld, and note that
> current x86_64 ld already has (c) installed, effectively disabling my
> pr15228 fix..
Urgh. The glibc issue sounds the most alarming. If we can't keep
back compatibility, isn't there a new bit/attribute we can put
somewhere to tag new binaries with protected symbols in a
way that existing systems just error out when loading them?