On Mon, Mar 2, 2015 at 11:09 AM, Alan Modra <amo...@gmail.com> wrote: > On Mon, Mar 02, 2015 at 09:40:01AM +0100, Richard Biener wrote: >> On Sat, Feb 28, 2015 at 5:42 PM, H.J. Lu <hjl.to...@gmail.com> wrote: >> > Ue copy relocation in PIE improves performance. But copy relocation >> > can't be used to access protected symbols defined in shared libaries >> > and linker in binutils 2.26 enforces doesn't allow it. GCC doesn't >> > know if an external definition is protected or not. This option adds >> > -mcopyreloc-in-pie to give user an option to turn it off to avoid problem >> > at link-time. OK for trunk? >> >> I wonder if the linker can fix this up? That is, turn the relocation into >> a valid one? > > No it can't (*), nor can the dynamic linker. Copy relocs aren't > really the issue. They are just a means of initializing a linker > generated variable to be used in place of a variable in a shared > library. The issue is the linker generated .dynbss variable itself. > > Consider an ELF executable linked against a shared library, with the > executable referencing (but not defining) a variable defined in the > shared library. You'd expect that the executable and shared library > would both use the same location for the variable. Indeed, that is > true. Both executable and shared library use the shared library's > variable. Except there is a wrinkle. If the executable is non-PIC, > code in the executable will require dynamic text relocations as the > variable's address isn't known until run time. To avoid that, some > clever person thought: "Why not have the linker define the variable in > the executable? ELF run time linking semantics mean the shared > library will now use the linker defined copy, so we'll still just be > using one copy of the variable". Any everyone was happy. At least > until ELF visibility was invented. > > When ELF visibility comes into play, a variable defined in a shared > library with non-default visibility is *not* overridden by another > definition in the executable, be it an actual definition or a linker > generated one. There is no problem of course if there is an actual > definition in the executable. In that case the programmer would > expect to see two different variables used. However, if the shared > library contains a protected visibility variable, and the linker > introduces a copy, then it has changed the meaning of the program. At > the source level we only had one definition of the variable, but at > run time we'd end up using two different locations. > > *) Except by avoiding .dynbss copies and hence requiring dynamic text > relocations.
Ah, I see (protected visibility has haunted us in the past...). So I think we need to turn the new option off by default. Richard. > -- > Alan Modra > Australia Development Lab, IBM
