Re: Controlling the linking of shared libraries

["Björn Persson" <bjorn@xn--rombobjrn-67a.se>]

I'll try to explain my problem in more detail and see if that makes it clear 
what I need.

First, please note that I'm not talking about compiling a program and 
linking it to a shared library. I'm talking about compiling the shared 
library itself. When you build a shared library there is a linking phase 
where you take all the .o files you just compiled and combine them into a 
.so file (or a .dll file if you're in Windows land, but I'm in GNU land 
here). I want to pass some options to the linker to control some details in 
how it generates the .so file.

Second, I'm writing this with my Fedora packager hat on. Packagers take 
programs and libraries that others have written and compile them into Fedora 
packages that the Fedora project distributes. Ideally we should be able to 
do this without modifying the upstream source code. Rather than writing 
custom patches for each and every package, we want to define a common set of 
command line parameters and/or environment variables that control how 
packages are built. In practice we often have to patch the upstream 
projects' build systems, as they arent written to be flexible enough, but 
the less we have to patch, and the more we can control the build with 
command line parameters and environment variables, the better.

The Fedora project recently decided to start using a linker option called 
"relro" in all packages as a security measure. This means that we need to 
pass the command line parameter "-Wl,-z,relro" to gcc so that gcc will in 
turn pass "-z relro" to ld. This parameter is not to be used in the 
compilation phase, only in the linking phase when gcc is invoked as a linker 
driver. This was the first distribution-wide linker option that was defined 
in Fedora. Previously the common set of parameters contained only compiler 
options, used in the compilation phase.

Now, some projects are built using makefiles and let the makefile handle the 
linking phase by invoking gcc directly. Those cases are not what I'm 
concerned over right now. Other projects have Gnat project files that 
control the whole build process, and in those cases I need to find a way to 
make Gnatmake or GPRbuild pass "-Wl,-z,relro" to gcc in the linking phase. I 
tried using "-largs -Wl,-z,relro" for this, and found that it has the 
desired effect when the project being built is a program. It does however 
not have any noticeable effect when the project being built is a shared 
library.

According to the manual the attribute Library_Options can be used in project 
files to specify linker options, but as I explained above patching each and 
every project file is undesirable. Therefore I came here to ask you guys: 
Does anyone know how to pass command line parameters to Gnatmake and 
GPRbuild that they should forward to gcc when they invoke gcc as a linker 
driver to build a shared library, given that -largs is not the answer? A 
solution using environment variables might also be preferable to patching.

-- 
Björn Persson
PGP key A88682FD