Packaging Static Libraries
Summary
MUST: Packages that install static library archives (.a files) must do one of:
- build them as PIC objects,
- build both PIC and non-PIC libraries, with a .pc file for each for dependent packages to use,
- document (in the spec file) why the static libraries are not built PIC.
Rationale
Some architectures (including i386, but not including x86_64) permit non-position-independent code. A static library may, through subsequent linking, be included in any of another static library, an application, or a shared library.
On architectures that permit it, non-PIC code comes with a tradeoff. Since the load address is assumed, the compiler has an additional register to use, which can be a - usually small - performance benefit. However, any otherwise relocatable object (shared library or PIE executable) that the non-PIC code is linked into will require text relocations. Text relocations impose a performance penalty on executable startup, and effectively make the code unshareable, thus negating the benefits of making the code into a shared library in the first place.
In general, it is preferable to err on the side of PIC. Text relocations are incompatible with some of the security measures in Fedora, and the performance benefit of actually being able to share your shared library code in memory outweighs any performance benefit from the additional register. Static libraries in particular should be built PIC, since if not their non-PIC-ness effectively poisons any shared libraries they are included in.
Exceptions
Non-PIE applications may legitimately include non-PIC code with no performance penalty. If a static library is only included in non-PIE applications, it need not be built PIC. This case is expected to be rare, limited mostly to numeric computation libraries like atlas or fftw where performance is critical.
In any case, non-PIC static libraries need explicit approval from FESCO.
Packages that install non-PIC static libraries should be explicitly listed in the wiki somewhere.
Documentation
Typically all that's needed to build a library PIC is to add -fPIC to CFLAGS. If a library contains assembly code, adding -fPIC won't make the assembly magically become PIC; code changes will be necessary in this case.
Autotools-based projects can usually disable static library creation by configuring with --disable-static, and can force PIC static libraries by configuring with --with-pic.
TODO: Add pc file templates for static and static-nonpic.
Open Issues
- Add howto info for common build systems to build static libraries PIC.
- Need a little more info for reviewers to be able to apply the Guidelines. Simple definition of PIC and PIE, for instance.
- Can we finish off the TODO in Documentation?