Background
Currently, binary packages' control files (<binpkg>.pkg/control in source
packages) must explicitly list library dependencies. Package maintainers can
manually see ELF files' DT_NEEDED entries with tools like readelf or
ldd and figure out which binary packages provide each SONAME (usually just
dropping .so from the middle of the SONAME, e.g. libz.1 provides
libz.so.1). Being a manual process, this method is error-prone: it's an extra
step that can be forgotten when preparing a new package or updating to a new
upstream version, it's possible to miss some ELF files in binary packages, etc.
An automated solution is clearly desirable.
Debian achieves this with a tool called dpkg-shlibdeps, which:
- Finds all ELF files in each binary package,
- Finds all libraries against which each ELF file is linked (i.e.
DT_NEEDED), - Finds dependency libraries on the system (resolving SONAMEs to file names),
- Looks up the packages that provide each dependency library (with
dpkg -S <lib-file-name>), and - Checks whether each ELF file uses symbols from its library dependencies (and warns of any useless/avoidable dependencies).
ProteanOS should have a similar tool. This document describes how this can be implemented.
Implementation
Changes need to be made in three places in ProteanOS: the Source Package Format specification, opkbuild, and opkhelper.
SPF 2.0
SPF 2.0 needs to be amended to specify tmp/<binpkg>.substvars files, like the
substvars file but generated during the build process (while commands from the
build makefile are run and before opkbuild tools complete the package
build) and specific to each binary package.
opkbuild
opkbuild needs to read the tmp/<binpkg>.substvars files and use them when
generating binary packages' control files.
opkhelper
opkhelper needs a new tool, which will be called oh-shlibdeps, to be run after oh-installfiles. This new tool will:
- Find all ELF files in each binary package (oh-strip already does something similar),
- Run the C library's ldd tool and parse its output (e.g. with
sed -n 's/^\t.* => \(.*\) (.*)$/\1/p;') to find the file names of all libraries against which each ELF file is linked (a combination of steps 2 and 3 above), and - Look up the packages that provide each dependency library (with
opkg search ${filename} | sed 's/ - .*$//;').
Detecting useless/avoidable dependencies is considered outside the scope of the current proposed design and may be considered in the future.
ProteanOS's opkhelper-3.0 binary package will need to ensure the existence of
the ldd tool. ldd is provided by the libc-bin package on the
any-any-glibc architectures (currently all architectures in ProteanOS).
musl's RTLD (a.k.a. dynamic linker) has ldd functionality built-in and
enabled when executed as ldd, so no separate package will be required. So the
needed dependencies will depend on the architecture, however opkhelper-3.0 is
Architecture: all, which means it can't have architecture-dependent
dependencies (which would be solved at build time by opkbuild, not at run time
by opkg). Therefore, opkhelper-3.0 will likely have to depend on
libc-bin | ldd, which will work on either any-any-glibc or any-any-musl
architectures if the musl library binary package Provides: ldd.