Musl porting notes/1.2.4

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Set of notes for musl 1.2.4 porting

See bug #903611 and bug #920477 for Musl 1.2.4 trackers.

What changed?

LFS64 hacks removed

Fixes

Note
If any of these bugs are too nontrivial to fix, a workaround to fix these is passing -D_LARGEFILE64_SOURCE as an argument. However, this will be removed in a future musl release.

error: LFS64 interfaces (*64 undeclared here, ex. pread64)

The Gentoo tracker bug for these issues is bug #903611.

The legacy "LFS64" ("large file support") interfaces, which were provided by macros remapping them to their standard names (#define stat64 stat and similar) have been deprecated and are no longer provided under the _GNU_SOURCE feature profile, only under explicit _LARGEFILE64_SOURCE. The latter will also be removed in a future version.

The correct fix is to adjust the code to use standard off_t types and then to cater for glibc by passing -D_FILE_OFFSET_BITS=64 to avoid regressing 32-bit glibc systems. In autoconf, this can be done with the AC_SYS_LARGEFILE macro. Meson does this by default. CMake has no built-in macro for this and it must be done manually.

As a temporary workaround, -D_LARGEFILE64_SOURCE can be appended to CPPFLAGS by doing

FILE borked.ebuild

inherit flag-o-matic
...
src_compile() {
    # Temporary workaround for musl-1.2.4 (upstream bug #123456, gentoo bug #123456)
    # XXX: This will stop working with future musl releases!
    append-cppflags "-D_LARGEFILE64_SOURCE"
    ...
}

Note
Report usage of these macros upstream.

FAQ

Why is the 64-bit supplementary interface not needed?

musl by default uses 64-bit types for the "LFS lot" (e.g. off_t, ino_t). It also does the same for time_t which is related but a separate topic.

In contrast, glibc on 32-bit systems needs -D_FILE_OFFSET_BITS=64 to be defined to widen its types. This however affects ABI and interfaces in public headers must be checked to see if they contain any of the changed types.

The dual-ABI approach was proposed years ago as a compromise to allow libraries to support both 32-bit off_t (and friends) and 64-bit LFS-capable off_t. The idea was that libraries would provide two versions of relevant functions, structures, etc rather than unilaterally changing ABI on a flag day. glibc did its part with providing e.g. off64_t guarded by -D_LARGEFILE_SOURCE (there's also -D_LARGEFILE64_SOURCE). Adoption of this was extremely poor and nowadays, use of these types is usually based on a misunderstanding of how to obtain wider types on glibc systems.

How do I reproduce these bugs?

If musl 1.2.4 is not already installed on the host, using a 32-bit chroot with a musl stage3 is the best way to reproduce.