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2024-03-03

Spent a little bit of time exploring linking and loading, and taking a break this week.

How to Write Shared Libraries -- Ulrich Drepper

I've been spending time reading Linkers and Loaders, but decided to take a detour and ended up reading a small book by Ulrich Drepper that went into a lot of detail on how shared objects are located and linked, alongwith the differences between RUNPATH and RPATH.

• LD_LIBRARY_PATH also honors $ORIGIN, to be able to choose paths relative to the location of the object.

• There are a lot of details on how the actual linking works, with offsets, tables and jump instructions that I need to re-read.

• Per symbol visibility in C can be added by using __attribute__ ((visibility ("hidden")))

• LD_PROFILE=<soname> will profile intefaces that go through a PLT call.

• API: Application Programming Interface; ABI: Application Binary Interface

  • ABI is the collection of all definitions available for use during the lifetime of the DSO

• DT_NEEDED in the object stores references to other files

• If both DT_RPATH and DT_RUNPATH are set, DT_RPATH is ignored

  • gcc -Wl,-rpath,/some/dir:/dir file.o --enable-new-dtags

• Accidentally adding an empty RPATH or LD_LIBRARY_PATH can have strange outcomes, and this is a mistake I've made myself.

• Additional variables

  • $LIB for lib32/lib64
  • $PLATFORM for an arch string

• --as-needed helps keep libs limited.

— Kunal