Subsection allocator might be useful

[stephenrkell]

Sometimes, especially for instrumentation, it's useful to be able to carve out little bits of space within a binary's segments. This could be used to hold trampolines, or static data they refer to.

Some such space is available in inter-section padding, so is already visible to liballocs. However, in larger objects there is a lot more space in linker-inserted gaps between sections. These are not currently visible, but would be visible if we could process the link map. Much as the liballocs toolchain forwards relocations (-Wl,-q), it could also forward the link map (-Wl,-Map,filename) for postprocessing into the -meta.so, after which the linker artifact can be deleted.

Probably the foundations for this would be best going in librunt, but it would be tied together here.

[stephenrkell]

It could also be useful in libsystrap for lazy instrumentation: make uninstrumented code "virtually executable" but turn off the X bit. On taking a trap, instrument a whole number of sections spanning one or more pages including the trap site.