DwarfTasks 4.3 says "relocatable values: smart reader". Here's the brain dump on what that means.

Right now, the libdw code is completely ignorant of the relocatability of the DWARF data. It reads straight DWARF encoding, and expects the values it gets that way to make sense. The libdwfl/relocate.c machinery does full relocation of all the data (blind to its formats) before libdw looks at it. This will change in libdw internals. (I don't think the libdw.h interface will change for this, thought some details in libdwfl and C++ will.)

The first step is to find each place in libdw that decodes a relocatable quantity. This is anything that is offset-size or address-size. (The dwarflint reloc work has recently enumerated all the spots not to forget.) The libdw code has lots of things like:

if (attrp->cu->offset_size == 8) off = read_8ubyte_unaligned (dbg, attrp->valp); else off = read_4ubyte_unaligned (dbg, attrp->valp);

Replace these with calls to new helpers, one for offset and one for address. Of each, also a *_inc variant. These will take the Dwarf * and also a size selection argument (either offset/address size, or bool of == 8). The offset decoders take an IDX_debug_* argument saying which section the offset points into. They need to be able to signal errors (e.g. return a bool saying they've done __libdw_seterrno). Perhaps also both (offset and address flavors) should take an IDX_debug_* argument saying where the decoded data comes from (that's probably just for debugging).

To start with, just make the helpers trivial inlines wrapping read_[48]ubyte_unaligned so nothing changes. Next, lift the offset validity checks from existing code into these helpers. If there are cases that are offset/address sized lengths instead of offsets/addrs, then we'll need variants of the helpers for that.

A good testing hack would be to make the wrappers emit somewhere (stderr, whatever) the idx:offset where each offset/address was decoded. Then you could walk the data with a eu-readelf run or whatnot, sort -u that log and compare it to what places dwarflint thinks relocs might be. Once we have the right calls to these helpers in all the right places, we can fill them in with hooks into reloc support.

I'll write more later on what the reloc support will be. For offset relocs in ET_REL, they are always resolvable inside the file. For addresses the idea is that it will do libdwfl for libdwfl-made Dwarf handles, old-fashioned dwarf_formaddr et al will fail with "needs relocation", and new C++ reloc-savvy interfaces will do something else.

Thanks, Roland