diff options
Diffstat (limited to '04_dynld_nostd/dynld.c')
| -rw-r--r-- | 04_dynld_nostd/dynld.c | 51 |
1 files changed, 47 insertions, 4 deletions
diff --git a/04_dynld_nostd/dynld.c b/04_dynld_nostd/dynld.c index d1ca815..fed4b62 100644 --- a/04_dynld_nostd/dynld.c +++ b/04_dynld_nostd/dynld.c @@ -397,10 +397,49 @@ static void resolve_reloc(const Dso* dso, const LinkMap* map, const Elf64Rela* r void* symaddr = 0; // FIXME: Should relocations of type `R_X86_64_64` only be looked up in `dso` directly? if (reloctype == R_X86_64_RELATIVE) { - // Symbols address is computed by re-basing the relative address based on the DSOs base address. + // Symbols address is computed by re-basing the relative address based + // on the DSOs base address. symaddr = (void*)(dso->base + reloc->addend); } else { - // TODO: Explain special handling of R_X86_64_COPY. + // Special handling of `R_X86_64_COPY` relocations. + // + // The `R_X86_64_COPY` relocation type is used in the main program when + // it references an object provided by a shared library (eg extern + // declared variable). + // The static linker will still allocate storage for the external + // object in the main programs `.bss` section and any reference to the + // object from the main program are resolved by the static linker to + // the location in the `.bss` section directly (relative addressing). + // During runtime, when resolving the `R_X86_64_COPY` relocation, the + // dynamic linker will copy the initial value from the shared library + // that actually provides the objects symbol into the location of the + // main program. References to the object by other shared library are + // resolved to the location in the main programs `.bss` section. + // + // LinkMap: Relocs: + // + // main program { sym: foo, type: R_X86_64_COPY } + // | + // v + // libso { sym: foo, type: R_X86_64_GLOB_DAT } + // // Also `foo` is defined in `libso`. + // + // main prog libso + // +-----------+ +-----------+ + // | .text | | .text | + // ref | | | | + // +--| ... [foo] | +-------------------| ... [foo] | + // | | | | R_X86_64_GLOB_DAT | | + // | +-----------+ | Resolve reference +-----------+ + // | | .bss | | to foo. | .data | + // | | | / | | + // +->| foo: ... |<--------------------| foo: ... | + // | | R_X86_64_COPY | | + // +-----------+ Copy initial value. +-----------+ + // + // + // The handling of `R_X86_64_COPY` relocation assumes that the main + // program is always the first entry in the link map. for (const LinkMap* lmap = (reloctype == R_X86_64_COPY ? map->next : map); lmap && symaddr == 0; lmap = lmap->next) { symaddr = lookup_sym(lmap->dso, symname); } @@ -451,8 +490,12 @@ static void resolve_relocs(const Dso* dso, const LinkMap* map) { // }}} // {{{ Dynamic Linking (lazy resolve) -// Mark `dynresolve_entry` as `naked` because we want to fully control the -// stack layout. +// Dynamic link handler for lazy resolve. +// This handler is installed in the GOT[2] entry of `Dso` objects which holds +// the address of the jump target for the PLT0 jump pad. +// +// Mark `dynresolve_entry` as `naked` because we don't want a prologue/epilogue +// being generated so we have full control over the stack layout. // // `noreturn` Function never returns. // `naked` Don't generate prologue/epilogue sequences. |