When opening an ET_REL file with dwarf_begin no relocations are resolved and no error checking is done if that is necessary. Causing some functions to produce errors or wrong results (because an index is wrongly assumed to be zero). When opening such a file through libdwfl we do resolve all cross-debug-section relocations using __libdwfl_relocate. It would be good to make this a public function. Maybe through libdwelf dwelf_relocation_debug_sections (Elf *elf). So users can make sure ET_REL files can be handled by libdw. And maybe add a check/flag in dwarf_begin so an error can be produced if a relocation value is needed for a correct result.
This is the current "workaround" I use when working with individual ET_REL DWARF files (also works for any other ELF type): static char *debuginfo_path = NULL; static const Dwfl_Callbacks dwfl_callbacks = { .find_elf = dwfl_build_id_find_elf, .find_debuginfo = dwfl_standard_find_debuginfo, .section_address = dwfl_offline_section_address, .debuginfo_path = &debuginfo_path, }; int main (int argc, char **argv) { Dwfl *dwfl = dwfl_begin (&dwfl_callbacks); Dwfl_Module *module = dwfl_report_elf (dwfl, argv[1], argv[1], -1, 0, false); if (module == NULL) printf ("%s\n", dwfl_errmsg (-1)); dwfl_report_end (dwfl, NULL, NULL); Dwarf_Addr bias; Dwarf *dwarf = dwfl_module_getdwarf (module, &bias); if (dwarf == NULL) printf ("%s\n", dwfl_errmsg (-1)); /* ... do something with dwarf ... */ dwfl_end (dwfl); // Also cleans up dwarf. } But this is obviously a little cumbersome if not really using any other libdwfl features.
Hey @mjw ``` $ eu-readelf -a qq.o | grep Type Type: REL (Relocatable file) $ eu-readelf -S qq.o | grep REL [ 2] .rela.text RELA 0000000000000000 000004b8 00000048 24 I 19 1 8 [ 6] .rela.debug_info RELA 0000000000000000 00000500 000000f0 24 I 19 5 8 [ 9] .rela.debug_aranges RELA 0000000000000000 000005f0 00000030 24 I 19 8 8 [11] .rela.debug_line RELA 0000000000000000 00000620 00000060 24 I 19 10 8 [18] .rela.eh_frame RELA 0000000000000000 00000680 00000030 24 I 19 17 8 ``` When applying relocation on a relocatable file (the above qq.o) with the new function dwelf_relocation_debug_sections (Elf *elf), it only needs to apply relocation on debug sections (which are [ 6] .rela.debug_info, [ 9] .rela.debug_aranges, [11] .rela.debug_line ), am I right?
Yes, just process the .rela.debug_* sections. Note that you don't need to depend on the exact name. You can follow the section header sh_info field, which points to the section (number) to which the relocations apply. If that section starts with .debug you can apply the relocations. Or better use ebl_debugscn_p, which checks against the list in libebl/eblopenbackend.c (default_debugscn_p). See also in strip.c, where it says: /* Make sure that this relocation section points to a section to relocate with contents, that isn't allocated and that is a debug section. */ Or libdwfl/relocate.c (relocate_section). Which contains similar logic.