Messages in this thread | | | Date | Tue, 21 Apr 2020 15:13:05 -0500 | From | Josh Poimboeuf <> | Subject | Re: [PATCH 3/3] objtool: optimize add_dead_ends for split sections |
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On Tue, Apr 21, 2020 at 11:07:24AM -0700, Sami Tolvanen wrote: > @@ -338,15 +351,13 @@ static int add_dead_ends(struct objtool_file *file) > if (insn) > insn = list_prev_entry(insn, list); > else if (rela->addend == rela->sym->sec->len) { > - found = false; > - list_for_each_entry_reverse(insn, &file->insn_list, list) { > - if (insn->sec == rela->sym->sec) { > - found = true; > - break; > - } > - } > + struct section_info *sec_info = (struct section_info *) > + rela->sym->sec->section_info; > + > + if (sec_info) > + insn = sec_info->last_insn; > > - if (!found) { > + if (!insn) { > WARN("can't find unreachable insn at %s+0x%x", > rela->sym->sec->name, rela->addend); > return -1;
Instead of the 'section_info' abstraction I think I'd rather just store the 'last_insn' pointer directly in the section struct.
Also, the unreachable annotation at the end of a section is really an edge case. I'm sort of wondering if there's a way to accomplish the same thing without storing the last_insn.
For example, I wonder if we could use find_insn() for some bytes at the end of the section. Most of the time I _think_ there will be a two-byte UD2 instruction there anyway. So maybe we could do something like:
for (offset = rela->sym->sec->len - 1; offset > rela->sym->sec->len - 10; offset --) {
insn = find_insn(file, rela->sym->sec, offset); if (insn) break; }
It's kind of ugly, but then we could maybe avoid the need for the last_insn thing.
BTW, just curious, what's your use case for -ffunction-sections? Is it for fgkaslr?
-- Josh
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