Commit 19526717 authored by Peter Zijlstra's avatar Peter Zijlstra
Browse files

objtool: Optimize elf_dirty_reloc_sym() 



When moving a symbol in the symtab its index changes and any reloc
referring that symtol-table-index will need to be rewritten too.

In order to facilitate this, objtool simply marks the whole reloc
section 'changed' which will cause the whole section to be
re-generated.

However, finding the relocs that use any given symbol is implemented
rather crudely -- a fully iteration of all sections and their relocs.
Given that some builds have over 20k sections (kallsyms etc..)
iterating all that for *each* symbol moved takes a bit of time.

Instead have each symbol keep a list of relocs that reference it.

This *vastly* improves build times for certain configs.

Reported-by: default avatarBorislav Petkov <bp@alien8.de>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/Y2LlRA7x+8UsE1xf@hirez.programming.kicks-ass.net
parent 0c3e806e

tools/objtool/elf.c

+10 −17
Original line number Diff line number Diff line
@@ -356,6 +356,7 @@ static void elf_add_symbol(struct elf *elf, struct symbol *sym) 
	struct rb_node *pnode;

	struct symbol *iter;



	INIT_LIST_HEAD(&sym->reloc_list);

	INIT_LIST_HEAD(&sym->pv_target);

	sym->alias = sym;
@@ -557,6 +558,7 @@ int elf_add_reloc(struct elf *elf, struct section *sec, unsigned long offset, 
	reloc->sym = sym;

	reloc->addend = addend;



	list_add_tail(&reloc->sym_reloc_entry, &sym->reloc_list);

	list_add_tail(&reloc->list, &sec->reloc->reloc_list);

	elf_hash_add(reloc, &reloc->hash, reloc_hash(reloc));
@@ -573,21 +575,10 @@ int elf_add_reloc(struct elf *elf, struct section *sec, unsigned long offset, 
 */

static void elf_dirty_reloc_sym(struct elf *elf, struct symbol *sym)

{

	struct section *sec;



	list_for_each_entry(sec, &elf->sections, list) {

	struct reloc *reloc;



		if (sec->changed)

			continue;



		list_for_each_entry(reloc, &sec->reloc_list, list) {

			if (reloc->sym == sym) {

				sec->changed = true;

				break;

			}

		}

	}

	list_for_each_entry(reloc, &sym->reloc_list, sym_reloc_entry)

		reloc->sec->changed = true;

}



/*
@@ -902,11 +893,12 @@ static int read_rela_reloc(struct section *sec, int i, struct reloc *reloc, unsi 


static int read_relocs(struct elf *elf)

{

	unsigned long nr_reloc, max_reloc = 0, tot_reloc = 0;

	struct section *sec;

	struct reloc *reloc;

	int i;

	unsigned int symndx;

	unsigned long nr_reloc, max_reloc = 0, tot_reloc = 0;

	struct symbol *sym;

	int i;



	if (!elf_alloc_hash(reloc, elf->text_size / 16))

		return -1;
@@ -947,13 +939,14 @@ static int read_relocs(struct elf *elf) 


			reloc->sec = sec;

			reloc->idx = i;

			reloc->sym = find_symbol_by_index(elf, symndx);

			reloc->sym = sym = find_symbol_by_index(elf, symndx);

			if (!reloc->sym) {

				WARN("can't find reloc entry symbol %d for %s",

				     symndx, sec->name);

				return -1;

			}



			list_add_tail(&reloc->sym_reloc_entry, &sym->reloc_list);

			list_add_tail(&reloc->list, &sec->reloc_list);

			elf_hash_add(reloc, &reloc->hash, reloc_hash(reloc));

tools/objtool/include/objtool/elf.h

+2 −0
Original line number Diff line number Diff line
@@ -62,6 +62,7 @@ struct symbol { 
	u8 fentry            : 1;

	u8 profiling_func    : 1;

	struct list_head pv_target;

	struct list_head reloc_list;

};



struct reloc {
@@ -73,6 +74,7 @@ struct reloc { 
	};

	struct section *sec;

	struct symbol *sym;

	struct list_head sym_reloc_entry;

	unsigned long offset;

	unsigned int type;

	s64 addend;