objtool: Speed up SHT_GROUP reindexing

After elf_update_group_sh_info() was introduced, a prototype version of
"objtool klp diff" went from taking ~1s to several minutes, due to
looping almost endlessly in elf_update_group_sh_info() while creating
thousands of local symbols in a file with thousands of sections.

Dramatically improve the performance by marking all symbols' correlated
SHT_GROUP sections while reading the object.  That way there's no need
to search for it every time a symbol gets reindexed.

Fixes: 2cb291596e2c ("objtool: Fix up st_info in COMDAT group section")
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Rong Xu <xur@google.com>
Link: https://lkml.kernel.org/r/2a33e583c87e3283706f346f9d59aac20653b7fd.1746662991.git.jpoimboe@kernel.org

diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index 8dffe68d705c6ae126bb32583ad43f59ca9b1468..ca5d77db692a23b7d4f7b9f8a69b94e32002b881 100644 (file)
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -572,28 +572,32 @@ err:
        return -1;
 }
 
-/*
- * @sym's idx has changed.  Update the sh_info in group sections.
- */
-static void elf_update_group_sh_info(struct elf *elf, Elf32_Word symtab_idx,
-                                    Elf32_Word new_idx, Elf32_Word old_idx)
+static int mark_group_syms(struct elf *elf)
 {
-       struct section *sec;
+       struct section *symtab, *sec;
+       struct symbol *sym;
+
+       symtab = find_section_by_name(elf, ".symtab");
+       if (!symtab) {
+               ERROR("no .symtab");
+               return -1;
+       }
 
        list_for_each_entry(sec, &elf->sections, list) {
-               if (sec->sh.sh_type != SHT_GROUP)
-                       continue;
-               if (sec->sh.sh_link == symtab_idx &&
-                   sec->sh.sh_info == old_idx) {
-                       sec->sh.sh_info = new_idx;
-                       mark_sec_changed(elf, sec, true);
-                       /*
-                        * Each ELF group should have a unique symbol key.
-                        * Return early on match.
-                        */
-                       return;
+               if (sec->sh.sh_type == SHT_GROUP &&
+                   sec->sh.sh_link == symtab->idx) {
+                       sym = find_symbol_by_index(elf, sec->sh.sh_info);
+                       if (!sym) {
+                               ERROR("%s: can't find SHT_GROUP signature symbol",
+                                     sec->name);
+                               return -1;
+                       }
+
+                       sym->group_sec = sec;
                }
        }
+
+       return 0;
 }
 
 /*
@@ -787,7 +791,11 @@ __elf_create_symbol(struct elf *elf, struct symbol *sym)
                if (elf_update_sym_relocs(elf, old))
                        return NULL;
 
-               elf_update_group_sh_info(elf, symtab->idx, new_idx, first_non_local);
+               if (old->group_sec) {
+                       old->group_sec->sh.sh_info = new_idx;
+                       mark_sec_changed(elf, old->group_sec, true);
+               }
+
                new_idx = first_non_local;
        }
 
@@ -1060,6 +1068,9 @@ struct elf *elf_open_read(const char *name, int flags)
        if (read_symbols(elf))
                goto err;
 
+       if (mark_group_syms(elf))
+               goto err;
+
        if (read_relocs(elf))
                goto err;
 

diff --git a/tools/objtool/include/objtool/elf.h b/tools/objtool/include/objtool/elf.h
index c7c4e87ebe8824f3768180b56eb0f26bf7154769..0a2fa3ac00793055a6a558640fa53916465c7e01 100644 (file)
--- a/tools/objtool/include/objtool/elf.h
+++ b/tools/objtool/include/objtool/elf.h
@@ -72,6 +72,7 @@ struct symbol {
        u8 ignore            : 1;
        struct list_head pv_target;
        struct reloc *relocs;
+       struct section *group_sec;
 };
 
 struct reloc {