[RFC 27/31] objtool: Fix weak symbol detection

[Josh Poimboeuf]

find_symbol_hole_containing() fails to find a symbol hole (aka stripped
weak symbol) if its section has no symbols before the hole.  This breaks
weak symbol detection if -ffunction-sections is enabled.

Fix it by allowing the interval tree to contain section symbols, which
are always at offset zero for a given section.

Fixes a bunch of (-ffunction-sections) warnings like:

  vmlinux.o: warning: objtool: .text.__x64_sys_io_setup+0x10: unreachable 
instruction
  vmlinux.o: warning: objtool: .text.__x64_sys_io_setup+0x14: unreachable 
instruction
  vmlinux.o: warning: objtool: .text.__x64_sys_io_setup+0x19: unreachable 
instruction
  vmlinux.o: warning: objtool: .text.__x64_sys_io_setup+0x20: unreachable 
instruction

Fixes: 4adb23686795 ("objtool: Ignore extra-symbol code")
Signed-off-by: Josh Poimboeuf <jpoim...@kernel.org>
---
 tools/objtool/elf.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index 471df0336aa7..3109277804cc 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -110,7 +110,7 @@ struct symbol_hole {
 };
 
 /*
- * Find !section symbol where @offset is after it.
+ * Find the last symbol before @offset.
  */
 static int symbol_hole_by_offset(const void *key, const struct rb_node *node)
 {
@@ -121,8 +121,7 @@ static int symbol_hole_by_offset(const void *key, const 
struct rb_node *node)
                return -1;
 
        if (sh->offset >= s->offset + s->len) {
-               if (s->type != STT_SECTION)
-                       sh->sym = s;
+               sh->sym = s;
                return 1;
        }
 
@@ -416,7 +415,8 @@ static void elf_add_symbol(struct elf *elf, struct symbol 
*sym)
        sym->len = sym->sym.st_size;
 
        __sym_for_each(s, &sym->sec->symbol_tree, sym->offset, sym->offset) {
-               if (s->offset == sym->offset && s->type == sym->type)
+               if (s->type == sym->type && s->offset == sym->offset &&
+                   s->len == sym->len)
                        s->alias = sym;
        }
 
@@ -433,9 +433,13 @@ static void elf_add_symbol(struct elf *elf, struct symbol 
*sym)
        /*
         * Don't store empty STT_NOTYPE symbols in the rbtree.  They
         * can exist within a function, confusing the sorting.
+        *
+        * TODO: is this still true?
         */
-       if (!sym->len)
+#if 0
+       if (sym->type == STT_NOTYPE && !sym->len)
                __sym_remove(sym, &sym->sec->symbol_tree);
+#endif
 }
 
 static void read_symbols(struct elf *elf)
-- 
2.45.2