objtool: Support GCC 8 '-fnoreorder-functions'

Since the following commit:

  cd77849a69cf ("objtool: Fix GCC 8 cold subfunction detection for aliased functions")

... if the kernel is built with EXTRA_CFLAGS='-fno-reorder-functions',
objtool can get stuck in an infinite loop.

That flag causes the new GCC 8 cold subfunctions to be placed in .text
instead of .text.unlikely.  But it also has an unfortunate quirk: in the
symbol table, the subfunction (e.g., nmi_panic.cold.7) is nested inside
the parent (nmi_panic).

That function overlap confuses objtool, and causes it to get into an
infinite loop in next_insn_same_func().  Here's Allan's description of
the loop:

  "Objtool iterates through the instructions in nmi_panic using
  next_insn_same_func. Once it reaches the end of nmi_panic at 0x534 it
  jumps to 0x528 as that's the start of nmi_panic.cold.7. However, since
  the instructions starting at 0x528 are still associated with nmi_panic
  objtool will get stuck in a loop, continually jumping back to 0x528
  after reaching 0x534."

Fix it by shortening the length of the parent function so that the
functions no longer overlap.

Reported-and-analyzed-by: Allan Xavier <[email protected]>
Signed-off-by: Josh Poimboeuf <[email protected]>
Cc: Allan Xavier <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/9e704c52bee651129b036be14feda317ae5606ae.1530136978.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <[email protected]>

diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index 4e60e105583ee803916589ca56df0e81e12b8fb3..0d1acb704f641df7762f12f159afd70313089cc0 100644 (file)
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -302,19 +302,34 @@ static int read_symbols(struct elf *elf)
                                continue;
                        sym->pfunc = sym->cfunc = sym;
                        coldstr = strstr(sym->name, ".cold.");
-                       if (coldstr) {
-                               coldstr[0] = '\0';
-                               pfunc = find_symbol_by_name(elf, sym->name);
-                               coldstr[0] = '.';
-
-                               if (!pfunc) {
-                                       WARN("%s(): can't find parent function",
-                                            sym->name);
-                                       goto err;
-                               }
-
-                               sym->pfunc = pfunc;
-                               pfunc->cfunc = sym;
+                       if (!coldstr)
+                               continue;
+
+                       coldstr[0] = '\0';
+                       pfunc = find_symbol_by_name(elf, sym->name);
+                       coldstr[0] = '.';
+
+                       if (!pfunc) {
+                               WARN("%s(): can't find parent function",
+                                    sym->name);
+                               goto err;
+                       }
+
+                       sym->pfunc = pfunc;
+                       pfunc->cfunc = sym;
+
+                       /*
+                        * Unfortunately, -fnoreorder-functions puts the child
+                        * inside the parent.  Remove the overlap so we can
+                        * have sane assumptions.
+                        *
+                        * Note that pfunc->len now no longer matches
+                        * pfunc->sym.st_size.
+                        */
+                       if (sym->sec == pfunc->sec &&
+                           sym->offset >= pfunc->offset &&
+                           sym->offset + sym->len == pfunc->offset + pfunc->len) {
+                               pfunc->len -= sym->len;
                        }
                }
        }