objtool: Allow alternatives to be ignored

Getting objtool to understand retpolines is going to be a bit of a
challenge.  For now, take advantage of the fact that retpolines are
patched in with alternatives.  Just read the original (sane)
non-alternative instruction, and ignore the patched-in retpoline.

This allows objtool to understand the control flow *around* the
retpoline, even if it can't yet follow what's inside.  This means the
ORC unwinder will fail to unwind from inside a retpoline, but will work
fine otherwise.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: gnomes@lxorguk.ukuu.org.uk
Cc: Rik van Riel <riel@redhat.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: thomas.lendacky@amd.com
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Kees Cook <keescook@google.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@linux-foundation.org>
Cc: Paul Turner <pjt@google.com>
Link: https://lkml.kernel.org/r/1515707194-20531-3-git-send-email-dwmw@amazon.co.uk

2 files changed, 57 insertions, 7 deletions

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index de053fb7049b..f40d46e24bcc 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -428,6 +428,40 @@ static void add_ignores(struct objtool_file *file)
 }
 
 /*
+ * FIXME: For now, just ignore any alternatives which add retpolines.  This is
+ * a temporary hack, as it doesn't allow ORC to unwind from inside a retpoline.
+ * But it at least allows objtool to understand the control flow *around* the
+ * retpoline.
+ */
+static int add_nospec_ignores(struct objtool_file *file)
+{
+        struct section *sec;
+        struct rela *rela;
+        struct instruction *insn;
+
+        sec = find_section_by_name(file->elf, ".rela.discard.nospec");
+        if (!sec)
+                return 0;
+
+        list_for_each_entry(rela, &sec->rela_list, list) {
+                if (rela->sym->type != STT_SECTION) {
+                        WARN("unexpected relocation symbol type in %s", sec->name);
+                        return -1;
+                }
+
+                insn = find_insn(file, rela->sym->sec, rela->addend);
+                if (!insn) {
+                        WARN("bad .discard.nospec entry");
+                        return -1;
+                }
+
+                insn->ignore_alts = true;
+        }
+
+        return 0;
+}
+
+/*
  * Find the destination instructions for all jumps.
  */
 static int add_jump_destinations(struct objtool_file *file)
@@ -509,11 +543,18 @@ static int add_call_destinations(struct objtool_file *file)
                         dest_off = insn->offset + insn->len + insn->immediate;
                         insn->call_dest = find_symbol_by_offset(insn->sec,
                                                                 dest_off);
+                        /*
+                         * FIXME: Thanks to retpolines, it's now considered
+                         * normal for a function to call within itself.  So
+                         * disable this warning for now.
+                         */
+#if 0
                         if (!insn->call_dest) {
                                 WARN_FUNC("can't find call dest symbol at offset 0x%lx",
                                           insn->sec, insn->offset, dest_off);
                                 return -1;
                         }
+#endif
                 } else if (rela->sym->type == STT_SECTION) {
                         insn->call_dest = find_symbol_by_offset(rela->sym->sec,
                                                                 rela->addend+4);
@@ -678,12 +719,6 @@ static int add_special_section_alts(struct objtool_file *file)
                 return ret;
 
         list_for_each_entry_safe(special_alt, tmp, &special_alts, list) {
-                alt = malloc(sizeof(*alt));
-                if (!alt) {
-                        WARN("malloc failed");
-                        ret = -1;
-                        goto out;
-                }
 
                 orig_insn = find_insn(file, special_alt->orig_sec,
                                       special_alt->orig_off);
@@ -694,6 +729,10 @@ static int add_special_section_alts(struct objtool_file *file)
                         goto out;
                 }
 
+                /* Ignore retpoline alternatives. */
+                if (orig_insn->ignore_alts)
+                        continue;
+
                 new_insn = NULL;
                 if (!special_alt->group || special_alt->new_len) {
                         new_insn = find_insn(file, special_alt->new_sec,
@@ -719,6 +758,13 @@ static int add_special_section_alts(struct objtool_file *file)
                                 goto out;
                 }
 
+                alt = malloc(sizeof(*alt));
+                if (!alt) {
+                        WARN("malloc failed");
+                        ret = -1;
+                        goto out;
+                }
+
                 alt->insn = new_insn;
                 list_add_tail(&alt->list, &orig_insn->alts);
 
@@ -1035,6 +1081,10 @@ static int decode_sections(struct objtool_file *file)
 
         add_ignores(file);
 
+        ret = add_nospec_ignores(file);
+        if (ret)
+                return ret;
+
         ret = add_jump_destinations(file);
         if (ret)
                 return ret;
diff --git a/tools/objtool/check.h b/tools/objtool/check.h
index 47d9ea70a83d..dbadb304a410 100644
--- a/tools/objtool/check.h
+++ b/tools/objtool/check.h
@@ -44,7 +44,7 @@ struct instruction {
         unsigned int len;
         unsigned char type;
         unsigned long immediate;
-        bool alt_group, visited, dead_end, ignore, hint, save, restore;
+        bool alt_group, visited, dead_end, ignore, hint, save, restore, ignore_alts;
         struct symbol *call_dest;
         struct instruction *jump_dest;
         struct list_head alts;