objtool: Improve detection of BUG() and other dead ends

commit d1091c7fa3d52ebce4dd3f15d04155b3469b2f90 upstream.

The BUG() macro's use of __builtin_unreachable() via the unreachable()
macro tells gcc that the instruction is a dead end, and that it's safe
to assume the current code path will not execute past the previous
instruction.

On x86, the BUG() macro is implemented with the 'ud2' instruction.  When
objtool's branch analysis sees that instruction, it knows the current
code path has come to a dead end.

Peter Zijlstra has been working on a patch to change the WARN macros to
use 'ud2'.  That patch will break objtool's assumption that 'ud2' is
always a dead end.

Generally it's best for objtool to avoid making those kinds of
assumptions anyway.  The more ignorant it is of kernel code internals,
the better.

So create a more generic way for objtool to detect dead ends by adding
an annotation to the unreachable() macro.  The annotation stores a
pointer to the end of the unreachable code path in an '__unreachable'
section.  Objtool can read that section to find the dead ends.

Tested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/41a6d33971462ebd944a1c60ad4bf5be86c17b77.1487712920.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index c7194e97c3d47975ef36e785d3a39e97eec47328..4ef267fb635adb47c2478385b0ade4af39bde7a9 100644 (file)
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -353,6 +353,7 @@ SECTIONS
        /DISCARD/ : {
                *(.eh_frame)
                *(__func_stack_frame_non_standard)
+               *(__unreachable)
        }
 }
 

diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index a1b1de17455c6832ca37063ec05f0caa836e01dc..accb5ec188cb2cf3936b4ffd0fc645b4894e1ce4 100644 (file)
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -203,6 +203,17 @@
 #endif
 #endif
 
+#ifdef CONFIG_STACK_VALIDATION
+#define annotate_unreachable() ({                                      \
+       asm("1:\t\n"                                                    \
+           ".pushsection __unreachable, \"a\"\t\n"                     \
+           ".long 1b\t\n"                                              \
+           ".popsection\t\n");                                         \
+})
+#else
+#define annotate_unreachable()
+#endif
+
 /*
  * Mark a position in code as unreachable.  This can be used to
  * suppress control flow warnings after asm blocks that transfer
@@ -212,7 +223,7 @@
  * this in the preprocessor, but we can live with this because they're
  * unreleased.  Really, we need to have autoconf for the kernel.
  */
-#define unreachable() __builtin_unreachable()
+#define unreachable() annotate_unreachable(); __builtin_unreachable()
 
 /* Mark a function definition as prohibited from being cloned. */
 #define __noclone      __attribute__((__noclone__, __optimize__("no-tracer")))

diff --git a/tools/objtool/arch.h b/tools/objtool/arch.h
index f7350fcedc70dc6c0ef04159d9fa6e33cc76bcdc..a59e061c0b4a0abcf3e9fc7a76e462fd2c41954b 100644 (file)
--- a/tools/objtool/arch.h
+++ b/tools/objtool/arch.h
@@ -31,9 +31,8 @@
 #define INSN_CALL_DYNAMIC      8
 #define INSN_RETURN            9
 #define INSN_CONTEXT_SWITCH    10
-#define INSN_BUG               11
-#define INSN_NOP               12
-#define INSN_OTHER             13
+#define INSN_NOP               11
+#define INSN_OTHER             12
 #define INSN_LAST              INSN_OTHER
 
 int arch_decode_instruction(struct elf *elf, struct section *sec,

diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
index 039636ffb6c8a3edb6c14fd9a2b3a854ab84f982..6ac99e3266eb8218aa485081501e9e0da17c4c77 100644 (file)
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -118,9 +118,6 @@ int arch_decode_instruction(struct elf *elf, struct section *sec,
                         op2 == 0x35)
                        /* sysenter, sysret */
                        *type = INSN_CONTEXT_SWITCH;
-               else if (op2 == 0x0b || op2 == 0xb9)
-                       /* ud2 */
-                       *type = INSN_BUG;
                else if (op2 == 0x0d || op2 == 0x1f)
                        /* nopl/nopw */
                        *type = INSN_NOP;

diff --git a/tools/objtool/builtin-check.c b/tools/objtool/builtin-check.c
index a688a857a7ae88ebde88548e83db19506f948744..2aa7313e31f24c9fdc301e5cfe903bb96072b96b 100644 (file)
--- a/tools/objtool/builtin-check.c
+++ b/tools/objtool/builtin-check.c
@@ -51,7 +51,7 @@ struct instruction {
        unsigned int len, state;
        unsigned char type;
        unsigned long immediate;
-       bool alt_group, visited, ignore_alts;
+       bool alt_group, visited, dead_end, ignore_alts;
        struct symbol *call_dest;
        struct instruction *jump_dest;
        struct list_head alts;
@@ -329,6 +329,54 @@ static int decode_instructions(struct objtool_file *file)
        return 0;
 }
 
+/*
+ * Find all uses of the unreachable() macro, which are code path dead ends.
+ */
+static int add_dead_ends(struct objtool_file *file)
+{
+       struct section *sec;
+       struct rela *rela;
+       struct instruction *insn;
+       bool found;
+
+       sec = find_section_by_name(file->elf, ".rela__unreachable");
+       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 .rela__unreachable");
+                       return -1;
+               }
+               insn = find_insn(file, rela->sym->sec, rela->addend);
+               if (insn)
+                       insn = list_prev_entry(insn, list);
+               else if (rela->addend == rela->sym->sec->len) {
+                       found = false;
+                       list_for_each_entry_reverse(insn, &file->insn_list, list) {
+                               if (insn->sec == rela->sym->sec) {
+                                       found = true;
+                                       break;
+                               }
+                       }
+
+                       if (!found) {
+                               WARN("can't find unreachable insn at %s+0x%x",
+                                    rela->sym->sec->name, rela->addend);
+                               return -1;
+                       }
+               } else {
+                       WARN("can't find unreachable insn at %s+0x%x",
+                            rela->sym->sec->name, rela->addend);
+                       return -1;
+               }
+
+               insn->dead_end = true;
+       }
+
+       return 0;
+}
+
 /*
  * Warnings shouldn't be reported for ignored functions.
  */
@@ -905,6 +953,10 @@ static int decode_sections(struct objtool_file *file)
        if (ret)
                return ret;
 
+       ret = add_dead_ends(file);
+       if (ret)
+               return ret;
+
        add_ignores(file);
 
        ret = add_nospec_ignores(file);
@@ -1103,13 +1155,13 @@ static int validate_branch(struct objtool_file *file,
 
                        return 0;
 
-               case INSN_BUG:
-                       return 0;
-
                default:
                        break;
                }
 
+               if (insn->dead_end)
+                       return 0;
+
                insn = next_insn_same_sec(file, insn);
                if (!insn) {
                        WARN("%s: unexpected end of section", sec->name);