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authorJosh Poimboeuf <jpoimboe@redhat.com>2017-02-21 15:35:32 -0600
committerIngo Molnar <mingo@kernel.org>2017-02-24 09:10:52 +0100
commitd1091c7fa3d52ebce4dd3f15d04155b3469b2f90 (patch)
treea56b8cc4e12f90c21c26bde0d49996c00c2341ef /tools
parent9f0c18aec620bc9d82268b3cb937568dd07b43ff (diff)
downloadlinux-d1091c7fa3d52ebce4dd3f15d04155b3469b2f90.tar.gz
objtool: Improve detection of BUG() and other dead ends
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>
Diffstat (limited to 'tools')
-rw-r--r--tools/objtool/arch.h5
-rw-r--r--tools/objtool/arch/x86/decode.c3
-rw-r--r--tools/objtool/builtin-check.c60
3 files changed, 58 insertions, 10 deletions
diff --git a/tools/objtool/arch.h b/tools/objtool/arch.h
index f7350fc..a59e061 100644
--- 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 039636f..6ac99e3 100644
--- 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 e8a1f69..5fc52ee 100644
--- 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;
+ bool alt_group, visited, dead_end;
struct symbol *call_dest;
struct instruction *jump_dest;
struct list_head alts;
@@ -330,6 +330,54 @@ static int decode_instructions(struct objtool_file *file)
}
/*
+ * 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.
*/
static void add_ignores(struct objtool_file *file)
@@ -843,6 +891,10 @@ 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_jump_destinations(file);
@@ -1037,13 +1089,13 @@ 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);