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Subject[PATCH 1/4] objtool: Assume unannotated 'ud2' instructions are dead ends
Date
Arnd reported some false positive warnings with GCC 7:

drivers/hid/wacom_wac.o: warning: objtool: wacom_bpt3_touch()+0x2a5: stack state mismatch: cfa1=7+8 cfa2=6+16
drivers/iio/adc/vf610_adc.o: warning: objtool: vf610_adc_calculate_rates() falls through to next function vf610_adc_sample_set()
drivers/pwm/pwm-hibvt.o: warning: objtool: hibvt_pwm_get_state() falls through to next function hibvt_pwm_remove()
drivers/pwm/pwm-mediatek.o: warning: objtool: mtk_pwm_config() falls through to next function mtk_pwm_enable()
drivers/spi/spi-bcm2835.o: warning: objtool: .text: unexpected end of section
drivers/spi/spi-bcm2835aux.o: warning: objtool: .text: unexpected end of section
drivers/watchdog/digicolor_wdt.o: warning: objtool: dc_wdt_get_timeleft() falls through to next function dc_wdt_restart()

When GCC 7 detects a potential divide-by-zero condition, it sometimes
inserts a 'ud2' instruction for the case where the divisor is zero,
instead of letting the hardware trap on the divide instruction.

Objtool doesn't consider 'ud2' to be fatal unless it's annotated with
unreachable(). So it considers the GCC-generated 'ud2' to be non-fatal,
and it tries to follow the control flow past the 'ud2' and gets
confused.

Previously, objtool *did* assume 'ud2' was always a dead end. That
changed with the following commit:

d1091c7fa3d5 ("objtool: Improve detection of BUG() and other dead ends")

The motivation behind that change was that Peter was planning on using
'ud2' for __WARN(), which is *not* a dead end. However, it turns out
that some emulators rely on 'ud2' being fatal, so he ended up using
'ud0' instead:

9a93848fe787 ("x86/debug: Implement __WARN() using UD0")

For GCC 4.5+, it should be safe to go back to the previous assumption
that 'ud2' is fatal, even when it's not annotated with unreachable().

But for pre-4.5 versions of GCC, the unreachable() macro isn't
supported, so such cases of 'ud2' need to be explicitly annotated as
reachable.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Fixes: d1091c7fa3d5 ("objtool: Improve detection of BUG() and other dead ends")
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
include/linux/compiler-gcc.h | 16 ------------
include/linux/compiler.h | 25 +++++++++++++++++-
tools/objtool/arch.h | 5 ++--
tools/objtool/arch/x86/decode.c | 17 ++++++++----
tools/objtool/check.c | 57 +++++++++++++++++++++++++++++++++++++++--
5 files changed, 94 insertions(+), 26 deletions(-)

diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index 2a34493e4d04..16d41de92ee3 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -128,22 +128,6 @@
#define __always_unused __attribute__((unused))
#define __mode(x) __attribute__((mode(x)))

-#ifdef CONFIG_STACK_VALIDATION
-#define annotate_unreachable() ({ \
- asm("%c0:\n\t" \
- ".pushsection .discard.unreachable\n\t" \
- ".long %c0b - .\n\t" \
- ".popsection\n\t" : : "i" (__LINE__)); \
-})
-#define ASM_UNREACHABLE \
- "999:\n\t" \
- ".pushsection .discard.unreachable\n\t" \
- ".long 999b - .\n\t" \
- ".popsection\n\t"
-#else
-#define annotate_unreachable()
-#endif
-
/* gcc version specific checks */

#if GCC_VERSION < 30200
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 2d2721756abf..dfaaeec4a32e 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -185,11 +185,34 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
#endif

/* Unreachable code */
+#ifdef CONFIG_STACK_VALIDATION
+#define annotate_reachable() ({ \
+ asm("%c0:\n\t" \
+ ".pushsection .discard.reachable\n\t" \
+ ".long %c0b - .\n\t" \
+ ".popsection\n\t" : : "i" (__LINE__)); \
+})
+#define annotate_unreachable() ({ \
+ asm("%c0:\n\t" \
+ ".pushsection .discard.unreachable\n\t" \
+ ".long %c0b - .\n\t" \
+ ".popsection\n\t" : : "i" (__LINE__)); \
+})
+#define ASM_UNREACHABLE \
+ "999:\n\t" \
+ ".pushsection .discard.unreachable\n\t" \
+ ".long 999b - .\n\t" \
+ ".popsection\n\t"
+#else
+#define annotate_reachable()
+#define annotate_unreachable()
+#endif
+
#ifndef ASM_UNREACHABLE
# define ASM_UNREACHABLE
#endif
#ifndef unreachable
-# define unreachable() do { } while (1)
+# define unreachable() do { annotate_reachable(); do { } while (1); } while (0)
#endif

/*
diff --git a/tools/objtool/arch.h b/tools/objtool/arch.h
index 21aeca874edb..b0d7dc3d71b5 100644
--- a/tools/objtool/arch.h
+++ b/tools/objtool/arch.h
@@ -31,8 +31,9 @@
#define INSN_RETURN 6
#define INSN_CONTEXT_SWITCH 7
#define INSN_STACK 8
-#define INSN_NOP 9
-#define INSN_OTHER 10
+#define INSN_BUG 9
+#define INSN_NOP 10
+#define INSN_OTHER 11
#define INSN_LAST INSN_OTHER

enum op_dest_type {
diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
index a36c2eba64e7..e4b400b67e41 100644
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -382,20 +382,27 @@ int arch_decode_instruction(struct elf *elf, struct section *sec,

case 0x0f:

- if (op2 >= 0x80 && op2 <= 0x8f)
+ if (op2 >= 0x80 && op2 <= 0x8f) {
+
*type = INSN_JUMP_CONDITIONAL;
- else if (op2 == 0x05 || op2 == 0x07 || op2 == 0x34 ||
- op2 == 0x35)
+
+ } else if (op2 == 0x05 || op2 == 0x07 || op2 == 0x34 ||
+ op2 == 0x35) {

/* sysenter, sysret */
*type = INSN_CONTEXT_SWITCH;

- else if (op2 == 0x0d || op2 == 0x1f)
+ } else if (op2 == 0x0b || op2 == 0xb9) {
+
+ /* ud2 */
+ *type = INSN_BUG;
+
+ } else if (op2 == 0x0d || op2 == 0x1f) {

/* nopl/nopw */
*type = INSN_NOP;

- else if (op2 == 0xa0 || op2 == 0xa8) {
+ } else if (op2 == 0xa0 || op2 == 0xa8) {

/* push fs/gs */
*type = INSN_STACK;
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 3436a942b606..d07bf4a62b45 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -296,7 +296,7 @@ static int decode_instructions(struct objtool_file *file)
}

/*
- * Find all uses of the unreachable() macro, which are code path dead ends.
+ * Mark "ud2" instructions and manually annotated dead ends.
*/
static int add_dead_ends(struct objtool_file *file)
{
@@ -305,9 +305,20 @@ static int add_dead_ends(struct objtool_file *file)
struct instruction *insn;
bool found;

+ /*
+ * By default, "ud2" is a dead end unless otherwise annotated, because
+ * GCC 7 inserts it for certain divide-by-zero cases.
+ */
+ for_each_insn(file, insn)
+ if (insn->type == INSN_BUG)
+ insn->dead_end = true;
+
+ /*
+ * Check for manually annotated dead ends.
+ */
sec = find_section_by_name(file->elf, ".rela.discard.unreachable");
if (!sec)
- return 0;
+ goto reachable;

list_for_each_entry(rela, &sec->rela_list, list) {
if (rela->sym->type != STT_SECTION) {
@@ -340,6 +351,48 @@ static int add_dead_ends(struct objtool_file *file)
insn->dead_end = true;
}

+reachable:
+ /*
+ * These manually annotated reachable checks are needed for GCC 4.4,
+ * where the Linux unreachable() macro isn't supported. In that case
+ * GCC doesn't know the "ud2" is fatal, so it generates code as if it's
+ * not a dead end.
+ */
+ sec = find_section_by_name(file->elf, ".rela.discard.reachable");
+ 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)
+ 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 reachable insn at %s+0x%x",
+ rela->sym->sec->name, rela->addend);
+ return -1;
+ }
+ } else {
+ WARN("can't find reachable insn at %s+0x%x",
+ rela->sym->sec->name, rela->addend);
+ return -1;
+ }
+
+ insn->dead_end = false;
+ }
+
return 0;
}

--
2.13.3
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