[PATCH 3/5] x86: Implement __WARN using UD2
From: Peter Zijlstra
Date: Fri Feb 03 2017 - 08:30:35 EST
By using "UD2" for WARNs we remove the function call and its possible
__FILE__ and __LINE__ immediate arguments from the instruction stream.
Total image size will not change much, what we win in the instrucion
stream we'll loose because of the __bug_table entries. Still, saves on
I$ footprint and the total image size does go down a bit.
text data bss dec hex filename size
10475740 4428992 974848 15879580 f24d9c defconfig-build/vmlinux.pre 25215824
10451804 4428992 974848 15855644 f1f01c defconfig-build/vmlinux.post 25211288
In particular this makes:
0000000000001490 <ihold>:
1490: 55 push %rbp
1491: 8b 87 48 01 00 00 mov 0x148(%rdi),%eax
1497: 48 89 e5 mov %rsp,%rbp
149a: eb 0a jmp 14a6 <ihold+0x16>
149c: f0 0f b1 97 48 01 00 lock cmpxchg %edx,0x148(%rdi)
14a3: 00
14a4: 74 20 je 14c6 <ihold+0x36>
14a6: 85 c0 test %eax,%eax
14a8: 8d 50 01 lea 0x1(%rax),%edx
14ab: 74 06 je 14b3 <ihold+0x23>
14ad: 85 d2 test %edx,%edx
14af: 75 eb jne 149c <ihold+0xc>
14b1: 5d pop %rbp
14b2: c3 retq
14b3: be 8d 00 00 00 mov $0x8d,%esi
14b8: 48 c7 c7 00 00 00 00 mov $0x0,%rdi
14bb: R_X86_64_32S .rodata.str1.1+0x35
14bf: e8 00 00 00 00 callq 14c4 <ihold+0x34>
14c0: R_X86_64_PC32 warn_slowpath_null-0x4
14c4: 5d pop %rbp
14c5: c3 retq
14c6: 83 fa ff cmp $0xffffffff,%edx
14c9: 75 e6 jne 14b1 <ihold+0x21>
14cb: be 80 00 00 00 mov $0x80,%esi
14d0: 48 c7 c7 00 00 00 00 mov $0x0,%rdi
14d3: R_X86_64_32S .rodata.str1.1+0x35
14d7: e8 00 00 00 00 callq 14dc <ihold+0x4c>
14d8: R_X86_64_PC32 warn_slowpath_null-0x4
14dc: 5d pop %rbp
14dd: c3 retq
14de: 66 90 xchg %ax,%ax
Look like:
0000000000001400 <ihold>:
1400: 55 push %rbp
1401: 8b 87 48 01 00 00 mov 0x148(%rdi),%eax
1407: 48 89 e5 mov %rsp,%rbp
140a: eb 0a jmp 1416 <ihold+0x16>
140c: f0 0f b1 97 48 01 00 lock cmpxchg %edx,0x148(%rdi)
1413: 00
1414: 74 11 je 1427 <ihold+0x27>
1416: 85 c0 test %eax,%eax
1418: 8d 50 01 lea 0x1(%rax),%edx
141b: 74 06 je 1423 <ihold+0x23>
141d: 85 d2 test %edx,%edx
141f: 75 eb jne 140c <ihold+0xc>
1421: 5d pop %rbp
1422: c3 retq
1423: 0f 0b ud2
1425: 5d pop %rbp
1426: c3 retq
1427: 83 fa ff cmp $0xffffffff,%edx
142a: 75 f5 jne 1421 <ihold+0x21>
142c: 0f 0b ud2
142e: 5d pop %rbp
142f: c3 retq
Note that custom x86 code could do better using the exception table
with a custom exception handler looking at the regs->ax value to
determine which of the failure cases was hit, removing a bunch of
compares from the actual code path.
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
arch/x86/include/asm/bug.h | 36 +++++++++++++++++++++++++++++-------
arch/x86/kernel/dumpstack.c | 3 ---
arch/x86/kernel/traps.c | 33 +++++++++++++++++++++++++++------
3 files changed, 56 insertions(+), 16 deletions(-)
--- a/arch/x86/include/asm/bug.h
+++ b/arch/x86/include/asm/bug.h
@@ -11,26 +11,48 @@
# define __BUG_C0 "2:\t.long 1b - 2b, %c0 - 2b\n"
#endif
-#define BUG() \
+#define _BUG_FLAGS(flags) \
do { \
asm volatile("1:\tud2\n" \
".pushsection __bug_table,\"a\"\n" \
__BUG_C0 \
- "\t.word %c1, 0\n" \
- "\t.org 2b+%c2\n" \
+ "\t.word %c1, %c2\n" \
+ "\t.org 2b+%c3\n" \
".popsection" \
: : "i" (__FILE__), "i" (__LINE__), \
- "i" (sizeof(struct bug_entry))); \
- unreachable(); \
+ "i" (flags), \
+ "i" (sizeof(struct bug_entry))); \
} while (0)
#else
+
+#ifdef CONFIG_X86_32
+# define __BUG_C0 "2:\t.long 1b\n"
+#else
+# define __BUG_C0 "2:\t.long 1b - 2b\n"
+#endif
+
+#define _BUG_FLAGS(flags) \
+do { \
+ asm volatile("1:\tud2\n" \
+ ".pushsection __bug_table,\"a\"\n" \
+ __BUG_C0 \
+ "\t.word %c0\n" \
+ "\t.org 2b+%c1\n" \
+ ".popsection" \
+ : : "i" (flags), \
+ "i" (sizeof(struct bug_entry))); \
+} while (0)
+
+#endif
+
#define BUG() \
do { \
- asm volatile("ud2"); \
+ _BUG_FLAGS(0); \
unreachable(); \
} while (0)
-#endif
+
+#define __WARN_TAINT(taint) _BUG_FLAGS(BUGFLAG_TAINT(taint))
#include <asm-generic/bug.h>
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -287,9 +287,6 @@ void die(const char *str, struct pt_regs
unsigned long flags = oops_begin();
int sig = SIGSEGV;
- if (!user_mode(regs))
- report_bug(regs->ip, regs);
-
if (__die(str, regs, err))
sig = 0;
oops_end(flags, regs, sig);
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -168,6 +168,24 @@ void ist_end_non_atomic(void)
preempt_disable();
}
+static int fixup_bug(struct pt_regs *regs, int trapnr)
+{
+ if (trapnr != X86_TRAP_UD)
+ return 0;
+
+ switch (report_bug(regs->ip, regs)) {
+ case BUG_TRAP_TYPE_NONE:
+ case BUG_TRAP_TYPE_BUG:
+ break;
+
+ case BUG_TRAP_TYPE_WARN:
+ regs->ip += 2;
+ return 1;
+ }
+
+ return 0;
+}
+
static nokprobe_inline int
do_trap_no_signal(struct task_struct *tsk, int trapnr, char *str,
struct pt_regs *regs, long error_code)
@@ -186,12 +204,15 @@ do_trap_no_signal(struct task_struct *ts
}
if (!user_mode(regs)) {
- if (!fixup_exception(regs, trapnr)) {
- tsk->thread.error_code = error_code;
- tsk->thread.trap_nr = trapnr;
- die(str, regs, error_code);
- }
- return 0;
+ if (fixup_exception(regs, trapnr))
+ return 0;
+
+ if (fixup_bug(regs, trapnr))
+ return 0;
+
+ tsk->thread.error_code = error_code;
+ tsk->thread.trap_nr = trapnr;
+ die(str, regs, error_code);
}
return -1;