Re: [PATCH 1/5] x86: Implement __WARN using UD0

[Peter Zijlstra <peterz@infradead.org>]

A little like so then.

---
Subject: x86: Implement __WARN using UD0
From: Peter Zijlstra <peterz@infradead.org>
Date: Thu Feb 2 14:43:51 CET 2017

By using "UD0" 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 instruction
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
10702123        4530992  843776 16076891         f5505b defconfig-build/vmlinux.0
10682460        4530992  843776 16057228         f5038c defconfig-build/vmlinux.1

(um didn't seem to use GENERIC_BUG at all, so remove it)

Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Richard Weinberger <richard.weinberger@gmail.com>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/um/Kconfig.common         |    5 --
 arch/x86/include/asm/bug.h     |   73 +++++++++++++++++++++++++++++++----------
 arch/x86/kernel/dumpstack.c    |    3 -
 arch/x86/kernel/dumpstack_32.c |   12 ------
 arch/x86/kernel/dumpstack_64.c |   10 -----
 arch/x86/kernel/traps.c        |   46 ++++++++++++++++++++++---
 arch/x86/um/Makefile           |    2 -
 arch/x86/um/bug.c              |   21 -----------
 8 files changed, 97 insertions(+), 75 deletions(-)

--- a/arch/um/Kconfig.common
+++ b/arch/um/Kconfig.common
@@ -50,11 +50,6 @@ config GENERIC_CALIBRATE_DELAY
 	bool
 	default y
 
-config GENERIC_BUG
-	bool
-	default y
-	depends on BUG
-
 config HZ
 	int
 	default 100
--- a/arch/x86/include/asm/bug.h
+++ b/arch/x86/include/asm/bug.h
@@ -1,36 +1,75 @@
 #ifndef _ASM_X86_BUG_H
 #define _ASM_X86_BUG_H
 
+#include <linux/stringify.h>
+
 #define HAVE_ARCH_BUG
 
-#ifdef CONFIG_DEBUG_BUGVERBOSE
+/*
+ * Since some emulators terminate on UD2, we cannot use it for WARN.
+ * Since various instruction decoders disagree on the length of UD1,
+ * we cannot use it either. So use UD0 for WARN.
+ *
+ * (binutils knows about "ud1" but {en,de}codes it as 2 bytes, whereas
+ *  our kernel decoder thinks it takes a ModRM byte, which seems consistent
+ *  with various things like the Intel SDM instruction encoding rules)
+ */
+
+#define ASM_UD0		".byte 0x0f, 0xff"
+#define ASM_UD1		".byte 0x0f, 0xb9" /* + ModRM */
+#define ASM_UD2		".byte 0x0f, 0x0b"
+
+#define INSN_UD0	0xff0f
+#define INSN_UD2	0x0b0f
+
+#define LEN_UD0		2
 
 #ifdef CONFIG_X86_32
-# define __BUG_C0	"2:\t.long 1b, %c0\n"
+# define __BUG_REL(val)	".long " __stringify(val)
 #else
-# define __BUG_C0	"2:\t.long 1b - 2b, %c0 - 2b\n"
+# define __BUG_REL(val)	".long " __stringify(val) " - 2b"
 #endif
 
-#define BUG()							\
-do {								\
-	asm volatile("1:\tud2\n"				\
-		     ".pushsection __bug_table,\"a\"\n"		\
-		     __BUG_C0					\
-		     "\t.word %c1, 0\n"				\
-		     "\t.org 2b+%c2\n"				\
-		     ".popsection"				\
-		     : : "i" (__FILE__), "i" (__LINE__),	\
-		     "i" (sizeof(struct bug_entry)));		\
-	unreachable();						\
+#ifdef CONFIG_DEBUG_BUGVERBOSE
+
+#define _BUG_FLAGS(ins, flags)						\
+do {									\
+	asm volatile("1:\t" ins "\n"					\
+		     ".pushsection __bug_table,\"a\"\n"			\
+		     "2:\t" __BUG_REL(1b) "\t# bug_entry::bug_addr\n"	\
+		     "\t"  __BUG_REL(%c0) "\t# bug_entry::file\n"	\
+		     "\t.word %c1"        "\t# bug_entry::line\n"	\
+		     "\t.word %c2"        "\t# bug_entry::flags\n"	\
+		     "\t.org 2b+%c3\n"					\
+		     ".popsection"					\
+		     : : "i" (__FILE__), "i" (__LINE__),		\
+			 "i" (flags),					\
+			 "i" (sizeof(struct bug_entry)));		\
 } while (0)
 
-#else
+#else /* !CONFIG_DEBUG_BUGVERBOSE */
+
+#define _BUG_FLAGS(ins, flags)						\
+do {									\
+	asm volatile("1:\t" ins "\n"					\
+		     ".pushsection __bug_table,\"a\"\n"			\
+		     "2:\t" __BUG_REL(1b) "\t# bug_entry::bug_addr\n"	\
+		     "\t.word %c0"        "\t# bug_entry::flags\n"	\
+		     "\t.org 2b+%c1\n"					\
+		     ".popsection"					\
+		     : : "i" (flags),					\
+			 "i" (sizeof(struct bug_entry)));		\
+} while (0)
+
+#endif /* CONFIG_DEBUG_BUGVERBOSE */
+
 #define BUG()							\
 do {								\
-	asm volatile("ud2");					\
+	_BUG_FLAGS(ASM_UD2, 0);					\
 	unreachable();						\
 } while (0)
-#endif
+
+#define __WARN_TAINT(taint)	_BUG_FLAGS(ASM_UD0, BUGFLAG_TAINT(taint))
 
 #include <asm-generic/bug.h>
 
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -289,9 +289,6 @@ 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/dumpstack_32.c
+++ b/arch/x86/kernel/dumpstack_32.c
@@ -162,15 +162,3 @@ void show_regs(struct pt_regs *regs)
 	}
 	pr_cont("\n");
 }
-
-int is_valid_bugaddr(unsigned long ip)
-{
-	unsigned short ud2;
-
-	if (ip < PAGE_OFFSET)
-		return 0;
-	if (probe_kernel_address((unsigned short *)ip, ud2))
-		return 0;
-
-	return ud2 == 0x0b0f;
-}
--- a/arch/x86/kernel/dumpstack_64.c
+++ b/arch/x86/kernel/dumpstack_64.c
@@ -178,13 +178,3 @@ void show_regs(struct pt_regs *regs)
 	}
 	pr_cont("\n");
 }
-
-int is_valid_bugaddr(unsigned long ip)
-{
-	unsigned short ud2;
-
-	if (__copy_from_user(&ud2, (const void __user *) ip, sizeof(ud2)))
-		return 0;
-
-	return ud2 == 0x0b0f;
-}
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -169,6 +169,37 @@ void ist_end_non_atomic(void)
 	preempt_disable();
 }
 
+int is_valid_bugaddr(unsigned long addr)
+{
+	unsigned short ud;
+
+	if (addr < TASK_SIZE_MAX)
+		return 0;
+
+	if (probe_kernel_address((unsigned short *)addr, ud))
+		return 0;
+
+	return ud == INSN_UD0 || ud == INSN_UD2;
+}
+
+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 += LEN_UD0;
+		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)
@@ -187,12 +218,15 @@ 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;
--- a/arch/x86/um/Makefile
+++ b/arch/x86/um/Makefile
@@ -8,7 +8,7 @@ else
 	BITS := 64
 endif
 
-obj-y = bug.o bugs_$(BITS).o delay.o fault.o ldt.o \
+obj-y = bugs_$(BITS).o delay.o fault.o ldt.o \
 	ptrace_$(BITS).o ptrace_user.o setjmp_$(BITS).o signal.o \
 	stub_$(BITS).o stub_segv.o \
 	sys_call_table_$(BITS).o sysrq_$(BITS).o tls_$(BITS).o \
--- a/arch/x86/um/bug.c
+++ /dev/null
@@ -1,21 +0,0 @@
-/*
- * Copyright (C) 2006 Jeff Dike (jdike@addtoit.com)
- * Licensed under the GPL V2
- */
-
-#include <linux/uaccess.h>
-
-/*
- * Mostly copied from i386/x86_86 - eliminated the eip < PAGE_OFFSET because
- * that's not relevant in skas mode.
- */
-
-int is_valid_bugaddr(unsigned long eip)
-{
-	unsigned short ud2;
-
-	if (probe_kernel_address((unsigned short __user *)eip, ud2))
-		return 0;
-
-	return ud2 == 0x0b0f;
-}