ARM: 6668/1: ptrace: remove single-step emulation code

PTRACE_SINGLESTEP is a ptrace request designed to offer single-stepping
support to userspace when the underlying architecture has hardware
support for this operation.

On ARM, we set arch_has_single_step() to 1 and attempt to emulate hardware
single-stepping by disassembling the current instruction to determine the
next pc and placing a software breakpoint on that location.

Unfortunately this has the following problems:

1.) Only a subset of ARMv7 instructions are supported
2.) Thumb-2 is unsupported
3.) The code is not SMP safe

We could try to fix this code, but it turns out that because of the above
issues it is rarely used in practice.  GDB, for example, uses PTRACE_POKETEXT
and PTRACE_PEEKTEXT to manage breakpoints itself and does not require any
kernel assistance.

This patch removes the single-step emulation code from ptrace meaning that
the PTRACE_SINGLESTEP request will return -EIO on ARM. Portable code must
check the return value from a ptrace call and handle the failure gracefully.

Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/include/asm/processor.h

@@ -29,19 +29,7 @@
 #define STACK_TOP_MAX	TASK_SIZE
 #endif
 
-union debug_insn {
-	u32	arm;
-	u16	thumb;
-};
-
-struct debug_entry {
-	u32			address;
-	union debug_insn	insn;
-};
-
 struct debug_info {
-	int			nsaved;
-	struct debug_entry	bp[2];
 #ifdef CONFIG_HAVE_HW_BREAKPOINT
 	struct perf_event	*hbp[ARM_MAX_HBP_SLOTS];
 #endif

arch/arm/include/asm/ptrace.h

@@ -130,8 +130,6 @@ struct pt_regs {
 
 #ifdef __KERNEL__
 
-#define arch_has_single_step()	(1)
-
 #define user_mode(regs)	\
 	(((regs)->ARM_cpsr & 0xf) == 0)
 

arch/arm/include/asm/traps.h

@@ -45,6 +45,7 @@ static inline int in_exception_text(unsigned long ptr)
 
 extern void __init early_trap_init(void);
 extern void dump_backtrace_entry(unsigned long where, unsigned long from, unsigned long frame);
+extern void ptrace_break(struct task_struct *tsk, struct pt_regs *regs);
 
 extern void *vectors_page;
 

arch/arm/kernel/ptrace.c

@@ -26,8 +26,6 @@
 #include <asm/system.h>
 #include <asm/traps.h>
 
-#include "ptrace.h"
-
 #define REG_PC	15
 #define REG_PSR	16
 /*
@@ -184,389 +182,12 @@ put_user_reg(struct task_struct *task, int offset, long data)
 	return ret;
 }
 
-static inline int
-read_u32(struct task_struct *task, unsigned long addr, u32 *res)
-{
-	int ret;
-
-	ret = access_process_vm(task, addr, res, sizeof(*res), 0);
-
-	return ret == sizeof(*res) ? 0 : -EIO;
-}
-
-static inline int
-read_instr(struct task_struct *task, unsigned long addr, u32 *res)
-{
-	int ret;
-
-	if (addr & 1) {
-		u16 val;
-		ret = access_process_vm(task, addr & ~1, &val, sizeof(val), 0);
-		ret = ret == sizeof(val) ? 0 : -EIO;
-		*res = val;
-	} else {
-		u32 val;
-		ret = access_process_vm(task, addr & ~3, &val, sizeof(val), 0);
-		ret = ret == sizeof(val) ? 0 : -EIO;
-		*res = val;
-	}
-	return ret;
-}
-
-/*
- * Get value of register `rn' (in the instruction)
- */
-static unsigned long
-ptrace_getrn(struct task_struct *child, unsigned long insn)
-{
-	unsigned int reg = (insn >> 16) & 15;
-	unsigned long val;
-
-	val = get_user_reg(child, reg);
-	if (reg == 15)
-		val += 8;
-
-	return val;
-}
-
-/*
- * Get value of operand 2 (in an ALU instruction)
- */
-static unsigned long
-ptrace_getaluop2(struct task_struct *child, unsigned long insn)
-{
-	unsigned long val;
-	int shift;
-	int type;
-
-	if (insn & 1 << 25) {
-		val = insn & 255;
-		shift = (insn >> 8) & 15;
-		type = 3;
-	} else {
-		val = get_user_reg (child, insn & 15);
-
-		if (insn & (1 << 4))
-			shift = (int)get_user_reg (child, (insn >> 8) & 15);
-		else
-			shift = (insn >> 7) & 31;
-
-		type = (insn >> 5) & 3;
-	}
-
-	switch (type) {
-	case 0:	val <<= shift;	break;
-	case 1:	val >>= shift;	break;
-	case 2:
-		val = (((signed long)val) >> shift);
-		break;
-	case 3:
- 		val = (val >> shift) | (val << (32 - shift));
-		break;
-	}
-	return val;
-}
-
-/*
- * Get value of operand 2 (in a LDR instruction)
- */
-static unsigned long
-ptrace_getldrop2(struct task_struct *child, unsigned long insn)
-{
-	unsigned long val;
-	int shift;
-	int type;
-
-	val = get_user_reg(child, insn & 15);
-	shift = (insn >> 7) & 31;
-	type = (insn >> 5) & 3;
-
-	switch (type) {
-	case 0:	val <<= shift;	break;
-	case 1:	val >>= shift;	break;
-	case 2:
-		val = (((signed long)val) >> shift);
-		break;
-	case 3:
- 		val = (val >> shift) | (val << (32 - shift));
-		break;
-	}
-	return val;
-}
-
-#define OP_MASK	0x01e00000
-#define OP_AND	0x00000000
-#define OP_EOR	0x00200000
-#define OP_SUB	0x00400000
-#define OP_RSB	0x00600000
-#define OP_ADD	0x00800000
-#define OP_ADC	0x00a00000
-#define OP_SBC	0x00c00000
-#define OP_RSC	0x00e00000
-#define OP_ORR	0x01800000
-#define OP_MOV	0x01a00000
-#define OP_BIC	0x01c00000
-#define OP_MVN	0x01e00000
-
-static unsigned long
-get_branch_address(struct task_struct *child, unsigned long pc, unsigned long insn)
-{
-	u32 alt = 0;
-
-	switch (insn & 0x0e000000) {
-	case 0x00000000:
-	case 0x02000000: {
-		/*
-		 * data processing
-		 */
-		long aluop1, aluop2, ccbit;
-
-	        if ((insn & 0x0fffffd0) == 0x012fff10) {
-		        /*
-			 * bx or blx
-			 */
-			alt = get_user_reg(child, insn & 15);
-			break;
-		}
-
-
-		if ((insn & 0xf000) != 0xf000)
-			break;
-
-		aluop1 = ptrace_getrn(child, insn);
-		aluop2 = ptrace_getaluop2(child, insn);
-		ccbit  = get_user_reg(child, REG_PSR) & PSR_C_BIT ? 1 : 0;
-
-		switch (insn & OP_MASK) {
-		case OP_AND: alt = aluop1 & aluop2;		break;
-		case OP_EOR: alt = aluop1 ^ aluop2;		break;
-		case OP_SUB: alt = aluop1 - aluop2;		break;
-		case OP_RSB: alt = aluop2 - aluop1;		break;
-		case OP_ADD: alt = aluop1 + aluop2;		break;
-		case OP_ADC: alt = aluop1 + aluop2 + ccbit;	break;
-		case OP_SBC: alt = aluop1 - aluop2 + ccbit;	break;
-		case OP_RSC: alt = aluop2 - aluop1 + ccbit;	break;
-		case OP_ORR: alt = aluop1 | aluop2;		break;
-		case OP_MOV: alt = aluop2;			break;
-		case OP_BIC: alt = aluop1 & ~aluop2;		break;
-		case OP_MVN: alt = ~aluop2;			break;
-		}
-		break;
-	}
-
-	case 0x04000000:
-	case 0x06000000:
-		/*
-		 * ldr
-		 */
-		if ((insn & 0x0010f000) == 0x0010f000) {
-			unsigned long base;
-
-			base = ptrace_getrn(child, insn);
-			if (insn & 1 << 24) {
-				long aluop2;
-
-				if (insn & 0x02000000)
-					aluop2 = ptrace_getldrop2(child, insn);
-				else
-					aluop2 = insn & 0xfff;
-
-				if (insn & 1 << 23)
-					base += aluop2;
-				else
-					base -= aluop2;
-			}
-			read_u32(child, base, &alt);
-		}
-		break;
-
-	case 0x08000000:
-		/*
-		 * ldm
-		 */
-		if ((insn & 0x00108000) == 0x00108000) {
-			unsigned long base;
-			unsigned int nr_regs;
-
-			if (insn & (1 << 23)) {
-				nr_regs = hweight16(insn & 65535) << 2;
-
-				if (!(insn & (1 << 24)))
-					nr_regs -= 4;
-			} else {
-				if (insn & (1 << 24))
-					nr_regs = -4;
-				else
-					nr_regs = 0;
-			}
-
-			base = ptrace_getrn(child, insn);
-
-			read_u32(child, base + nr_regs, &alt);
-			break;
-		}
-		break;
-
-	case 0x0a000000: {
-		/*
-		 * bl or b
-		 */
-		signed long displ;
-		/* It's a branch/branch link: instead of trying to
-		 * figure out whether the branch will be taken or not,
-		 * we'll put a breakpoint at both locations.  This is
-		 * simpler, more reliable, and probably not a whole lot
-		 * slower than the alternative approach of emulating the
-		 * branch.
-		 */
-		displ = (insn & 0x00ffffff) << 8;
-		displ = (displ >> 6) + 8;
-		if (displ != 0 && displ != 4)
-			alt = pc + displ;
-	    }
-	    break;
-	}
-
-	return alt;
-}
-
-static int
-swap_insn(struct task_struct *task, unsigned long addr,
-	  void *old_insn, void *new_insn, int size)
-{
-	int ret;
-
-	ret = access_process_vm(task, addr, old_insn, size, 0);
-	if (ret == size)
-		ret = access_process_vm(task, addr, new_insn, size, 1);
-	return ret;
-}
-
-static void
-add_breakpoint(struct task_struct *task, struct debug_info *dbg, unsigned long addr)
-{
-	int nr = dbg->nsaved;
-
-	if (nr < 2) {
-		u32 new_insn = BREAKINST_ARM;
-		int res;
-
-		res = swap_insn(task, addr, &dbg->bp[nr].insn, &new_insn, 4);
-
-		if (res == 4) {
-			dbg->bp[nr].address = addr;
-			dbg->nsaved += 1;
-		}
-	} else
-		printk(KERN_ERR "ptrace: too many breakpoints\n");
-}
-
-/*
- * Clear one breakpoint in the user program.  We copy what the hardware
- * does and use bit 0 of the address to indicate whether this is a Thumb
- * breakpoint or an ARM breakpoint.
- */
-static void clear_breakpoint(struct task_struct *task, struct debug_entry *bp)
-{
-	unsigned long addr = bp->address;
-	union debug_insn old_insn;
-	int ret;
-
-	if (addr & 1) {
-		ret = swap_insn(task, addr & ~1, &old_insn.thumb,
-				&bp->insn.thumb, 2);
-
-		if (ret != 2 || old_insn.thumb != BREAKINST_THUMB)
-			printk(KERN_ERR "%s:%d: corrupted Thumb breakpoint at "
-				"0x%08lx (0x%04x)\n", task->comm,
-				task_pid_nr(task), addr, old_insn.thumb);
-	} else {
-		ret = swap_insn(task, addr & ~3, &old_insn.arm,
-				&bp->insn.arm, 4);
-
-		if (ret != 4 || old_insn.arm != BREAKINST_ARM)
-			printk(KERN_ERR "%s:%d: corrupted ARM breakpoint at "
-				"0x%08lx (0x%08x)\n", task->comm,
-				task_pid_nr(task), addr, old_insn.arm);
-	}
-}
-
-void ptrace_set_bpt(struct task_struct *child)
-{
-	struct pt_regs *regs;
-	unsigned long pc;
-	u32 insn;
-	int res;
-
-	regs = task_pt_regs(child);
-	pc = instruction_pointer(regs);
-
-	if (thumb_mode(regs)) {
-		printk(KERN_WARNING "ptrace: can't handle thumb mode\n");
-		return;
-	}
-
-	res = read_instr(child, pc, &insn);
-	if (!res) {
-		struct debug_info *dbg = &child->thread.debug;
-		unsigned long alt;
-
-		dbg->nsaved = 0;
-
-		alt = get_branch_address(child, pc, insn);
-		if (alt)
-			add_breakpoint(child, dbg, alt);
-
-		/*
-		 * Note that we ignore the result of setting the above
-		 * breakpoint since it may fail.  When it does, this is
-		 * not so much an error, but a forewarning that we may
-		 * be receiving a prefetch abort shortly.
-		 *
-		 * If we don't set this breakpoint here, then we can
-		 * lose control of the thread during single stepping.
-		 */
-		if (!alt || predicate(insn) != PREDICATE_ALWAYS)
-			add_breakpoint(child, dbg, pc + 4);
-	}
-}
-
-/*
- * Ensure no single-step breakpoint is pending.  Returns non-zero
- * value if child was being single-stepped.
- */
-void ptrace_cancel_bpt(struct task_struct *child)
-{
-	int i, nsaved = child->thread.debug.nsaved;
-
-	child->thread.debug.nsaved = 0;
-
-	if (nsaved > 2) {
-		printk("ptrace_cancel_bpt: bogus nsaved: %d!\n", nsaved);
-		nsaved = 2;
-	}
-
-	for (i = 0; i < nsaved; i++)
-		clear_breakpoint(child, &child->thread.debug.bp[i]);
-}
-
-void user_disable_single_step(struct task_struct *task)
-{
-	task->ptrace &= ~PT_SINGLESTEP;
-	ptrace_cancel_bpt(task);
-}
-
-void user_enable_single_step(struct task_struct *task)
-{
-	task->ptrace |= PT_SINGLESTEP;
-}
-
 /*
  * Called by kernel/ptrace.c when detaching..
  */
 void ptrace_disable(struct task_struct *child)
 {
-	user_disable_single_step(child);
+	/* Nothing to do. */
 }
 
 /*
@@ -576,8 +197,6 @@ void ptrace_break(struct task_struct *tsk, struct pt_regs *regs)
 {
 	siginfo_t info;
 
-	ptrace_cancel_bpt(tsk);
-
 	info.si_signo = SIGTRAP;
 	info.si_errno = 0;
 	info.si_code  = TRAP_BRKPT;

arch/arm/kernel/ptrace.h

@@ -1,37 +0,0 @@
-/*
- *  linux/arch/arm/kernel/ptrace.h
- *
- *  Copyright (C) 2000-2003 Russell King
- *
- * This program is free software; you can redistribute it and/or modify
- * it under the terms of the GNU General Public License version 2 as
- * published by the Free Software Foundation.
- */
-#include <linux/ptrace.h>
-
-extern void ptrace_cancel_bpt(struct task_struct *);
-extern void ptrace_set_bpt(struct task_struct *);
-extern void ptrace_break(struct task_struct *, struct pt_regs *);
-
-/*
- * Send SIGTRAP if we're single-stepping
- */
-static inline void single_step_trap(struct task_struct *task)
-{
-	if (task->ptrace & PT_SINGLESTEP) {
-		ptrace_cancel_bpt(task);
-		send_sig(SIGTRAP, task, 1);
-	}
-}
-
-static inline void single_step_clear(struct task_struct *task)
-{
-	if (task->ptrace & PT_SINGLESTEP)
-		ptrace_cancel_bpt(task);
-}
-
-static inline void single_step_set(struct task_struct *task)
-{
-	if (task->ptrace & PT_SINGLESTEP)
-		ptrace_set_bpt(task);
-}

arch/arm/kernel/signal.c

@@ -20,7 +20,6 @@
 #include <asm/unistd.h>
 #include <asm/vfp.h>
 
-#include "ptrace.h"
 #include "signal.h"
 
 #define _BLOCKABLE (~(sigmask(SIGKILL) | sigmask(SIGSTOP)))
@@ -348,8 +347,6 @@ asmlinkage int sys_sigreturn(struct pt_regs *regs)
 	if (restore_sigframe(regs, frame))
 		goto badframe;
 
-	single_step_trap(current);
-
 	return regs->ARM_r0;
 
 badframe:
@@ -383,8 +380,6 @@ asmlinkage int sys_rt_sigreturn(struct pt_regs *regs)
 	if (do_sigaltstack(&frame->sig.uc.uc_stack, NULL, regs->ARM_sp) == -EFAULT)
 		goto badframe;
 
-	single_step_trap(current);
-
 	return regs->ARM_r0;
 
 badframe:
@@ -704,8 +699,6 @@ static void do_signal(struct pt_regs *regs, int syscall)
 	if (try_to_freeze())
 		goto no_signal;
 
-	single_step_clear(current);
-
 	signr = get_signal_to_deliver(&info, &ka, regs, NULL);
 	if (signr > 0) {
 		sigset_t *oldset;
@@ -724,7 +717,6 @@ static void do_signal(struct pt_regs *regs, int syscall)
 			if (test_thread_flag(TIF_RESTORE_SIGMASK))
 				clear_thread_flag(TIF_RESTORE_SIGMASK);
 		}
-		single_step_set(current);
 		return;
 	}
 
@@ -770,7 +762,6 @@ static void do_signal(struct pt_regs *regs, int syscall)
 			sigprocmask(SIG_SETMASK, &current->saved_sigmask, NULL);
 		}
 	}
-	single_step_set(current);
 }
 
 asmlinkage void

arch/arm/kernel/traps.c

@@ -23,6 +23,7 @@
 #include <linux/kexec.h>
 #include <linux/delay.h>
 #include <linux/init.h>
+#include <linux/sched.h>
 
 #include <asm/atomic.h>
 #include <asm/cacheflush.h>
@@ -32,7 +33,6 @@
 #include <asm/unwind.h>
 #include <asm/tls.h>
 
-#include "ptrace.h"
 #include "signal.h"
 
 static const char *handler[]= { "prefetch abort", "data abort", "address exception", "interrupt" };