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>

7 files changed, 3 insertions, 443 deletions

diff --git a/arch/arm/include/asm/processor.h b/arch/arm/include/asm/processor.h
index 67357baaeee..b439b41aeac 100644
--- a/arch/arm/include/asm/processor.h
+++ b/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
diff --git a/arch/arm/include/asm/ptrace.h b/arch/arm/include/asm/ptrace.h
index 783d50f3261..a8ff22b2a39 100644
--- a/arch/arm/include/asm/ptrace.h
+++ b/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)
 
diff --git a/arch/arm/include/asm/traps.h b/arch/arm/include/asm/traps.h
index 1b960d5ef6a..f90756dc16d 100644
--- a/arch/arm/include/asm/traps.h
+++ b/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;
 
diff --git a/arch/arm/kernel/ptrace.c b/arch/arm/kernel/ptrace.c
index 19c6816db61..eace844511f 100644
--- a/arch/arm/kernel/ptrace.c
+++ b/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;
diff --git a/arch/arm/kernel/ptrace.h b/arch/arm/kernel/ptrace.h
deleted file mode 100644
index 3926605b82e..00000000000
--- a/arch/arm/kernel/ptrace.h
+++ /dev/null
@@ -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);
-}
diff --git a/arch/arm/kernel/signal.c b/arch/arm/kernel/signal.c
index 907d5a620bc..7709668c484 100644
--- a/arch/arm/kernel/signal.c
+++ b/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
diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c
index 7f53c3651c5..21ac43f1c2d 100644
--- a/arch/arm/kernel/traps.c
+++ b/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" };