step.c

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/*
 * x86 single-step support code, common to 32-bit and 64-bit.
 */
#include <linux/sched.h>
#include <linux/mm.h>
#include <linux/ptrace.h>
#include <asm/desc.h>

unsigned long convert_ip_to_linear(struct task_struct *child, struct pt_regs *regs)
{
    unsigned long addr, seg;

    addr = regs->ip;
    seg = regs->cs & 0xffff;
    if (v8086_mode(regs)) {
        addr = (addr & 0xffff) + (seg << 4);
        return addr;
    }

    /*
     * We'll assume that the code segments in the GDT
     * are all zero-based. That is largely true: the
     * TLS segments are used for data, and the PNPBIOS
     * and APM bios ones we just ignore here.
     */
    if ((seg & SEGMENT_TI_MASK) == SEGMENT_LDT) {
        struct desc_struct *desc;
        unsigned long base;

        seg &= ~7UL;

        mutex_lock(&child->mm->context.lock);
        if (unlikely((seg >> 3) >= child->mm->context.size))
            addr = -1L; /* bogus selector, access would fault */
        else {
            desc = child->mm->context.ldt + seg;
            base = get_desc_base(desc);

            /* 16-bit code segment? */
            if (!desc->d)
                addr &= 0xffff;
            addr += base;
        }
        mutex_unlock(&child->mm->context.lock);
    }

    return addr;
}

static int is_setting_trap_flag(struct task_struct *child, struct pt_regs *regs)
{
    int i, copied;
    unsigned char opcode[15];
    unsigned long addr = convert_ip_to_linear(child, regs);

    copied = access_process_vm(child, addr, opcode, sizeof(opcode), 0);
    for (i = 0; i < copied; i++) {
        switch (opcode[i]) {
        /* popf and iret */
        case 0x9d: case 0xcf:
            return 1;

            /* CHECKME: 64 65 */

        /* opcode and address size prefixes */
        case 0x66: case 0x67:
            continue;
        /* irrelevant prefixes (segment overrides and repeats) */
        case 0x26: case 0x2e:
        case 0x36: case 0x3e:
        case 0x64: case 0x65:
        case 0xf0: case 0xf2: case 0xf3:
            continue;

#ifdef CONFIG_X86_64
        case 0x40 ... 0x4f:
            if (!user_64bit_mode(regs))
                /* 32-bit mode: register increment */
                return 0;
            /* 64-bit mode: REX prefix */
            continue;
#endif

            /* CHECKME: f2, f3 */

        /*
         * pushf: NOTE! We should probably not let
         * the user see the TF bit being set. But
         * it's more pain than it's worth to avoid
         * it, and a debugger could emulate this
         * all in user space if it _really_ cares.
         */
        case 0x9c:
        default:
            return 0;
        }
    }
    return 0;
}

/*
 * Enable single-stepping.  Return nonzero if user mode is not using TF itself.
 */
static int enable_single_step(struct task_struct *child)
{
    struct pt_regs *regs = task_pt_regs(child);
    unsigned long oflags;

    /*
     * If we stepped into a sysenter/syscall insn, it trapped in
     * kernel mode; do_debug() cleared TF and set TIF_SINGLESTEP.
     * If user-mode had set TF itself, then it's still clear from
     * do_debug() and we need to set it again to restore the user
     * state so we don't wrongly set TIF_FORCED_TF below.
     * If enable_single_step() was used last and that is what
     * set TIF_SINGLESTEP, then both TF and TIF_FORCED_TF are
     * already set and our bookkeeping is fine.
     */
    if (unlikely(test_tsk_thread_flag(child, TIF_SINGLESTEP)))
        regs->flags |= X86_EFLAGS_TF;

    /*
     * Always set TIF_SINGLESTEP - this guarantees that
     * we single-step system calls etc..  This will also
     * cause us to set TF when returning to user mode.
     */
    set_tsk_thread_flag(child, TIF_SINGLESTEP);

    oflags = regs->flags;

    /* Set TF on the kernel stack.. */
    regs->flags |= X86_EFLAGS_TF;

    /*
     * ..but if TF is changed by the instruction we will trace,
     * don't mark it as being "us" that set it, so that we
     * won't clear it by hand later.
     *
     * Note that if we don't actually execute the popf because
     * of a signal arriving right now or suchlike, we will lose
     * track of the fact that it really was "us" that set it.
     */
    if (is_setting_trap_flag(child, regs)) {
        clear_tsk_thread_flag(child, TIF_FORCED_TF);
        return 0;
    }

    /*
     * If TF was already set, check whether it was us who set it.
     * If not, we should never attempt a block step.
     */
    if (oflags & X86_EFLAGS_TF)
        return test_tsk_thread_flag(child, TIF_FORCED_TF);

    set_tsk_thread_flag(child, TIF_FORCED_TF);

    return 1;
}

void set_task_blockstep(struct task_struct *task, bool on)
{
    unsigned long debugctl;

    /*
     * Ensure irq/preemption can't change debugctl in between.
     * Note also that both TIF_BLOCKSTEP and debugctl should
     * be changed atomically wrt preemption.
     * FIXME: this means that set/clear TIF_BLOCKSTEP is simply
     * wrong if task != current, SIGKILL can wakeup the stopped
     * tracee and set/clear can play with the running task, this
     * can confuse the next __switch_to_xtra().
     */
    local_irq_disable();
    debugctl = get_debugctlmsr();
    if (on) {
        debugctl |= DEBUGCTLMSR_BTF;
        set_tsk_thread_flag(task, TIF_BLOCKSTEP);
    } else {
        debugctl &= ~DEBUGCTLMSR_BTF;
        clear_tsk_thread_flag(task, TIF_BLOCKSTEP);
    }
    if (task == current)
        update_debugctlmsr(debugctl);
    local_irq_enable();
}

/*
 * Enable single or block step.
 */
static void enable_step(struct task_struct *child, bool block)
{
    /*
     * Make sure block stepping (BTF) is not enabled unless it should be.
     * Note that we don't try to worry about any is_setting_trap_flag()
     * instructions after the first when using block stepping.
     * So no one should try to use debugger block stepping in a program
     * that uses user-mode single stepping itself.
     */
    if (enable_single_step(child) && block)
        set_task_blockstep(child, true);
    else if (test_tsk_thread_flag(child, TIF_BLOCKSTEP))
        set_task_blockstep(child, false);
}

void user_enable_single_step(struct task_struct *child)
{
    enable_step(child, 0);
}

void user_enable_block_step(struct task_struct *child)
{
    enable_step(child, 1);
}

void user_disable_single_step(struct task_struct *child)
{
    /*
     * Make sure block stepping (BTF) is disabled.
     */
    if (test_tsk_thread_flag(child, TIF_BLOCKSTEP))
        set_task_blockstep(child, false);

    /* Always clear TIF_SINGLESTEP... */
    clear_tsk_thread_flag(child, TIF_SINGLESTEP);

    /* But touch TF only if it was set by us.. */
    if (test_and_clear_tsk_thread_flag(child, TIF_FORCED_TF))
        task_pt_regs(child)->flags &= ~X86_EFLAGS_TF;
}