// SPDX-License-Identifier: GPL-2.0
/*
 * x86 single-step support code, common to 32-bit and 64-bit.
 */
#include <linux/sched.h>
#include <linux/sched/task_stack.h>
#include <linux/mm.h>
#include <linux/ptrace.h>

#include <asm/desc.h>
#include <asm/debugreg.h>
#include <asm/mmu_context.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;
        if (v8086_mode(regs)) {
                addr = (addr & 0xffff) + (seg << 4);
                return addr;
        }

#ifdef CONFIG_MODIFY_LDT_SYSCALL
        /*
         * 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 >>= 3;

                mutex_lock(&child->mm->context.lock);
                if (unlikely(!child->mm->context.ldt ||
                             seg >= child->mm->context.ldt->nr_entries))
                        addr = -1L; /* bogus selector, access would fault */
                else {
                        desc = &child->mm->context.ldt->entries[seg];
                        base = get_desc_base(desc);

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

        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),
                        FOLL_FORCE);
        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 will also
         * cause us to set TF when returning to user mode.
         */
        set_tsk_thread_flag(child, TIF_SINGLESTEP);

        /*
         * Ensure that a trap is triggered once stepping out of a system
         * call prior to executing any user instruction.
         */
        set_task_syscall_work(child, SYSCALL_EXIT_TRAP);

        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.
         *
         * NOTE: this means that set/clear TIF_BLOCKSTEP is only safe if
         * task is current or it can't be running, otherwise we can race
         * with __switch_to_xtra(). We rely on ptrace_freeze_traced().
         */
        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);
        clear_task_syscall_work(child, SYSCALL_EXIT_TRAP);

        /* 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;
}