swp_emulate.c

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/*
 *  linux/arch/arm/kernel/swp_emulate.c
 *
 *  Copyright (C) 2009 ARM Limited
 *  __user_* functions adapted from include/asm/uaccess.h
 *
 * 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.
 *
 *  Implements emulation of the SWP/SWPB instructions using load-exclusive and
 *  store-exclusive for processors that have them disabled (or future ones that
 *  might not implement them).
 *
 *  Syntax of SWP{B} instruction: SWP{B}<c> <Rt>, <Rt2>, [<Rn>]
 *  Where: Rt  = destination
 *     Rt2 = source
 *     Rn  = address
 */

#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/proc_fs.h>
#include <linux/sched.h>
#include <linux/syscalls.h>
#include <linux/perf_event.h>

#include <asm/opcodes.h>
#include <asm/traps.h>
#include <asm/uaccess.h>

/*
 * Error-checking SWP macros implemented using ldrex{b}/strex{b}
 */
#define __user_swpX_asm(data, addr, res, temp, B)       \
    __asm__ __volatile__(                   \
    "   mov     %2, %1\n"           \
    "0: ldrex"B"    %1, [%3]\n"         \
    "1: strex"B"    %0, %2, [%3]\n"         \
    "   cmp     %0, #0\n"           \
    "   movne       %0, %4\n"           \
    "2:\n"                          \
    "   .section     .fixup,\"ax\"\n"       \
    "   .align      2\n"                \
    "3: mov     %0, %5\n"           \
    "   b       2b\n"               \
    "   .previous\n"                    \
    "   .section     __ex_table,\"a\"\n"        \
    "   .align      3\n"                \
    "   .long       0b, 3b\n"           \
    "   .long       1b, 3b\n"           \
    "   .previous"                  \
    : "=&r" (res), "+r" (data), "=&r" (temp)        \
    : "r" (addr), "i" (-EAGAIN), "i" (-EFAULT)      \
    : "cc", "memory")

#define __user_swp_asm(data, addr, res, temp) \
    __user_swpX_asm(data, addr, res, temp, "")
#define __user_swpb_asm(data, addr, res, temp) \
    __user_swpX_asm(data, addr, res, temp, "b")

/*
 * Macros/defines for extracting register numbers from instruction.
 */
#define EXTRACT_REG_NUM(instruction, offset) \
    (((instruction) & (0xf << (offset))) >> (offset))
#define RN_OFFSET  16
#define RT_OFFSET  12
#define RT2_OFFSET  0
/*
 * Bit 22 of the instruction encoding distinguishes between
 * the SWP and SWPB variants (bit set means SWPB).
 */
#define TYPE_SWPB (1 << 22)

static unsigned long swpcounter;
static unsigned long swpbcounter;
static unsigned long abtcounter;
static pid_t         previous_pid;

#ifdef CONFIG_PROC_FS
static int proc_read_status(char *page, char **start, off_t off, int count,
                int *eof, void *data)
{
    char *p = page;
    int len;

    p += sprintf(p, "Emulated SWP:\t\t%lu\n", swpcounter);
    p += sprintf(p, "Emulated SWPB:\t\t%lu\n", swpbcounter);
    p += sprintf(p, "Aborted SWP{B}:\t\t%lu\n", abtcounter);
    if (previous_pid != 0)
        p += sprintf(p, "Last process:\t\t%d\n", previous_pid);

    len = (p - page) - off;
    if (len < 0)
        len = 0;

    *eof = (len <= count) ? 1 : 0;
    *start = page + off;

    return len;
}
#endif

/*
 * Set up process info to signal segmentation fault - called on access error.
 */
static void set_segfault(struct pt_regs *regs, unsigned long addr)
{
    siginfo_t info;

    if (find_vma(current->mm, addr) == NULL)
        info.si_code = SEGV_MAPERR;
    else
        info.si_code = SEGV_ACCERR;

    info.si_signo = SIGSEGV;
    info.si_errno = 0;
    info.si_addr  = (void *) instruction_pointer(regs);

    pr_debug("SWP{B} emulation: access caused memory abort!\n");
    arm_notify_die("Illegal memory access", regs, &info, 0, 0);

    abtcounter++;
}

static int emulate_swpX(unsigned int address, unsigned int *data,
            unsigned int type)
{
    unsigned int res = 0;

    if ((type != TYPE_SWPB) && (address & 0x3)) {
        /* SWP to unaligned address not permitted */
        pr_debug("SWP instruction on unaligned pointer!\n");
        return -EFAULT;
    }

    while (1) {
        unsigned long temp;

        /*
         * Barrier required between accessing protected resource and
         * releasing a lock for it. Legacy code might not have done
         * this, and we cannot determine that this is not the case
         * being emulated, so insert always.
         */
        smp_mb();

        if (type == TYPE_SWPB)
            __user_swpb_asm(*data, address, res, temp);
        else
            __user_swp_asm(*data, address, res, temp);

        if (likely(res != -EAGAIN) || signal_pending(current))
            break;

        cond_resched();
    }

    if (res == 0) {
        /*
         * Barrier also required between acquiring a lock for a
         * protected resource and accessing the resource. Inserted for
         * same reason as above.
         */
        smp_mb();

        if (type == TYPE_SWPB)
            swpbcounter++;
        else
            swpcounter++;
    }

    return res;
}

/*
 * swp_handler logs the id of calling process, dissects the instruction, sanity
 * checks the memory location, calls emulate_swpX for the actual operation and
 * deals with fixup/error handling before returning
 */
static int swp_handler(struct pt_regs *regs, unsigned int instr)
{
    unsigned int address, destreg, data, type;
    unsigned int res = 0;

    perf_sw_event(PERF_COUNT_SW_EMULATION_FAULTS, 1, regs, regs->ARM_pc);

    res = arm_check_condition(instr, regs->ARM_cpsr);
    switch (res) {
    case ARM_OPCODE_CONDTEST_PASS:
        break;
    case ARM_OPCODE_CONDTEST_FAIL:
        /* Condition failed - return to next instruction */
        regs->ARM_pc += 4;
        return 0;
    case ARM_OPCODE_CONDTEST_UNCOND:
        /* If unconditional encoding - not a SWP, undef */
        return -EFAULT;
    default:
        return -EINVAL;
    }

    if (current->pid != previous_pid) {
        pr_debug("\"%s\" (%ld) uses deprecated SWP{B} instruction\n",
             current->comm, (unsigned long)current->pid);
        previous_pid = current->pid;
    }

    address = regs->uregs[EXTRACT_REG_NUM(instr, RN_OFFSET)];
    data    = regs->uregs[EXTRACT_REG_NUM(instr, RT2_OFFSET)];
    destreg = EXTRACT_REG_NUM(instr, RT_OFFSET);

    type = instr & TYPE_SWPB;

    pr_debug("addr in r%d->0x%08x, dest is r%d, source in r%d->0x%08x)\n",
         EXTRACT_REG_NUM(instr, RN_OFFSET), address,
         destreg, EXTRACT_REG_NUM(instr, RT2_OFFSET), data);

    /* Check access in reasonable access range for both SWP and SWPB */
    if (!access_ok(VERIFY_WRITE, (address & ~3), 4)) {
        pr_debug("SWP{B} emulation: access to %p not allowed!\n",
             (void *)address);
        res = -EFAULT;
    } else {
        res = emulate_swpX(address, &data, type);
    }

    if (res == 0) {
        /*
         * On successful emulation, revert the adjustment to the PC
         * made in kernel/traps.c in order to resume execution at the
         * instruction following the SWP{B}.
         */
        regs->ARM_pc += 4;
        regs->uregs[destreg] = data;
    } else if (res == -EFAULT) {
        /*
         * Memory errors do not mean emulation failed.
         * Set up signal info to return SEGV, then return OK
         */
        set_segfault(regs, address);
    }

    return 0;
}

/*
 * Only emulate SWP/SWPB executed in ARM state/User mode.
 * The kernel must be SWP free and SWP{B} does not exist in Thumb/ThumbEE.
 */
static struct undef_hook swp_hook = {
    .instr_mask = 0x0fb00ff0,
    .instr_val  = 0x01000090,
    .cpsr_mask  = MODE_MASK | PSR_T_BIT | PSR_J_BIT,
    .cpsr_val   = USR_MODE,
    .fn     = swp_handler
};

/*
 * Register handler and create status file in /proc/cpu
 * Invoked as late_initcall, since not needed before init spawned.
 */
static int __init swp_emulation_init(void)
{
#ifdef CONFIG_PROC_FS
    struct proc_dir_entry *res;

    res = create_proc_entry("cpu/swp_emulation", S_IRUGO, NULL);

    if (!res)
        return -ENOMEM;

    res->read_proc = proc_read_status;
#endif /* CONFIG_PROC_FS */

    printk(KERN_NOTICE "Registering SWP/SWPB emulation handler\n");
    register_undef_hook(&swp_hook);

    return 0;
}

late_initcall(swp_emulation_init);