drm/vc4: Add support for feeding DSI encoders from the pixel valve.

[mirror_ubuntu-zesty-kernel.git] / kernel / trace / trace_probe.c

/*
 * Common code for probe-based Dynamic events.
 *
 * 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.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 *
 * This code was copied from kernel/trace/trace_kprobe.c written by
 * Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
 *
 * Updates to make this generic:
 * Copyright (C) IBM Corporation, 2010-2011
 * Author:     Srikar Dronamraju
 */

#include "trace_probe.h"

const char *reserved_field_names[] = {
        "common_type",
        "common_flags",
        "common_preempt_count",
        "common_pid",
        "common_tgid",
        FIELD_STRING_IP,
        FIELD_STRING_RETIP,
        FIELD_STRING_FUNC,
};

/* Printing  in basic type function template */
#define DEFINE_BASIC_PRINT_TYPE_FUNC(tname, type, fmt)                  \
int PRINT_TYPE_FUNC_NAME(tname)(struct trace_seq *s, const char *name,  \
                                void *data, void *ent)                  \
{                                                                       \
        trace_seq_printf(s, " %s=" fmt, name, *(type *)data);           \
        return !trace_seq_has_overflowed(s);                            \
}                                                                       \
const char PRINT_TYPE_FMT_NAME(tname)[] = fmt;                          \
NOKPROBE_SYMBOL(PRINT_TYPE_FUNC_NAME(tname));

DEFINE_BASIC_PRINT_TYPE_FUNC(u8,  u8,  "%u")
DEFINE_BASIC_PRINT_TYPE_FUNC(u16, u16, "%u")
DEFINE_BASIC_PRINT_TYPE_FUNC(u32, u32, "%u")
DEFINE_BASIC_PRINT_TYPE_FUNC(u64, u64, "%Lu")
DEFINE_BASIC_PRINT_TYPE_FUNC(s8,  s8,  "%d")
DEFINE_BASIC_PRINT_TYPE_FUNC(s16, s16, "%d")
DEFINE_BASIC_PRINT_TYPE_FUNC(s32, s32, "%d")
DEFINE_BASIC_PRINT_TYPE_FUNC(s64, s64, "%Ld")
DEFINE_BASIC_PRINT_TYPE_FUNC(x8,  u8,  "0x%x")
DEFINE_BASIC_PRINT_TYPE_FUNC(x16, u16, "0x%x")
DEFINE_BASIC_PRINT_TYPE_FUNC(x32, u32, "0x%x")
DEFINE_BASIC_PRINT_TYPE_FUNC(x64, u64, "0x%Lx")

/* Print type function for string type */
int PRINT_TYPE_FUNC_NAME(string)(struct trace_seq *s, const char *name,
                                 void *data, void *ent)
{
        int len = *(u32 *)data >> 16;

        if (!len)
                trace_seq_printf(s, " %s=(fault)", name);
        else
                trace_seq_printf(s, " %s=\"%s\"", name,
                                 (const char *)get_loc_data(data, ent));
        return !trace_seq_has_overflowed(s);
}
NOKPROBE_SYMBOL(PRINT_TYPE_FUNC_NAME(string));

const char PRINT_TYPE_FMT_NAME(string)[] = "\\\"%s\\\"";

#define CHECK_FETCH_FUNCS(method, fn)                   \
        (((FETCH_FUNC_NAME(method, u8) == fn) ||        \
          (FETCH_FUNC_NAME(method, u16) == fn) ||       \
          (FETCH_FUNC_NAME(method, u32) == fn) ||       \
          (FETCH_FUNC_NAME(method, u64) == fn) ||       \
          (FETCH_FUNC_NAME(method, string) == fn) ||    \
          (FETCH_FUNC_NAME(method, string_size) == fn)) \
         && (fn != NULL))

/* Data fetch function templates */
#define DEFINE_FETCH_reg(type)                                          \
void FETCH_FUNC_NAME(reg, type)(struct pt_regs *regs, void *offset, void *dest) \
{                                                                       \
        *(type *)dest = (type)regs_get_register(regs,                   \
                                (unsigned int)((unsigned long)offset)); \
}                                                                       \
NOKPROBE_SYMBOL(FETCH_FUNC_NAME(reg, type));
DEFINE_BASIC_FETCH_FUNCS(reg)
/* No string on the register */
#define fetch_reg_string        NULL
#define fetch_reg_string_size   NULL

#define DEFINE_FETCH_retval(type)                                       \
void FETCH_FUNC_NAME(retval, type)(struct pt_regs *regs,                \
                                   void *dummy, void *dest)             \
{                                                                       \
        *(type *)dest = (type)regs_return_value(regs);                  \
}                                                                       \
NOKPROBE_SYMBOL(FETCH_FUNC_NAME(retval, type));
DEFINE_BASIC_FETCH_FUNCS(retval)
/* No string on the retval */
#define fetch_retval_string             NULL
#define fetch_retval_string_size        NULL

/* Dereference memory access function */
struct deref_fetch_param {
        struct fetch_param      orig;
        long                    offset;
        fetch_func_t            fetch;
        fetch_func_t            fetch_size;
};

#define DEFINE_FETCH_deref(type)                                        \
void FETCH_FUNC_NAME(deref, type)(struct pt_regs *regs,                 \
                                  void *data, void *dest)               \
{                                                                       \
        struct deref_fetch_param *dprm = data;                          \
        unsigned long addr;                                             \
        call_fetch(&dprm->orig, regs, &addr);                           \
        if (addr) {                                                     \
                addr += dprm->offset;                                   \
                dprm->fetch(regs, (void *)addr, dest);                  \
        } else                                                          \
                *(type *)dest = 0;                                      \
}                                                                       \
NOKPROBE_SYMBOL(FETCH_FUNC_NAME(deref, type));
DEFINE_BASIC_FETCH_FUNCS(deref)
DEFINE_FETCH_deref(string)

void FETCH_FUNC_NAME(deref, string_size)(struct pt_regs *regs,
                                         void *data, void *dest)
{
        struct deref_fetch_param *dprm = data;
        unsigned long addr;

        call_fetch(&dprm->orig, regs, &addr);
        if (addr && dprm->fetch_size) {
                addr += dprm->offset;
                dprm->fetch_size(regs, (void *)addr, dest);
        } else
                *(string_size *)dest = 0;
}
NOKPROBE_SYMBOL(FETCH_FUNC_NAME(deref, string_size));

static void update_deref_fetch_param(struct deref_fetch_param *data)
{
        if (CHECK_FETCH_FUNCS(deref, data->orig.fn))
                update_deref_fetch_param(data->orig.data);
        else if (CHECK_FETCH_FUNCS(symbol, data->orig.fn))
                update_symbol_cache(data->orig.data);
}
NOKPROBE_SYMBOL(update_deref_fetch_param);

static void free_deref_fetch_param(struct deref_fetch_param *data)
{
        if (CHECK_FETCH_FUNCS(deref, data->orig.fn))
                free_deref_fetch_param(data->orig.data);
        else if (CHECK_FETCH_FUNCS(symbol, data->orig.fn))
                free_symbol_cache(data->orig.data);
        kfree(data);
}
NOKPROBE_SYMBOL(free_deref_fetch_param);

/* Bitfield fetch function */
struct bitfield_fetch_param {
        struct fetch_param      orig;
        unsigned char           hi_shift;
        unsigned char           low_shift;
};

#define DEFINE_FETCH_bitfield(type)                                     \
void FETCH_FUNC_NAME(bitfield, type)(struct pt_regs *regs,              \
                                     void *data, void *dest)            \
{                                                                       \
        struct bitfield_fetch_param *bprm = data;                       \
        type buf = 0;                                                   \
        call_fetch(&bprm->orig, regs, &buf);                            \
        if (buf) {                                                      \
                buf <<= bprm->hi_shift;                                 \
                buf >>= bprm->low_shift;                                \
        }                                                               \
        *(type *)dest = buf;                                            \
}                                                                       \
NOKPROBE_SYMBOL(FETCH_FUNC_NAME(bitfield, type));
DEFINE_BASIC_FETCH_FUNCS(bitfield)
#define fetch_bitfield_string           NULL
#define fetch_bitfield_string_size      NULL

static void
update_bitfield_fetch_param(struct bitfield_fetch_param *data)
{
        /*
         * Don't check the bitfield itself, because this must be the
         * last fetch function.
         */
        if (CHECK_FETCH_FUNCS(deref, data->orig.fn))
                update_deref_fetch_param(data->orig.data);
        else if (CHECK_FETCH_FUNCS(symbol, data->orig.fn))
                update_symbol_cache(data->orig.data);
}

static void
free_bitfield_fetch_param(struct bitfield_fetch_param *data)
{
        /*
         * Don't check the bitfield itself, because this must be the
         * last fetch function.
         */
        if (CHECK_FETCH_FUNCS(deref, data->orig.fn))
                free_deref_fetch_param(data->orig.data);
        else if (CHECK_FETCH_FUNCS(symbol, data->orig.fn))
                free_symbol_cache(data->orig.data);

        kfree(data);
}

void FETCH_FUNC_NAME(comm, string)(struct pt_regs *regs,
                                          void *data, void *dest)
{
        int maxlen = get_rloc_len(*(u32 *)dest);
        u8 *dst = get_rloc_data(dest);
        long ret;

        if (!maxlen)
                return;

        ret = strlcpy(dst, current->comm, maxlen);
        *(u32 *)dest = make_data_rloc(ret, get_rloc_offs(*(u32 *)dest));
}
NOKPROBE_SYMBOL(FETCH_FUNC_NAME(comm, string));

void FETCH_FUNC_NAME(comm, string_size)(struct pt_regs *regs,
                                               void *data, void *dest)
{
        *(u32 *)dest = strlen(current->comm) + 1;
}
NOKPROBE_SYMBOL(FETCH_FUNC_NAME(comm, string_size));

static const struct fetch_type *find_fetch_type(const char *type,
                                                const struct fetch_type *ftbl)
{
        int i;

        if (!type)
                type = DEFAULT_FETCH_TYPE_STR;

        /* Special case: bitfield */
        if (*type == 'b') {
                unsigned long bs;

                type = strchr(type, '/');
                if (!type)
                        goto fail;

                type++;
                if (kstrtoul(type, 0, &bs))
                        goto fail;

                switch (bs) {
                case 8:
                        return find_fetch_type("u8", ftbl);
                case 16:
                        return find_fetch_type("u16", ftbl);
                case 32:
                        return find_fetch_type("u32", ftbl);
                case 64:
                        return find_fetch_type("u64", ftbl);
                default:
                        goto fail;
                }
        }

        for (i = 0; ftbl[i].name; i++) {
                if (strcmp(type, ftbl[i].name) == 0)
                        return &ftbl[i];
        }

fail:
        return NULL;
}

/* Special function : only accept unsigned long */
static void fetch_kernel_stack_address(struct pt_regs *regs, void *dummy, void *dest)
{
        *(unsigned long *)dest = kernel_stack_pointer(regs);
}
NOKPROBE_SYMBOL(fetch_kernel_stack_address);

static void fetch_user_stack_address(struct pt_regs *regs, void *dummy, void *dest)
{
        *(unsigned long *)dest = user_stack_pointer(regs);
}
NOKPROBE_SYMBOL(fetch_user_stack_address);

static fetch_func_t get_fetch_size_function(const struct fetch_type *type,
                                            fetch_func_t orig_fn,
                                            const struct fetch_type *ftbl)
{
        int i;

        if (type != &ftbl[FETCH_TYPE_STRING])
                return NULL;    /* Only string type needs size function */

        for (i = 0; i < FETCH_MTD_END; i++)
                if (type->fetch[i] == orig_fn)
                        return ftbl[FETCH_TYPE_STRSIZE].fetch[i];

        WARN_ON(1);     /* This should not happen */

        return NULL;
}

/* Split symbol and offset. */
int traceprobe_split_symbol_offset(char *symbol, unsigned long *offset)
{
        char *tmp;
        int ret;

        if (!offset)
                return -EINVAL;

        tmp = strchr(symbol, '+');
        if (tmp) {
                /* skip sign because kstrtoul doesn't accept '+' */
                ret = kstrtoul(tmp + 1, 0, offset);
                if (ret)
                        return ret;

                *tmp = '\0';
        } else
                *offset = 0;

        return 0;
}

#define PARAM_MAX_STACK (THREAD_SIZE / sizeof(unsigned long))

static int parse_probe_vars(char *arg, const struct fetch_type *t,
                            struct fetch_param *f, bool is_return,
                            bool is_kprobe)
{
        int ret = 0;
        unsigned long param;

        if (strcmp(arg, "retval") == 0) {
                if (is_return)
                        f->fn = t->fetch[FETCH_MTD_retval];
                else
                        ret = -EINVAL;
        } else if (strncmp(arg, "stack", 5) == 0) {
                if (arg[5] == '\0') {
                        if (strcmp(t->name, DEFAULT_FETCH_TYPE_STR))
                                return -EINVAL;

                        if (is_kprobe)
                                f->fn = fetch_kernel_stack_address;
                        else
                                f->fn = fetch_user_stack_address;
                } else if (isdigit(arg[5])) {
                        ret = kstrtoul(arg + 5, 10, &param);
                        if (ret || (is_kprobe && param > PARAM_MAX_STACK))
                                ret = -EINVAL;
                        else {
                                f->fn = t->fetch[FETCH_MTD_stack];
                                f->data = (void *)param;
                        }
                } else
                        ret = -EINVAL;
        } else if (strcmp(arg, "comm") == 0) {
                if (strcmp(t->name, "string") != 0 &&
                    strcmp(t->name, "string_size") != 0)
                        return -EINVAL;
                f->fn = t->fetch[FETCH_MTD_comm];
        } else
                ret = -EINVAL;

        return ret;
}

/* Recursive argument parser */
static int parse_probe_arg(char *arg, const struct fetch_type *t,
                     struct fetch_param *f, bool is_return, bool is_kprobe,
                     const struct fetch_type *ftbl)
{
        unsigned long param;
        long offset;
        char *tmp;
        int ret = 0;

        switch (arg[0]) {
        case '$':
                ret = parse_probe_vars(arg + 1, t, f, is_return, is_kprobe);
                break;

        case '%':       /* named register */
                ret = regs_query_register_offset(arg + 1);
                if (ret >= 0) {
                        f->fn = t->fetch[FETCH_MTD_reg];
                        f->data = (void *)(unsigned long)ret;
                        ret = 0;
                }
                break;

        case '@':       /* memory, file-offset or symbol */
                if (isdigit(arg[1])) {
                        ret = kstrtoul(arg + 1, 0, &param);
                        if (ret)
                                break;

                        f->fn = t->fetch[FETCH_MTD_memory];
                        f->data = (void *)param;
                } else if (arg[1] == '+') {
                        /* kprobes don't support file offsets */
                        if (is_kprobe)
                                return -EINVAL;

                        ret = kstrtol(arg + 2, 0, &offset);
                        if (ret)
                                break;

                        f->fn = t->fetch[FETCH_MTD_file_offset];
                        f->data = (void *)offset;
                } else {
                        /* uprobes don't support symbols */
                        if (!is_kprobe)
                                return -EINVAL;

                        ret = traceprobe_split_symbol_offset(arg + 1, &offset);
                        if (ret)
                                break;

                        f->data = alloc_symbol_cache(arg + 1, offset);
                        if (f->data)
                                f->fn = t->fetch[FETCH_MTD_symbol];
                }
                break;

        case '+':       /* deref memory */
                arg++;  /* Skip '+', because kstrtol() rejects it. */
        case '-':
                tmp = strchr(arg, '(');
                if (!tmp)
                        break;

                *tmp = '\0';
                ret = kstrtol(arg, 0, &offset);

                if (ret)
                        break;

                arg = tmp + 1;
                tmp = strrchr(arg, ')');

                if (tmp) {
                        struct deref_fetch_param        *dprm;
                        const struct fetch_type         *t2;

                        t2 = find_fetch_type(NULL, ftbl);
                        *tmp = '\0';
                        dprm = kzalloc(sizeof(struct deref_fetch_param), GFP_KERNEL);

                        if (!dprm)
                                return -ENOMEM;

                        dprm->offset = offset;
                        dprm->fetch = t->fetch[FETCH_MTD_memory];
                        dprm->fetch_size = get_fetch_size_function(t,
                                                        dprm->fetch, ftbl);
                        ret = parse_probe_arg(arg, t2, &dprm->orig, is_return,
                                                        is_kprobe, ftbl);
                        if (ret)
                                kfree(dprm);
                        else {
                                f->fn = t->fetch[FETCH_MTD_deref];
                                f->data = (void *)dprm;
                        }
                }
                break;
        }
        if (!ret && !f->fn) {   /* Parsed, but do not find fetch method */
                pr_info("%s type has no corresponding fetch method.\n", t->name);
                ret = -EINVAL;
        }

        return ret;
}

#define BYTES_TO_BITS(nb)       ((BITS_PER_LONG * (nb)) / sizeof(long))

/* Bitfield type needs to be parsed into a fetch function */
static int __parse_bitfield_probe_arg(const char *bf,
                                      const struct fetch_type *t,
                                      struct fetch_param *f)
{
        struct bitfield_fetch_param *bprm;
        unsigned long bw, bo;
        char *tail;

        if (*bf != 'b')
                return 0;

        bprm = kzalloc(sizeof(*bprm), GFP_KERNEL);
        if (!bprm)
                return -ENOMEM;

        bprm->orig = *f;
        f->fn = t->fetch[FETCH_MTD_bitfield];
        f->data = (void *)bprm;
        bw = simple_strtoul(bf + 1, &tail, 0);  /* Use simple one */

        if (bw == 0 || *tail != '@')
                return -EINVAL;

        bf = tail + 1;
        bo = simple_strtoul(bf, &tail, 0);

        if (tail == bf || *tail != '/')
                return -EINVAL;

        bprm->hi_shift = BYTES_TO_BITS(t->size) - (bw + bo);
        bprm->low_shift = bprm->hi_shift + bo;

        return (BYTES_TO_BITS(t->size) < (bw + bo)) ? -EINVAL : 0;
}

/* String length checking wrapper */
int traceprobe_parse_probe_arg(char *arg, ssize_t *size,
                struct probe_arg *parg, bool is_return, bool is_kprobe,
                const struct fetch_type *ftbl)
{
        const char *t;
        int ret;

        if (strlen(arg) > MAX_ARGSTR_LEN) {
                pr_info("Argument is too long.: %s\n",  arg);
                return -ENOSPC;
        }
        parg->comm = kstrdup(arg, GFP_KERNEL);
        if (!parg->comm) {
                pr_info("Failed to allocate memory for command '%s'.\n", arg);
                return -ENOMEM;
        }
        t = strchr(parg->comm, ':');
        if (t) {
                arg[t - parg->comm] = '\0';
                t++;
        }
        /*
         * The default type of $comm should be "string", and it can't be
         * dereferenced.
         */
        if (!t && strcmp(arg, "$comm") == 0)
                t = "string";
        parg->type = find_fetch_type(t, ftbl);
        if (!parg->type) {
                pr_info("Unsupported type: %s\n", t);
                return -EINVAL;
        }
        parg->offset = *size;
        *size += parg->type->size;
        ret = parse_probe_arg(arg, parg->type, &parg->fetch, is_return,
                              is_kprobe, ftbl);

        if (ret >= 0 && t != NULL)
                ret = __parse_bitfield_probe_arg(t, parg->type, &parg->fetch);

        if (ret >= 0) {
                parg->fetch_size.fn = get_fetch_size_function(parg->type,
                                                              parg->fetch.fn,
                                                              ftbl);
                parg->fetch_size.data = parg->fetch.data;
        }

        return ret;
}

/* Return 1 if name is reserved or already used by another argument */
int traceprobe_conflict_field_name(const char *name,
                               struct probe_arg *args, int narg)
{
        int i;

        for (i = 0; i < ARRAY_SIZE(reserved_field_names); i++)
                if (strcmp(reserved_field_names[i], name) == 0)
                        return 1;

        for (i = 0; i < narg; i++)
                if (strcmp(args[i].name, name) == 0)
                        return 1;

        return 0;
}

void traceprobe_update_arg(struct probe_arg *arg)
{
        if (CHECK_FETCH_FUNCS(bitfield, arg->fetch.fn))
                update_bitfield_fetch_param(arg->fetch.data);
        else if (CHECK_FETCH_FUNCS(deref, arg->fetch.fn))
                update_deref_fetch_param(arg->fetch.data);
        else if (CHECK_FETCH_FUNCS(symbol, arg->fetch.fn))
                update_symbol_cache(arg->fetch.data);
}

void traceprobe_free_probe_arg(struct probe_arg *arg)
{
        if (CHECK_FETCH_FUNCS(bitfield, arg->fetch.fn))
                free_bitfield_fetch_param(arg->fetch.data);
        else if (CHECK_FETCH_FUNCS(deref, arg->fetch.fn))
                free_deref_fetch_param(arg->fetch.data);
        else if (CHECK_FETCH_FUNCS(symbol, arg->fetch.fn))
                free_symbol_cache(arg->fetch.data);

        kfree(arg->name);
        kfree(arg->comm);
}

int traceprobe_command(const char *buf, int (*createfn)(int, char **))
{
        char **argv;
        int argc, ret;

        argc = 0;
        ret = 0;
        argv = argv_split(GFP_KERNEL, buf, &argc);
        if (!argv)
                return -ENOMEM;

        if (argc)
                ret = createfn(argc, argv);

        argv_free(argv);

        return ret;
}

#define WRITE_BUFSIZE  4096

ssize_t traceprobe_probes_write(struct file *file, const char __user *buffer,
                                size_t count, loff_t *ppos,
                                int (*createfn)(int, char **))
{
        char *kbuf, *tmp;
        int ret = 0;
        size_t done = 0;
        size_t size;

        kbuf = kmalloc(WRITE_BUFSIZE, GFP_KERNEL);
        if (!kbuf)
                return -ENOMEM;

        while (done < count) {
                size = count - done;

                if (size >= WRITE_BUFSIZE)
                        size = WRITE_BUFSIZE - 1;

                if (copy_from_user(kbuf, buffer + done, size)) {
                        ret = -EFAULT;
                        goto out;
                }
                kbuf[size] = '\0';
                tmp = strchr(kbuf, '\n');

                if (tmp) {
                        *tmp = '\0';
                        size = tmp - kbuf + 1;
                } else if (done + size < count) {
                        pr_warn("Line length is too long: Should be less than %d\n",
                                WRITE_BUFSIZE);
                        ret = -EINVAL;
                        goto out;
                }
                done += size;
                /* Remove comments */
                tmp = strchr(kbuf, '#');

                if (tmp)
                        *tmp = '\0';

                ret = traceprobe_command(kbuf, createfn);
                if (ret)
                        goto out;
        }
        ret = done;

out:
        kfree(kbuf);

        return ret;
}

static int __set_print_fmt(struct trace_probe *tp, char *buf, int len,
                           bool is_return)
{
        int i;
        int pos = 0;

        const char *fmt, *arg;

        if (!is_return) {
                fmt = "(%lx)";
                arg = "REC->" FIELD_STRING_IP;
        } else {
                fmt = "(%lx <- %lx)";
                arg = "REC->" FIELD_STRING_FUNC ", REC->" FIELD_STRING_RETIP;
        }

        /* When len=0, we just calculate the needed length */
#define LEN_OR_ZERO (len ? len - pos : 0)

        pos += snprintf(buf + pos, LEN_OR_ZERO, "\"%s", fmt);

        for (i = 0; i < tp->nr_args; i++) {
                pos += snprintf(buf + pos, LEN_OR_ZERO, " %s=%s",
                                tp->args[i].name, tp->args[i].type->fmt);
        }

        pos += snprintf(buf + pos, LEN_OR_ZERO, "\", %s", arg);

        for (i = 0; i < tp->nr_args; i++) {
                if (strcmp(tp->args[i].type->name, "string") == 0)
                        pos += snprintf(buf + pos, LEN_OR_ZERO,
                                        ", __get_str(%s)",
                                        tp->args[i].name);
                else
                        pos += snprintf(buf + pos, LEN_OR_ZERO, ", REC->%s",
                                        tp->args[i].name);
        }

#undef LEN_OR_ZERO

        /* return the length of print_fmt */
        return pos;
}

int set_print_fmt(struct trace_probe *tp, bool is_return)
{
        int len;
        char *print_fmt;

        /* First: called with 0 length to calculate the needed length */
        len = __set_print_fmt(tp, NULL, 0, is_return);
        print_fmt = kmalloc(len + 1, GFP_KERNEL);
        if (!print_fmt)
                return -ENOMEM;

        /* Second: actually write the @print_fmt */
        __set_print_fmt(tp, print_fmt, len + 1, is_return);
        tp->call.print_fmt = print_fmt;

        return 0;
}