libfswrap/fswrap.c

/*
 * Copyright 2023 Xavier Del Campo Romero
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#define _POSIX_C_SOURCE 200809L

#include <fswrap.h>
#include <errno.h>
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

struct secret_file
{
    FILE *f;
    const void *buf;
    long offset, sz;
    int eof, error;
};

FILE *__wrap_fopen(const char *restrict const pathname,
    const char *restrict const flags)
{
    FILE *__real_fopen(const char *, const char *);
    struct secret_file *const s = malloc(sizeof *s);

    if (!s)
        return NULL;

    *s = (const struct secret_file){0};

    for (size_t i = 0; i < fswrap_entries->n; i++)
    {
        const struct fswrap_entry *const e = &fswrap_entries->entries[i];

        if (!strcmp(pathname, e->pathname))
        {
            if (strcmp(flags, "r") && strcmp(flags, "rb"))
            {
                fprintf(stderr, "%s: unsupported flags %s\n", __func__, flags);
                return NULL;
            }

            s->buf = e->buf;
            s->sz = e->sz;
            return (FILE *)s;
        }
    }

    if (!(s->f = __real_fopen(pathname, flags)))
        goto failure;

    return (FILE *)s;

failure:
    free(s);
    return NULL;
}

int __wrap_fclose(FILE *const f)
{
    int ret;
    int __real_fclose(FILE *);
    struct secret_file *const s = (struct secret_file *)f;

    if (s->f)
        ret = __real_fclose(s->f);

    free(s);
    return ret;
}

static int read_item(void *restrict *const ptr, const size_t size,
    struct secret_file *restrict const s)
{
    if (size > s->sz - s->offset) {
        s->eof = 1;
        return -1;
    }

    memcpy(*ptr, &((const char *restrict)(s->buf))[s->offset], size);
    *ptr = (char *restrict)*ptr + size;
    s->offset += size;
    return 0;
}

size_t __wrap_fread(void *restrict ptr, const size_t size,
    const size_t nitems, FILE *restrict const stream)
{
    size_t __real_fread(void *restrict, size_t, size_t, FILE *restrict);

    if (((struct secret_file *restrict )stream)->f)
        return __real_fread(ptr, size, nitems,
            ((struct secret_file *restrict)stream)->f);

    for (size_t i = 0; i < nitems; i++)
        if (read_item(&ptr, size, (struct secret_file *restrict)stream))
            return i;

    return nitems;
}

int __wrap_feof(FILE *const stream)
{
    int __real_feof(FILE *);
    struct secret_file *const s = (struct secret_file *)stream;

    if (s->f)
        return __real_feof(s->f);

    return s->eof;
}

int __wrap_ferror(FILE *const stream)
{
    int __real_ferror(FILE *);
    struct secret_file *const s = (struct secret_file *)stream;

    if (s->f)
        return __real_ferror(s->f);

    return s->error;
}

static int file_seek(struct secret_file *const s, const long offset,
    const int whence)
{
    switch (whence)
    {
        case SEEK_SET:
            if (offset < 0)
            {
                errno = EINVAL;
                return -1;
            }

            s->offset = offset;
            break;

        case SEEK_CUR:
            if (offset > LONG_MAX - s->offset)
            {
                errno = EOVERFLOW;
                return -1;
            }
            else if (offset < -s->offset)
            {
                errno = EINVAL;
                return -1;
            }

            s->offset += offset;
            break;

        case SEEK_END:
            if (offset > LONG_MAX - s->sz)
            {
                errno = EOVERFLOW;
                return -1;
            }
            else if (offset < -s->sz)
            {
                errno = EINVAL;
                return -1;
            }

            s->offset = s->sz + offset;
            break;

        default:
            errno = EINVAL;
            return -1;
    }

    s->eof = 0;
    return 0;
}

int __wrap_fseek(FILE *const stream, const long offset, const int whence)
{
    int __real_fseek(FILE *, long, int);
    struct secret_file *const s = (struct secret_file *)stream;

    if (s->f)
        return __real_fseek(stream, offset, whence);

    return file_seek(s, offset, whence);
}

void __wrap_clearerr(FILE *const stream)
{
    int __real_clearerr(FILE *);
    struct secret_file *const s = (struct secret_file *)stream;

    if (s->f)
        __real_clearerr(s->f);
    else
        s->eof = s->error = 0;
}

int __wrap_access(const char *const pathname, const int mode)
{
    return -1;
}