wasi-libc/libc-bottom-half/mman/mman.c

/*
 * Userspace emulation of mmap and munmap. Restrictions apply.
 *
 * This is meant to be complete enough to be compatible with code that uses
 * mmap for simple file I/O. It just allocates memory with malloc and reads
 * and writes data with pread and pwrite.
 */

#define _WASI_EMULATED_MMAN
#include <stdlib.h>
#include <errno.h>
#include <unistd.h>
#include <string.h>
#include <sys/mman.h>
#include <sys/types.h>

struct map {
    int prot;
    int flags;
    off_t offset;
    size_t length;
    char body[];
};

void *mmap(void *addr, size_t length, int prot, int flags,
           int fd, off_t offset) {
    // Check for unsupported flags.
    if ((flags & MAP_FIXED) != 0 ||
#ifdef MAP_SHARED_VALIDATE
        (flags & MAP_SHARED_VALIDATE) != 0 ||
#endif
#ifdef MAP_NORESERVE
        (flags & MAP_NORESERVE) != 0 ||
#endif
#ifdef MAP_GROWSDOWN
        (flags & MAP_GROWSDOWN) != 0 ||
#endif
#ifdef MAP_HUGETLB
        (flags & MAP_HUGETLB) != 0 ||
#endif
#ifdef MAP_FIXED_NOREPLACE
        (flags & MAP_FIXED_NOREPLACE) != 0 ||
#endif
        0)
    {
        errno = EINVAL;
        return MAP_FAILED;
    }

    // Check for unsupported protection requests.
    if (prot == PROT_NONE ||
#ifdef PROT_EXEC
        (prot & PROT_EXEC) != 0 ||
#endif
        0)
    {
        errno = EINVAL;
        return MAP_FAILED;
    }

    // Allocate the memory.
    struct map *map = malloc(sizeof(struct map) + length);
    if (!map) {
        errno = ENOMEM;
        return MAP_FAILED;
    }

    // Initialize the header.
    map->prot = prot;
    map->flags = flags;
    map->offset = offset;
    map->length = length;

    // Initialize the main memory buffer, either with the contents of a file,
    // or with zeros.
    if ((flags & MAP_ANON) == 0) {
        char *body = map->body;
        while (length > 0) {
            ssize_t nread = pread(fd, body, length, offset);
            if (nread < 0) {
                if (errno == EINTR)
                    continue;
                return MAP_FAILED;
            }
            if (nread == 0)
                break;
            length -= (size_t)nread;
            offset += (size_t)nread;
            body += (size_t)nread;
        }
    } else {
        memset(map->body, 0, length);
    }

    return map->body;
}

int munmap(void *addr, size_t length) {
    struct map *map = (struct map *)addr - 1;
    off_t offset = map->offset;
    int flags = map->flags;
    int prot = map->prot;
    
    // We don't support partial munmapping.
    if (map->length != length) {
        errno = EINVAL;
        return -1;
    }

    // Release the memory.
    free(map);

    // Success!
    return 0;
}