libc-bottom-half/mman/mman.c

   1 // Userspace emulation of mmap and munmap. Restrictions apply.
   2 //
   3 // This is meant to be complete enough to be compatible with code that uses
   4 // mmap for simple file I/O. It just allocates memory with malloc and reads
   5 // and writes data with pread and pwrite.
   6
   7 #define _WASI_EMULATED_MMAN
   8 #include <stdlib.h>
   9 #include <errno.h>
  10 #include <unistd.h>
  11 #include <string.h>
  12 #include <sys/mman.h>
  13 #include <sys/types.h>
  14
  15 struct map {
  16     int prot;
  17     int flags;
  18     off_t offset;
  19     size_t length;
  20 };
  21
  22 void *mmap(void *addr, size_t length, int prot, int flags,
  23            int fd, off_t offset) {
  24     // Check for unsupported flags.
  25     if ((flags & (MAP_PRIVATE | MAP_SHARED)) == 0 ||
  26         (flags & MAP_FIXED) != 0 ||
  27 #ifdef MAP_SHARED_VALIDATE
  28         (flags & MAP_SHARED_VALIDATE) == MAP_SHARED_VALIDATE ||
  29 #endif
  30 #ifdef MAP_NORESERVE
  31         (flags & MAP_NORESERVE) != 0 ||
  32 #endif
  33 #ifdef MAP_GROWSDOWN
  34         (flags & MAP_GROWSDOWN) != 0 ||
  35 #endif
  36 #ifdef MAP_HUGETLB
  37         (flags & MAP_HUGETLB) != 0 ||
  38 #endif
  39 #ifdef MAP_FIXED_NOREPLACE
  40         (flags & MAP_FIXED_NOREPLACE) != 0 ||
  41 #endif
  42         0)
  43     {
  44         errno = EINVAL;
  45         return MAP_FAILED;
  46     }
  47
  48     // Check for unsupported protection requests.
  49     if (prot == PROT_NONE ||
  50 #ifdef PROT_EXEC
  51         (prot & PROT_EXEC) != 0 ||
  52 #endif
  53         0)
  54     {
  55         errno = EINVAL;
  56         return MAP_FAILED;
  57     }
  58
  59     //  To be consistent with POSIX.
  60     if (length == 0) {
  61         errno = EINVAL;
  62         return MAP_FAILED;
  63     }
  64
  65     // Check for integer overflow.
  66     size_t buf_len = 0;
  67     if (__builtin_add_overflow(length, sizeof(struct map), &buf_len)) {
  68         errno = ENOMEM;
  69         return MAP_FAILED;
  70     }
  71
  72     // Allocate the memory.
  73     struct map *map = malloc(buf_len);
  74     if (!map) {
  75         errno = ENOMEM;
  76         return MAP_FAILED;
  77     }
  78
  79     // Initialize the header.
  80     map->prot = prot;
  81     map->flags = flags;
  82     map->offset = offset;
  83     map->length = length;
  84
  85     // Initialize the main memory buffer, either with the contents of a file,
  86     // or with zeros.
  87     addr = map + 1;
  88     if ((flags & MAP_ANON) == 0) {
  89         char *body = (char *)addr;
  90         while (length > 0) {
  91             const ssize_t nread = pread(fd, body, length, offset);
  92             if (nread < 0) {
  93                 if (errno == EINTR)
  94                     continue;
  95                 return MAP_FAILED;
  96             }
  97             if (nread == 0)
  98                 break;
  99             length -= (size_t)nread;
 100             offset += (size_t)nread;
 101             body += (size_t)nread;
 102         }
 103     } else {
 104         memset(addr, 0, length);
 105     }
 106
 107     return addr;
 108 }
 109
 110 int munmap(void *addr, size_t length) {
 111     struct map *map = (struct map *)addr - 1;
 112
 113     // We don't support partial munmapping.
 114     if (map->length != length) {
 115         errno = EINVAL;
 116         return -1;
 117     }
 118
 119     // Release the memory.
 120     free(map);
 121
 122     // Success!
 123     return 0;
 124 }