add support for the msync syscall

README.md

@@ -21,6 +21,9 @@ additional extensions:
 - pipe and event support (`pipe`, `event` )
 - DNS resolution support (`resolve` )
 
+# NOTES
+- Memory mapping is currently not compatible with threads
+
 # WASI(X) Extensions Spec
 
 [WASIX](https://wasix.org) is maintained by wasix.org [here](https://github.com/wasix-org/wasix-witx)

expected/wasm32-wasi-threads/defined-symbols.txt

@@ -1155,6 +1155,7 @@ modf
 modff
 modfl
 mrand48
+msync
 mtx_destroy
 mtx_init
 mtx_lock

expected/wasm32-wasi/defined-symbols.txt

@@ -1189,6 +1189,7 @@ modf
 modff
 modfl
 mrand48
+msync
 mtx_destroy
 mtx_init
 mtx_lock

expected/wasm64-wasi/defined-symbols.txt

@@ -1189,6 +1189,7 @@ modf
 modff
 modfl
 mrand48
+msync
 mtx_destroy
 mtx_init
 mtx_lock

libc-bottom-half/mman/mman.c

@@ -21,6 +21,7 @@ struct map {
     int flags;
     off_t offset;
     size_t length;
+    int fd;
 };
 
 void *mmap(void *addr, size_t length, int prot, int flags,
@@ -90,6 +91,17 @@ void *mmap(void *addr, size_t length, int prot, int flags,
     // or with zeros.
     addr = map + 1;
     if ((flags & MAP_ANON) == 0) {
+        int new_fd = dup(fd);
+
+        if (new_fd < 0) {
+            errno = EINVAL;
+            free(map);
+
+            return NULL;
+        }
+
+        map->fd = new_fd;
+
         char *body = (char *)addr;
         while (length > 0) {
             const ssize_t nread = pread(fd, body, length, offset);
@@ -105,6 +117,7 @@ void *mmap(void *addr, size_t length, int prot, int flags,
             body += (size_t)nread;
         }
     } else {
+        map->fd = -1;
         memset(addr, 0, length);
     }
 
@@ -123,6 +136,54 @@ int munmap(void *addr, size_t length) {
     // Release the memory.
     free(map);
 
+    // Write the data back to the backing file and close
+    // the file handle
+    if (map->fd > 0) {
+        if ((map->prot & PROT_WRITE) != 0) {
+            msync(addr, length, MS_SYNC);
+        }
+
+        close(map->fd);
+    }
+
     // Success!
     return 0;
 }
+
+int msync (void *addr, size_t length, int flags) {
+    struct map *map = (struct map *)addr - 1;
+    size_t map_flags = map->flags;
+    off_t map_offset = map->offset;
+    size_t map_length = map->length;
+    int fd = map->fd;
+
+    if (length > map_length) {
+        errno = EINVAL;
+        return -1;
+    }
+
+    if ((map->prot & PROT_WRITE) != 0) {
+        errno = EINVAL;
+        return -1;
+    }
+
+    if ((map_flags & MAP_ANON) == 0) {
+        char *body = (char *)addr;
+
+        while (length > 0) {
+            const ssize_t nwrite = pwrite(fd, body, length, map_offset);
+
+            if (nwrite > 0) {
+                length -= (size_t)nwrite;
+                map_offset += (size_t)nwrite;
+                body += (size_t)nwrite;
+            } else if (errno == EINTR) {
+                continue;
+            } else {
+                return -1;
+            }
+        }
+    }
+
+    return 0;
+}