linux-sgx: Use non-destructive modes for opening files using SGX IPFS (#1645)

Refer to https://github.com/bytecodealliance/wasm-micro-runtime/pull/1645

core/shared/platform/linux-sgx/sgx_ipfs.c

@@ -260,22 +260,27 @@ ipfs_fopen(int fd, const char *filename, int flags)
     bool write_only = (flags & O_ACCMODE) == O_WRONLY;
     bool read_write = (flags & O_ACCMODE) == O_RDWR;
 
-    // The mapping of the mode are described in the table in the official
+    // The mapping of the mode is similar to the table in the official
     // specifications:
     // https://pubs.opengroup.org/onlinepubs/9699919799/functions/fopen.html
+    // Note that POSIX has obtained a file descriptor beforehand.
+    // If opened with a destructive mode ("w" or "w+"), the truncate operation
+    // already occurred and must not be repeated because this will invalidate
+    // the file descriptor obtained by POSIX. Therefore, we do NOT map to the
+    // modes that truncate the file ("w" and "w+"). Instead, we map to a
+    // non-destructive mode ("r+").
+
     if (read_only)
         mode = "r";
     else if (write_only && must_create && must_truncate)
-        mode = "w";
+        // Rather than "w", we map to a non-destructive mode
+        mode = "r+";
     else if (write_only && must_create && must_append)
         mode = "a";
-    else if (read_write && must_create && must_truncate)
-        mode = "w+";
     else if (read_write && must_create && must_append)
         mode = "a+";
-    else if (read_write && must_create)
-        mode = "w+";
     else if (read_write)
+        // Rather than "w+", we map to a non-destructive mode
         mode = "r+";
     else
         mode = NULL;

samples/file/wasm-app/main.c

@@ -26,6 +26,7 @@ main(int argc, char **argv)
     const char *text = FILE_TEXT;
     char buffer[1000];
     int ret;
+    long long stat_size;
 
     // Test: File opening (fopen)
     printf("Opening a file..\n");
@@ -114,18 +115,26 @@ main(int argc, char **argv)
     assert(ftell(file) == strlen(text) + 2 * ADDITIONAL_SPACE);
     printf("[Test] Extension of the file size passed.\n");
 
+    // Display some debug information
+    printf("Getting the size of the file on disk..\n");
+    struct stat st;
+    stat(PATH_TEST_FILE, &st);
+    stat_size = st.st_size;
+    assert(stat_size != 0);
+
+    // Compare with the size from fstat
+    fstat(fileno(file), &st);
+    printf("The file size is: %lld (stat), %lld (fstat).\n", stat_size,
+           st.st_size);
+    assert(stat_size != 0);
+    assert(stat_size == st.st_size);
+
     // Test: closing the file (fclose)
     printf("Closing from the file..\n");
     ret = fclose(file);
     assert(ret == 0);
     printf("[Test] Closing file passed.\n");
 
-    // Display some debug information
-    printf("Getting the size of the file on disk..\n");
-    struct stat st;
-    stat(PATH_TEST_FILE, &st);
-    printf("The file size is %lld.\n", st.st_size);
-
     printf("All the tests passed!\n");
 
     return 0;