Add a new fwrite test that exercises buffer overflow

Exercises fwrite's internal buffer when doing a file operation.
The new test, exercises 2 overflow behaviors:

1. Call fwrite multiple times making usage of fwrite's internal buffer.
   The total number of bytes written is larger than fwrite's internal
   buffer, forcing an automatic flush.

2. Call fwrite a single time with an amount of data that is larger than
   fwrite's internal buffer.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

stdio-common/Makefile

@@ -231,6 +231,7 @@ tests := \
   tst-fseek \
   tst-fwrite \
   tst-fwrite-memstrm \
+  tst-fwrite-overflow \
   tst-fwrite-ro \
   tst-getline \
   tst-getline-enomem \

stdio-common/tst-fwrite-overflow.c

@@ -0,0 +1,130 @@
+/* Test the overflow of fwrite's internal buffer.
+   Copyright (C) 2024 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+/* stdio.h provides BUFSIZ, which is the size of fwrite's internal buffer.  */
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <support/check.h>
+#include <support/temp_file.h>
+#include <support/support.h>
+#include <support/xstdio.h>
+#include <support/xunistd.h>
+
+/* Length of the buffers in bytes.  */
+#define RWBUF_SIZE (2 * BUFSIZ)
+
+void
+test_one_rw (const char *in, size_t size, size_t nmemb, size_t blocks)
+{
+  int fd;
+  FILE *f;
+  char *out;
+  size_t written, to_write;
+  const size_t requested = size * nmemb;
+
+  printf ("Testing with size = %zd, nmemb = %zd, blocks = %zd\n",
+	  size, nmemb, blocks);
+
+  TEST_VERIFY_EXIT (requested <= RWBUF_SIZE);
+  /* Ensure fwrite's internal buffer will overflow.  */
+  TEST_VERIFY_EXIT (requested > BUFSIZ);
+
+  /* Create a temporary file and open it for reading and writing.  */
+  fd = create_temp_file ("tst-fwrite-overflow", NULL);
+  TEST_VERIFY_EXIT (fd != -1);
+  f = fdopen (fd, "w+");
+  TEST_VERIFY_EXIT (f != NULL);
+
+  /* Call fwrite() as many times as needed, until all data is written,
+     limiting the amount of data written per call to block items.  */
+  for (written = 0; written < nmemb; written += to_write)
+    {
+      if (written + blocks <= nmemb)
+	to_write = blocks;
+      else
+	to_write = nmemb - written;
+      /* Check if fwrite() returns the expected value.  No errors are
+	 expected.  */
+      TEST_COMPARE (fwrite (in + size * written, size, to_write, f),
+		    to_write);
+      TEST_COMPARE (ferror (f), 0);
+    }
+  TEST_VERIFY_EXIT (written == nmemb);
+
+  /* Ensure all the data is flushed to file.  */
+  TEST_COMPARE (fflush (f), 0);
+
+  /* We have to check if the contents in the file are correct.  Go back to
+     the beginning of the file.  */
+  rewind (f);
+  /* Try to allocate a buffer and save the contents of the generated file to
+     it.  */
+  out = xmalloc (RWBUF_SIZE);
+  TEST_COMPARE (fread (out, size, nmemb, f), nmemb);
+
+  /* Ensure the output has the expected contents.  */
+  TEST_COMPARE (memcmp (out, in, requested), 0);
+
+  xfclose (f);
+  free (out);
+}
+
+static int
+do_test (void)
+{
+  char * in;
+  int i, j;
+  size_t nmemb[] = {BUFSIZ + 1, RWBUF_SIZE, 0};
+  /* Maximum number of items written for each fwrite call.  */
+  size_t block[] = {100, 1024, 2047, 0};
+  /* The largest block must fit entirely in fwrite's buffer.  */
+  _Static_assert (2047 < BUFSIZ,
+		  "a block must fit in fwrite's internal buffer");
+
+  in = xmalloc (RWBUF_SIZE);
+  for (i = 0; i < RWBUF_SIZE; i++)
+    in[i] = i % 0xff;
+
+  for (i = 0; nmemb[i] != 0; i++)
+    for (j = 0; block[j] != 0; j++)
+      {
+	/* Run a test with an array of nmemb bytes.  Write at most block
+	   items per fwrite call.  */
+	test_one_rw (in, 1, nmemb[i], block[j]);
+	/* Run a test that overflows fwrite's internal buffer in a single call
+	   by writting a single item of nmemb bytes.
+	   This call should not use the buffer and should be written directly
+	   to the file.  */
+	test_one_rw (in, nmemb[i], 1, nmemb[i]);
+      }
+
+  for (j = 0; block[j] != 0; j++)
+    {
+      /* Run a test with size=2 and the minimum nmemb value that still
+	 overflows the buffer.  Write at most block items per fwrite call.  */
+      test_one_rw (in, 2, BUFSIZ / 2 + 1, block[j]);
+      /* Likewise, but size=3.  */
+      test_one_rw (in, 3, BUFSIZ / 3 + 1, block[j]);
+    }
+
+  free (in);
+  return 0;
+}
+
+#include <support/test-driver.c>