libio: Fix fmemopen append mode failure (BZ# 20012)

The fmemopen implementation does not account the file position correctly in
append mode. The following example shows the failure:

===
int main ()
{
  char buf[10] = "test";
  FILE *fp = fmemopen (buf, 10, "a+");
  fseek (fp, 0, SEEK_SET);

  int gr;
  if ((gr = getc (fp)) != 't' ||
      (gr = getc (fp)) != 'e' ||
      (gr = getc (fp)) != 's' ||
      (gr = getc (fp)) != 't' ||
      (gr = getc (fp)) != EOF)
    {
      printf ("%s: getc failed returned %i\n", __FUNCTION__, gr);
      return 1;
    }

  return 0;
}
===

This is due both how read and write operation update the buffer position,
taking in consideration buffer lenght instead of maximum position defined
by the open mode.  This patch fixes it and also fixes fseek not returning
EINVAL for invalid whence modes.

Tested on x86_64 and i686.

	[BZ #20012]
	* libio/fmemopen.c (fmemopen_read): Use buffer maximum position, not
	length to calculate the buffer to read.
	(fmemopen_write): Set the buffer position based on bytes written.
	(fmemopen_seek): Return EINVAL for invalid whence modes.

1 files changed, 117 insertions, 1 deletions

diff --git a/stdio-common/tst-fmemopen3.c b/stdio-common/tst-fmemopen3.c
index 250f9ecc75..054b0697b3 100644
--- a/stdio-common/tst-fmemopen3.c
+++ b/stdio-common/tst-fmemopen3.c
@@ -25,8 +25,13 @@ static void
 print_buffer (const char *s, size_t n)
 {
   size_t i;
+  printf ("{");
   for (i=0; i<n; ++i)
-    printf ("0x%02X (%c), ", s[i], s[i]);
+    {
+      printf ("0x%02X (%c)", s[i], s[i]);
+      if (i != n)
+	printf (", ");
+    }
 }
 
 /* This test check append mode initial position (a/a+) based on POSIX defition
@@ -186,6 +191,112 @@ do_test_read_seek_negative (void)
 }
 
 static int
+do_test_write_append_2 (const char *str)
+{
+  char buf[10];
+  size_t strn = strlen (str);
+  strcpy (buf, str);
+
+  FILE *fp = fmemopen (buf, sizeof (buf), "a+");
+  size_t r = ftell (fp);
+  size_t e = strlen (buf);
+  if (r != e)
+    {
+      printf ("%s: ftell returned %zu, expected %zu\n", __FUNCTION__, r, e);
+      return 1;
+    }
+
+  if (fseek (fp, 0, SEEK_SET) == -1)
+    {
+      printf ("%s: fseek returned -1\n", __FUNCTION__);
+      return 1;
+    }
+
+  int gr;
+  for (int i=0; i<strn; ++i)
+    {
+      if ((gr = getc (fp)) != str[i])
+	{
+	  printf ("%s: getc failed returned %d, expected %d\n", __FUNCTION__,
+		  gr, str[i]);
+	  return 1;
+        }
+    }
+  if ((gr = getc (fp)) != EOF)
+    {
+      printf ("%s: getc failed returned %d, expected EOF\n", __FUNCTION__,
+	      gr);
+      return 1;
+    }
+
+  if (fseek (fp, e+1, SEEK_SET) == -1)
+    {
+      printf ("%s: fseek returned -1\n", __FUNCTION__);
+      return 1;
+    }
+
+  if ((r = ftell (fp)) != e+1)
+    {
+      printf ("%s: ftell returned %zu, expected %zu\n", __FUNCTION__, r, e+1);
+      return 1;
+    }
+
+  if ((gr = getc (fp)) != EOF)
+    {
+      printf ("%s: getc failed returned %i\n", __FUNCTION__, gr);
+      return 1;
+    }
+
+  /* Check if internal position is not changed with a getc returning EOF.  */
+  if ((r = ftell (fp)) != e+1)
+    {
+      printf ("%s: ftell returned %zu, expected %zu\n", __FUNCTION__, r, e+1);
+      return 1;
+    }
+
+  if (fseek (fp, 0, SEEK_CUR) == -1)
+    {
+      printf ("%s: fseek returned -1\n", __FUNCTION__);
+      return 1;
+    }
+
+  /* This should be overwritten by fprintf + fflush.  */
+  buf[e+2] = 'X';
+
+  if ((r = fprintf (fp, "%d", 101)) != 3)
+    {
+      printf ("%s: fprintf returned %zu, expected %d\n", __FUNCTION__, r, 3);
+      return 1;
+    }
+
+  fflush (fp);
+
+  /* Check if internal position is changed by 3 (strlen of '101').  */
+  if ((r = ftell (fp)) != e+3)
+    {
+      printf ("%s: ftell returned %zu, expected %zu\n", __FUNCTION__, r, e+3);
+      return 1;
+    }
+
+  char exp[20];
+  sprintf (exp, "%s%d", str,  101);
+  if (memcmp (buf, exp, strlen (exp)) != 0)
+    {
+      printf ("%s: check failed:", __FUNCTION__);
+      printf ("\nexpected: ");
+      print_buffer (buf, sizeof (buf));
+      printf ("\nbuffer:   ");
+      print_buffer (exp, sizeof (exp));
+      printf ("\n");
+      return 1;
+    }
+
+  fclose(fp);
+
+  return 0;
+}
+
+static int
 do_test (void)
 {
   int ret = 0;
@@ -199,6 +310,11 @@ do_test (void)
 
   ret += do_test_read_seek_negative ();
 
+  /* First test plus addend will fit in the define buffer of size 10.  */
+  ret += do_test_write_append_2 ("test");
+  /* The second test will also fit, but not the final '\0'.  */
+  ret += do_test_write_append_2 ("testing");
+
   return ret;
 }