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.

This is a backport of b65b205fbcabbb02463e31df17f5cabf7556f892.

	[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, 15 insertions, 13 deletions

diff --git a/libio/fmemopen.c b/libio/fmemopen.c
index 9264b724ee..0f655909e8 100644
--- a/libio/fmemopen.c
+++ b/libio/fmemopen.c
@@ -50,16 +50,14 @@ fmemopen_read (void *cookie, char *b, size_t s)
 
   if (c->pos + s > c->maxpos)
     {
-      if ((size_t) c->pos == c->maxpos)
-	return 0;
-      s = c->size - c->pos;
+      s = c->maxpos - c->pos;
+      if ((size_t) c->pos > c->maxpos)
+	s = 0;
     }
 
   memcpy (b, &(c->buffer[c->pos]), s);
 
   c->pos += s;
-  if ((size_t) c->pos > c->maxpos)
-    c->maxpos = c->pos;
 
   return s;
 }
@@ -70,28 +68,29 @@ fmemopen_write (void *cookie, const char *b, size_t s)
 {
   fmemopen_cookie_t *c = (fmemopen_cookie_t *) cookie;;
   _IO_off64_t pos = c->append ? c->maxpos : c->pos;
-  int addnullc;
-
-  addnullc = (s == 0 || b[s - 1] != '\0');
+  int addnullc = (s == 0 || b[s - 1] != '\0');
 
-  if (pos + s + addnullc > c->size)
+  if (pos + s > c->size)
     {
       if ((size_t) (c->pos + addnullc) >= c->size)
 	{
 	  __set_errno (ENOSPC);
 	  return 0;
 	}
-      s = c->size - pos - addnullc;
+      s = c->size - pos;
     }
 
   memcpy (&(c->buffer[pos]), b, s);
 
-  c->pos += s;
+  c->pos = pos + s;
   if ((size_t) c->pos > c->maxpos)
     {
       c->maxpos = c->pos;
-      if (addnullc)
+      if (c->maxpos < c->size && addnullc)
 	c->buffer[c->maxpos] = '\0';
+      /* A null byte is written in a stream open for update iff it fits.  */
+      else if (c->append == 0 && addnullc != 0)
+	c->buffer[c->size-1] = '\0';
     }
 
   return s;
@@ -123,7 +122,10 @@ fmemopen_seek (void *cookie, _IO_off64_t *p, int w)
     }
 
   if (np < 0 || (size_t) np > c->size)
-    return -1;
+    {
+      __set_errno (EINVAL);
+      return -1;
+    }
 
   *p = c->pos = np;