flex_array: fix get function for elements in base starting at non-zero

If all array elements fit into the base structure and data is copied using
flex_array_put() starting at a non-zero index, flex_array_get() will fail
to return the data.

This fixes the bug by only checking for NULL parts when all elements do
not fit in the base structure when flex_array_get() is used.  Otherwise,
fa_element_to_part_nr() will always be 0 since there are no parts
structures needed and such element may never have been put.  Thus, it will
remain NULL due to the kzalloc() of the base.

Additionally, flex_array_put() now only checks for a NULL part when all
elements do not fit in the base structure.  This is otherwise unnecessary
since the base structure is guaranteed to exist (or we would have already
hit a NULL pointer).
Signed-off-by: David Rientjes <rientjes@google.com>
Acked-by: Dave Hansen <dave@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/flex_array.c

@@ -198,10 +198,11 @@ int flex_array_put(struct flex_array *fa, int element_nr, void *src, gfp_t flags
 		return -ENOSPC;
 	if (elements_fit_in_base(fa))
 		part = (struct flex_array_part *)&fa->parts[0];
-	else
+	else {
 		part = __fa_get_part(fa, part_nr, flags);
 		if (!part)
 			return -ENOMEM;
+	}
 	dst = &part->elements[index_inside_part(fa, element_nr)];
 	memcpy(dst, src, fa->element_size);
 	return 0;
@@ -257,11 +258,12 @@ void *flex_array_get(struct flex_array *fa, int element_nr)
 
 	if (element_nr >= fa->total_nr_elements)
 		return NULL;
-	if (!fa->parts[part_nr])
-		return NULL;
 	if (elements_fit_in_base(fa))
 		part = (struct flex_array_part *)&fa->parts[0];
-	else
+	else {
 		part = fa->parts[part_nr];
+		if (!part)
+			return NULL;
+	}
 	return &part->elements[index_inside_part(fa, element_nr)];
 }