bpf: allow access into map value arrays

Suppose you have a map array value that is something like this

struct foo {
	unsigned iter;
	int array[SOME_CONSTANT];
};

You can easily insert this into an array, but you cannot modify the contents of
foo->array[] after the fact.  This is because we have no way to verify we won't
go off the end of the array at verification time.  This patch provides a start
for this work.  We accomplish this by keeping track of a minimum and maximum
value a register could be while we're checking the code.  Then at the time we
try to do an access into a MAP_VALUE we verify that the maximum offset into that
region is a valid access into that memory region.  So in practice, code such as
this

unsigned index = 0;

if (foo->iter >= SOME_CONSTANT)
	foo->iter = index;
else
	index = foo->iter++;
foo->array[index] = bar;

would be allowed, as we can verify that index will always be between 0 and
SOME_CONSTANT-1.  If you wish to use signed values you'll have to have an extra
check to make sure the index isn't less than 0, or do something like index %=
SOME_CONSTANT.

Signed-off-by: Josef Bacik <jbacik@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 7 insertions, 0 deletions

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 5691fdc83819..c201017b5730 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -139,6 +139,13 @@ enum bpf_reg_type {
 	 */
 	PTR_TO_PACKET,
 	PTR_TO_PACKET_END,	 /* skb->data + headlen */
+
+	/* PTR_TO_MAP_VALUE_ADJ is used for doing pointer math inside of a map
+	 * elem value.  We only allow this if we can statically verify that
+	 * access from this register are going to fall within the size of the
+	 * map element.
+	 */
+	PTR_TO_MAP_VALUE_ADJ,
 };
 
 struct bpf_prog;