On 11/24/23 09:28, Gustavo A. R. Silva wrote:
On 11/24/23 04:24, Joey Gouly wrote:
Hi all,
I just hit a boot crash on v6.7-rc2 (arm64, FVP model):
[..]
Checking `struct neighbour`:
struct neighbour {
struct neighbour __rcu *next;
struct neigh_table *tbl;
.. fields ..
u8 primary_key[0];
} __randomize_layout;
Due to the `__randomize_layout`, `primary_key` field is being placed before `tbl` (actually it's the same address since it's a 0 length array). That means the
memcpy() corrupts the tbl pointer.
I think I just got unlucky with my CONFIG_RANDSTRUCT seed (I can provide it if
needed), it doesn't look as if it's a new issue.
It seems the issue is caused by this change that was recently added to -rc2:
commit 1ee60356c2dc ("gcc-plugins: randstruct: Only warn about true flexible
arrays")
Previously, one-element and zero-length arrays were treated as true flexible
arrays
(however, they are "fake" flex arrays), and __randomize_layout would leave them
untouched at the end of the struct; the same for proper C99 flex-array members.
But
after the commit above, that's no longer the case: Only C99 flex-array members
will
behave correctly (remaining untouched at end of the struct), and the other two
types
of arrays will be randomized.
Kees,
I think we should complement the changes in commit 1ee60356c2dc with the
following
update:
diff --git a/scripts/gcc-plugins/randomize_layout_plugin.c
b/scripts/gcc-plugins/randomize_layout_plugin.c
index 910bd21d08f4..746ff2d272f2 100644
--- a/scripts/gcc-plugins/randomize_layout_plugin.c
+++ b/scripts/gcc-plugins/randomize_layout_plugin.c
@@ -339,8 +339,7 @@ static int relayout_struct(tree type)
/*
* enforce that we don't randomize the layout of the last
- * element of a struct if it's a 0 or 1-length array
- * or a proper flexible array
+ * element of a struct if it's a proper flexible array
*/
if (is_flexible_array(newtree[num_fields - 1])) {
has_flexarray = true;
--
Gustavo
I couldn't reproduce directly on v6.6 (the offsets for `tbl` and `primary_key`
didn't overlap).
However I tried changing the zero-length-array to a flexible one:
+ DECLARE_FLEX_ARRAY(u8, primary_key);
+ u8 primary_key[0];
Then the field offsets ended up overlapping, and I also got the same crash on
v6.6.
The right approach is to transform the zero-length array into a C99 flex-array
member,
like this:
diff --git a/include/net/neighbour.h b/include/net/neighbour.h
index 07022bb0d44d..0d28172193fa 100644
--- a/include/net/neighbour.h
+++ b/include/net/neighbour.h
@@ -162,7 +162,7 @@ struct neighbour {
struct rcu_head rcu;
struct net_device *dev;
netdevice_tracker dev_tracker;
- u8 primary_key[0];
+ u8 primary_key[];
} __randomize_layout;
struct neigh_ops {
--
Gustavo
