When a struct containing a flexible array is included in another struct,
and there is a member after the struct-with-flex-array, there is a
possibility of memory overlap. These cases must be audited [1]. See:

struct inner {
	...
	int flex[];
};

struct outer {
	...
	struct inner header;
	int overlap;
	...
};

This is the scenario for all the "struct *_filter" structures that are
included in the following "struct ib_flow_spec_*" structures:

struct ib_flow_spec_eth
struct ib_flow_spec_ib
struct ib_flow_spec_ipv4
struct ib_flow_spec_ipv6
struct ib_flow_spec_tcp_udp
struct ib_flow_spec_tunnel
struct ib_flow_spec_esp
struct ib_flow_spec_gre
struct ib_flow_spec_mpls

The pattern is like the one shown below:

struct *_filter {
	...
	u8 real_sz[];
};

struct ib_flow_spec_* {
	...
	struct *_filter val;
	struct *_filter mask;
};

In this case, the trailing flexible array "real_sz" is never allocated
and is only used to calculate the size of the structures. Here the use
of the "offsetof" helper can be changed by the "sizeof" operator because
the goal is to get the size of these structures. Therefore, the trailing
flexible arrays can also be removed.

However, due to the trailing padding that can be induced in structs it
is possible that the:

offsetof(struct *_filter, real_sz) != sizeof(struct *_filter)

This situation happens with the "struct ib_flow_ipv6_filter" and to
avoid it the "__packed" macro is used in this structure. But now, the
"sizeof(struct ib_flow_ipv6_filter)" has changed. This is not a problem
since this size is not used in the code.

The situation now is that "sizeof(struct ib_flow_spec_ipv6)" has also
changed (this struct contains the struct ib_flow_ipv6_filter). This is
also not a problem since it is only used to set the size of the "union
ib_flow_spec", which can store all the "ib_flow_spec_*" structures.

Link: https://lore.kernel.org/r/20240217142913.4285-1-erick.archer@gmx.comSigned-off-by: Erick Archer <erick.archer@gmx.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>