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xsk: Require XDP_UMEM_TX_METADATA_LEN to actuate tx_metadata_len

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Julian reports that commit 341ac980ea ("xsk: Support tx_metadata_len")
can break existing use cases which don't zero-initialize xdp_umem_reg
padding. Introduce new XDP_UMEM_TX_METADATA_LEN to make sure we
interpret the padding as tx_metadata_len only when being explicitly
asked.

Fixes: 341ac980ea ("xsk: Support tx_metadata_len")
Reported-by: Julian Schindel <mail@arctic-alpaca.de>
Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/bpf/20240713015253.121248-2-sdf@fomichev.me
This commit is contained in:
Stanislav Fomichev 2024-07-12 18:52:51 -07:00 committed by Daniel Borkmann
parent fa5ef65561
commit d5e726d914
3 changed files with 20 additions and 9 deletions
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16
Documentation/networking/xsk-tx-metadata.rst
View file

@ -11,12 +11,16 @@ metadata on the receive side.
General Design
==============
The headroom for the metadata is reserved via ``tx_metadata_len`` in
``struct xdp_umem_reg``. The metadata length is therefore the same for
every socket that shares the same umem. The metadata layout is a fixed UAPI,
refer to ``union xsk_tx_metadata`` in ``include/uapi/linux/if_xdp.h``.
Thus, generally, the ``tx_metadata_len`` field above should contain
``sizeof(union xsk_tx_metadata)``.
The headroom for the metadata is reserved via ``tx_metadata_len`` and
``XDP_UMEM_TX_METADATA_LEN`` flag in ``struct xdp_umem_reg``. The metadata
length is therefore the same for every socket that shares the same umem.
The metadata layout is a fixed UAPI, refer to ``union xsk_tx_metadata`` in
``include/uapi/linux/if_xdp.h``. Thus, generally, the ``tx_metadata_len``
field above should contain ``sizeof(union xsk_tx_metadata)``.
Note that in the original implementation the ``XDP_UMEM_TX_METADATA_LEN``
flag was not required. Applications might attempt to create a umem
with a flag first and if it fails, do another attempt without a flag.
The headroom and the metadata itself should be located right before
``xdp_desc->addr`` in the umem frame. Within a frame, the metadata

4
include/uapi/linux/if_xdp.h
View file

@ -41,6 +41,10 @@
*/
#define XDP_UMEM_TX_SW_CSUM (1 << 1)
/* Request to reserve tx_metadata_len bytes of per-chunk metadata.
*/
#define XDP_UMEM_TX_METADATA_LEN (1 << 2)
struct sockaddr_xdp {
__u16 sxdp_family;
__u16 sxdp_flags;

9
net/xdp/xdp_umem.c
View file

@ -151,6 +151,7 @@ static int xdp_umem_account_pages(struct xdp_umem *umem)
#define XDP_UMEM_FLAGS_VALID ( \
XDP_UMEM_UNALIGNED_CHUNK_FLAG | \
XDP_UMEM_TX_SW_CSUM | \
XDP_UMEM_TX_METADATA_LEN | \
0)
static int xdp_umem_reg(struct xdp_umem *umem, struct xdp_umem_reg *mr)
@ -204,8 +205,11 @@ static int xdp_umem_reg(struct xdp_umem *umem, struct xdp_umem_reg *mr)
if (headroom >= chunk_size - XDP_PACKET_HEADROOM)
return -EINVAL;
if (mr->tx_metadata_len >= 256 || mr->tx_metadata_len % 8)
return -EINVAL;
if (mr->flags & XDP_UMEM_TX_METADATA_LEN) {
if (mr->tx_metadata_len >= 256 || mr->tx_metadata_len % 8)
return -EINVAL;
umem->tx_metadata_len = mr->tx_metadata_len;
}
umem->size = size;
umem->headroom = headroom;
@ -215,7 +219,6 @@ static int xdp_umem_reg(struct xdp_umem *umem, struct xdp_umem_reg *mr)
umem->pgs = NULL;
umem->user = NULL;
umem->flags = mr->flags;
umem->tx_metadata_len = mr->tx_metadata_len;
INIT_LIST_HEAD(&umem->xsk_dma_list);
refcount_set(&umem->users, 1);