[PATCH RFC v1 11/15] xdp: xdp_frame add member frame_sz and handle in convert_to_xdp_frame

Jesper Dangaard Brouer
2020-03-17T17:30:21Z

Use hole in struct xdp_frame, when adding member frame_sz, which keeps
same sizeof struct (32 bytes)

Drivers ixgbe and sfc had bug cases where the necessary/expected
tailroom was not reserved. This can lead to some hard to catch memory
corruption issues. Having the drivers frame_sz this can be detected when
packet length/end via xdp->data_end exceed the xdp_data_hard_end
pointer, which accounts for the reserved the tailroom.

When detecting this driver issue, simply fail the conversion with NULL,
which results in feedback to driver (failing xdp_do_redirect()) causing
driver to drop packet. Given the lack of consistent XDP stats, this can
be hard to troubleshoot. And given this is a driver bug, we want to
generate some more noise in form of a WARN stack dump (to ID the driver
code that inlined convert_to_xdp_frame).

Inlining the WARN macro is problematic, because it adds an asm
instruction (on Intel CPUs ud2) what influence instruction cache
prefetching. Thus, introduce xdp_warn and macro XDP_WARN, to avoid this
and at the same time make identifying the function and line of this
inlined function easier.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
include/net/xdp.h | 14 +++++++++++++-
net/core/xdp.c | 7 +++++++
2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/include/net/xdp.h b/include/net/xdp.h
index 99f4374f6214..55a885aa4e53 100644
--- a/include/net/xdp.h
+++ b/include/net/xdp.h
@@ -93,7 +93,8 @@ struct xdp_frame {
void *data;
u16 len;
u16 headroom;
- u16 metasize;
+ u32 metasize:8;
+ u32 frame_sz:24;
/* Lifetime of xdp_rxq_info is limited to NAPI/enqueue time,
* while mem info is valid on remote CPU.
*/
@@ -108,6 +109,10 @@ static inline void xdp_scrub_frame(struct xdp_frame *frame)
frame->dev_rx = NULL;
}

+/* Avoids inlining WARN macro in fast-path */
+void xdp_warn(const char* msg, const char* func, const int line);
+#define XDP_WARN(msg) xdp_warn(msg, __func__, __LINE__)
+
struct xdp_frame *xdp_convert_zc_to_xdp_frame(struct xdp_buff *xdp);

/* Convert xdp_buff to xdp_frame */
@@ -128,6 +133,12 @@ struct xdp_frame *convert_to_xdp_frame(struct xdp_buff *xdp)
if (unlikely((headroom - metasize) < sizeof(*xdp_frame)))
return NULL;

+ /* Catch if driver didn't reserve tailroom for skb_shared_info */
+ if (unlikely(xdp->data_end > xdp_data_hard_end(xdp))) {
+ XDP_WARN("Driver BUG: missing reserved tailroom");
+ return NULL;
+ }
+
/* Store info in top of packet */
xdp_frame = xdp->data_hard_start;

@@ -135,6 +146,7 @@ struct xdp_frame *convert_to_xdp_frame(struct xdp_buff *xdp)
xdp_frame->len = xdp->data_end - xdp->data;
xdp_frame->headroom = headroom - sizeof(*xdp_frame);
xdp_frame->metasize = metasize;
+ xdp_frame->frame_sz = xdp->frame_sz;

/* rxq only valid until napi_schedule ends, convert to xdp_mem_info */
xdp_frame->mem = xdp->rxq->mem;
diff --git a/net/core/xdp.c b/net/core/xdp.c
index 4c7ea85486af..4bc3026ae218 100644
--- a/net/core/xdp.c
+++ b/net/core/xdp.c
@@ -11,6 +11,7 @@
#include <linux/slab.h>
#include <linux/idr.h>
#include <linux/rhashtable.h>
+#include <linux/bug.h>
#include <net/page_pool.h>

#include <net/xdp.h>
@@ -496,3 +497,9 @@ struct xdp_frame *xdp_convert_zc_to_xdp_frame(struct xdp_buff *xdp)
return xdpf;
}
EXPORT_SYMBOL_GPL(xdp_convert_zc_to_xdp_frame);
+
+/* Used by XDP_WARN macro, to avoid inlining WARN() in fast-path */
+void xdp_warn(const char* msg, const char* func, const int line) {
+ WARN(1, "XDP_WARN: %s(line:%d): %s\n", func, line, msg);
+};
+EXPORT_SYMBOL_GPL(xdp_warn);