bpf, events: fix offset in skb copy handler

This patch fixes the __output_custom() routine we currently use with
bpf_skb_copy(). I missed that when len is larger than the size of the
current handle, we can issue multiple invocations of copy_func, and
__output_custom() advances destination but also source buffer by the
written amount of bytes. When we have __output_custom(), this is actually
wrong since in that case the source buffer points to a non-linear object,
in our case an skb, which the copy_func helper is supposed to walk.
Therefore, since this is non-linear we thus need to pass the offset into
the helper, so that copy_func can use it for extracting the data from
the source object.

Therefore, adjust the callback signatures properly and pass offset
into the skb_header_pointer() invoked from bpf_skb_copy() callback. The
__DEFINE_OUTPUT_COPY_BODY() is adjusted to accommodate for two things:
i) to pass in whether we should advance source buffer or not; this is
a compile-time constant condition, ii) to pass in the offset for
__output_custom(), which we do with help of __VA_ARGS__, so everything
can stay inlined as is currently. Both changes allow for adapting the
__output_* fast-path helpers w/o extra overhead.

Fixes: 555c8a8623a3 ("bpf: avoid stack copy and use skb ctx for event output")
Fixes: 7e3f977edd0b ("perf, events: add non-linear data support for raw records")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 2 deletions

diff --git a/net/core/filter.c b/net/core/filter.c
index 0b521353008d..5708999f8a79 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -2026,9 +2026,9 @@ bool bpf_helper_changes_skb_data(void *func)
 }
 
 static unsigned long bpf_skb_copy(void *dst_buff, const void *skb,
-				  unsigned long len)
+				  unsigned long off, unsigned long len)
 {
-	void *ptr = skb_header_pointer(skb, 0, len, dst_buff);
+	void *ptr = skb_header_pointer(skb, off, len, dst_buff);
 
 	if (unlikely(!ptr))
 		return len;