In your particular example, since you intend to copy xdp_md->data, you
can directly
access that from xdp_md->data pointer, there is no need to copy ctx
which is not
what you want.
Thanks for your answer, but I think you misunderstood me. I need to
store the packet's payload in a map (not the xdp_md structure itself),
because my use case forces me to do so.
I write a program that reassembles split packets into a single one.
Therefore I have to buffer packet fragments until all have been arrived.
The only way in eBPF to realize such a buffer is a map, so I have to put
the packet's payload in there. My problem is, that I have no clue how to
do it properly as there is no direct way to put the payload into a map.
How would you put a packet with a size of 700 bytes into a map? What
would be your strategy when you can only access your packet via the
xdp_md structure? My strategy (and that's the best I have found so far)
is to split this packet into two packets of size 350 bytes, so that I
can process them on the stack consecutively.
On 4/16/24 5:22 AM, Yonghong Song wrote:
On 4/15/24 1:25 PM, Fabian Pfitzner wrote:
Looks like you intend to copy packet data. So from the above,
'expected=fp,pkt,pkt_meta...', you can just put the first argument
with xdp->data, right?
Yes, I intend to copy packet data. What do you mean by "first
argument"? I'd like to put the whole data that is depicted by
xdp->data into a map that stores them as raw bytes (by using a char
array as map element to store the data).
Sorry, typo. 'first argument' should be 'third argument'.
Verifer rejects to 'ctx' since 'ctx' contents are subject to
verifier rewrite. So actual 'ctx' contents/layouts may not match
uapi definition.
Sorry but I do not understand what you mean by "subject to verifier
rewrite". What kind of rewrite happens when using the ctx as
argument? Furthermore, am I correct that you assume that the uapi may
dictate the structure of the data that can be stored in a map? How is
it different to the case when first storing it on the stack and then
putting it into a map?
The UAPI xdp_md struct:
struct xdp_md {
__u32 data;
__u32 data_end;
__u32 data_meta;
/* Below access go through struct xdp_rxq_info */
__u32 ingress_ifindex; /* rxq->dev->ifindex */
__u32 rx_queue_index; /* rxq->queue_index */
__u32 egress_ifindex; /* txq->dev->ifindex */
};
The actual kernel representation of xdp_md:
struct xdp_buff {
void *data;
void *data_end;
void *data_meta;
void *data_hard_start;
struct xdp_rxq_info *rxq;
struct xdp_txq_info *txq;
u32 frame_sz; /* frame size to deduce data_hard_end/reserved
tailroom*/
u32 flags; /* supported values defined in xdp_buff_flags */
};
You can see they are quite different. So to use pointee of 'ctx' as
the key, we
need to allocate a space of sizeof(struct_md) to the stack and copy
necessary
stuff to that structure. For example, xdp_md->ingress_ifindex =
xdp_buff->rxq->dev->ifindex, etc.
Some fields actually does not make sense for copying, e.g.,
data/data_end/data_meta in 64bit
architecture. Since stack allocation is needed any way, so disabling
ctx and requires
user explicit using stack make sense (if they want to use *ctx as map
update value).
In your particular example, since you intend to copy xdp_md->data, you
can directly
access that from xdp_md->data pointer, there is no need to copy ctx
which is not
what you want.
On 4/15/24 6:01 PM, Yonghong Song wrote:
On 4/14/24 2:34 PM, Fabian Pfitzner wrote:
Hello,
is there a specific reason why it is not allowed to copy data from
ctx directly into a map via the bpf_map_update_elem helper?
I develop a XDP program where I need to store incoming packets
(including the whole payload) into a map in order to buffer them.
I thought I could simply put them into a map via the mentioned
helper function, but the verifier complains about expecting another
type as "ctx" (R3 type=ctx expected=fp, pkt, pkt_meta, .....).
Looks like you intend to copy packet data. So from the above,
'expected=fp,pkt,pkt_meta...', you can just put the first argument
with xdp->data, right?
Verifer rejects to 'ctx' since 'ctx' contents are subject to
verifier rewrite. So actual 'ctx' contents/layouts may not match
uapi definition.
I was able to circumvent this error by first putting the packet
onto the stack (via xdp->data) and then write it into the map.
The only limitation with this is that I cannot store packets larger
than 512 bytes due to the maximum stack size.
I was also able to circumvent this by slicing chunks, that are
smaller than 512 bytes, out of the packet so that I can use the
stack as a clipboard before putting them into the map. This is a
really ugly solution, but I have not found a better one yet.
So my question is: Why does this limitation exist? I am not sure if
its only related to XDP programs as this restriction is defined
inside of the bpf_map_update_elem_proto struct (arg3_type restricts
this), so I think it is a general limitation that affects all
program types.
Best regards,
Fabian Pfitzner
