Re: modifying packets in XDP

Yonghong Song

On Thu, Aug 24, 2017 at 12:17 PM, Ilya Baldin <ibaldin@...> wrote:
Thank you, everyone for the replies. This explains about bpf_probe_read.

I tried the approach below (changed the check to offset +1) however that
didn’t help, I still get the same error and BCC refuses to load the program

R1=inv,min_value=6,max_value=65541,min_align=1,aux_off_align=2 R2=pkt_end
R6=inv,min_value=4,max_value=65539,min_align=1,aux_off_align=2 R10=fp
65: (69) r1 = *(u16 *)(r6 +0)
R6 invalid mem access 'inv'
Note that:
"inv" means unknown value.

Could you double check your program?

HINT: The invalid mem access 'inv' error can happen if you try to
dereference memory without first using bpf_probe_read() to copy it to the
BPF stack. Sometimes the bpf_probe_read is automatic by the bcc rewriter,
other times you'll need to be explicit.

and it is the ‘write’ statements that are causing it as in when I try to
write into the packet

start[off1] = val;

is when I get the error.

Is the solution to use the tc loader instead of bcc? Less convenient, but
doable, I suppose.


Ilya Baldin

On Aug 24, 2017, at 12:43 PM, Brenden Blanco <bblanco@...> wrote:

On Thu, Aug 24, 2017 at 9:26 AM, Daniel Borkmann via iovisor-dev
<iovisor-dev@...> wrote:

On 08/24/2017 06:17 PM, Y Song wrote:

CC to bcc mailing list as well.

bpf_probe_read is not allowed in XDP programs.

Your comparison may need to comparison not just starting offset, but
also including the memory size so the
whole write won't fall beyond the "data_end".

Regarding to bcc translates "start[off2]" to bpf_probe_read, it is
possible that bcc rewriter tries to infer bpf_probe candidate and
finds this one ...

I was wondering about this last one, and where this suggestion
comes from exactly (bcc for sure?). How does the error message
look like?

The error comes from here:

But as Yonghong said, you cannot use bpf_probe_read from XDP and hence
should ignore the warning.

I would suggest rewriting your check to be something like:

if (((void*)&start[off1 + 1] > data_end) || ((void*)&start[off2 + 1]


return 0;

On Thu, Aug 24, 2017 at 8:22 AM, Ilya Baldin <ibaldin@...> wrote:

Hello everyone,

A couple of questions. I’m using XDP with BCC on a Fedora 25 with kernel
4.13.0-0.rc5.git0.2.fc27.x86_64 with e1000 driver. Basic XDP examples appear
to work, I was able to create my own simple example counting TCP SYN
packets, so the setup is at least partially correct.

I’m playing around now with modifying TCP payload on the fly and failing
miserably. The function that is supposed to swap two u16s in TCP payload is

static inline int swapu16(uint16_t *start, void *data_end, int off1, int
off2) {
int len = (uint16_t*)data_end - start;
uint16_t poff1, poff2;

if (((void*)&start[off1] > data_end) || ((void*)&start[off2] >
return 0;

poff1 = start[off1];
poff2 = start[off2];

//start[off2] = poff1;
//start[off1] = poff2;

return 1;

and it gets called with start pointing to the beginning of the payload
(I’m reasonably sure that is correct).

If I *uncomment* either of the two lines in the function I get verifier
errors suggesting I use bpf_probe_read. First question

1. Is what I’m attempting even possible - am I allowed to modify the
packet in XDP hook? If no this may be a short conversation.

2. If it is possible, is there a canonical way of doing it compared to
what I’m trying to do?

3. I actually attempted to use bpf_probe_read() (even though with the
code structured as above, it appears they are inserted automatically,
because the BPF program is installed properly with those two lines commented
out), like in the code shown below:

static inline int swapu16(uint16_t *start, void *data_end, int off1, int
off2) {
int len = (uint16_t*)data_end - start;
uint16_t poff1, poff2;

if (((void*)&start[off1] > data_end) || ((void*)&start[off2] >
return 0;

bpf_probe_read(&poff1, sizeof(uint16_t), (void*)&start[off1]);
//poff1 = start[off1];
//poff2 = start[off2];

//start[off2] = poff1;
//start[off1] = poff2;

return 1;

I get a verifier error
60: (2d) if r1 > r2 goto pc+5
R2=pkt_end R3=inv,min_value=2,max_value=65537,min_align=1,aux_off_align=2
R5=pkt(id=0,off=42,r=42) R6=pkt(id=1,off=0,r=20),aux_off_align=2
R7=imm0,min_value=0,max_value=0,min_align=2147483648 R10=fp
61: (bf) r1 = r10
62: (07) r1 += -32
63: (b7) r2 = 2
64: (85) call bpf_probe_read#4
unknown func bpf_probe_read#4

which is really strange. In fact it appears I’m unable to invoke any of
the helper bpf functions explicitly.

Many thanks for your suggestions.


Ilya Baldin

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