Re: [PATCH bpf 1/2] bpf: Fix partial copy of non-linear skb test_run output

[Paul Chaignon <paul.chaignon@gmail.com>]

On Mon, Jun 15, 2026 at 03:38:55PM +0800, Sun Jian wrote:
> For non-linear skbs, bpf_test_finish() derives the linear head copy
> length from copy_size - frag_size. This only matches the skb head length
> when copy_size is the full packet size.
> 
> When userspace provides a short data_out buffer, copy_size is clamped to
> that buffer size. If copy_size is smaller than frag_size, the computed
> length becomes negative and bpf_test_finish() returns -ENOSPC before
> copying the packet prefix or updating data_size_out.

Thanks for fixing this!

> 
> Compute the linear head length from the skb layout instead, and clamp the
> head copy length to copy_size. This preserves the expected partial-copy
> semantics: return -ENOSPC, copy the packet prefix that fits in data_out,
> and report the full packet length through data_size_out.
> 
> Fixes: 838baa351cee ("bpf: Craft non-linear skbs in BPF_PROG_TEST_RUN")

Wouldn't this bug actually go back to 7855e0db150ad ("bpf: test_run:
add xdp_shared_info pointer in bpf_test_finish signature") and also
affect the XDP bpf_prog_test_run_xdp()? If so, could you also add a
selftest that covers it for XDP?

> Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
> ---
>  net/bpf/test_run.c | 11 ++++-------
>  1 file changed, 4 insertions(+), 7 deletions(-)
> 
> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> index 2bc04feadfab..976e8fa31bc9 100644
> --- a/net/bpf/test_run.c
> +++ b/net/bpf/test_run.c
> @@ -453,19 +453,16 @@ static int bpf_test_finish(const union bpf_attr *kattr,
>  	}
>  
>  	if (data_out) {
> -		int len = sinfo ? copy_size - frag_size : copy_size;
> -
> -		if (len < 0) {
> -			err = -ENOSPC;
> -			goto out;
> -		}
> +		u32 head_len = size - frag_size;
> +		u32 len = min(copy_size, head_len);
>  
>  		if (copy_to_user(data_out, data, len))
>  			goto out;
>  
>  		if (sinfo) {
> -			int i, offset = len;
> +			u32 offset = len;
>  			u32 data_len;
> +			int i;
>  
>  			for (i = 0; i < sinfo->nr_frags; i++) {
>  				skb_frag_t *frag = &sinfo->frags[i];
> -- 
> 2.43.0
>