On Fri, Nov 16, 2018 at 12:54 PM Lorenz Bauer <lmb@xxxxxxxxxxxxxx> wrote:
>
> Use data_size_out as a size hint when copying test output to user space.
> A program using BPF_PERF_OUTPUT can compare its own buffer length with
> data_size_out after the syscall to detect whether truncation has taken
> place. Callers which so far did not set data_size_in are not affected.
>
> Signed-off-by: Lorenz Bauer <lmb@xxxxxxxxxxxxxx>
> ---
> net/bpf/test_run.c | 9 ++++++++-
> 1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> index c89c22c49015..30c57b7f4ba4 100644
> --- a/net/bpf/test_run.c
> +++ b/net/bpf/test_run.c
> @@ -74,8 +74,15 @@ static int bpf_test_finish(const union bpf_attr *kattr,
> {
> void __user *data_out = u64_to_user_ptr(kattr->test.data_out);
> int err = -EFAULT;
> + u32 copy_size = size;
>
> - if (data_out && copy_to_user(data_out, data, size))
> + /* Clamp copy if the user has provided a size hint, but copy the full
> + * buffer if not to retain old behaviour.
> + */
> + if (kattr->test.data_size_out && copy_size > kattr->test.data_size_out)
> + copy_size = kattr->test.data_size_out;
> +
> + if (data_out && copy_to_user(data_out, data, copy_size))
> goto out;
> if (copy_to_user(&uattr->test.data_size_out, &size, sizeof(size)))
> goto out;
if copy_size < size, maybe we should return -ENOSPC so user space is aware
of insufficient size and takes proper action? This behavior will then
be consistent
with BPF_PROG_QUERY subcommand where prog_cnt is the in/out parameter and
-ENOSPC is returned if kernel does not have enough space to copy out
the whole data.
Also, since data_size_out field now has more complex semantics, could you add
some comments in uapi/linux/bpf.h so it will be relatively clear from
uapi header
how this field will be used?
> --
> 2.17.1
>
