Handle non-utf8 strings in protobuf structures

[kkourt]

BPF/kernel strings are C strings which are a sequence of null-terminated
bytes. They may or may not be valid utf-8 strings.

For example, execve() can accept invalid utf-8 strings. Similarly,
filenames are not required to be utf-8 encoded.

This becomes an issue because we define fields like Binary, Arguments,
and CWD as strings in the proto descriptions.

According to the protobuf spec:
"A string must always contain UTF-8 encoded or 7-bit ASCII text, and
cannot be longer than 2^32."

As a result, when passing non-utf8 data as strings, protobuf clients
(e.g., the JSON writer and the gRPC client) cannot handle the event. For
example, running tetra getevents and executing a program with invalid
utf8 arguments leads the following error:
msg="Failed to receive events" error="rpc error: code = Internal desc = grpc: error while marshaling: string field contains invalid UTF-8"

There are a number of different approaches we can use to address this
problem. One is that we can try to encode arbitrary bytes in the
string. In the past, we have used base64 to do that (specifically, for
bytes arguments). An alternative (better?) solution would be to quote
the string, using strconv.Quote() or similar.

Quoting, however, means that we need to always parse the data and quote
them, which has a performance cost. To avoid this cost this patch
uses strings.ToValidUTF8 instead, which has small overhead (minimal in
case where the data are actually valid utf8), to replace invalid runes
with "�". This means that we loose information (what the actual bytes
were) but we also keep backwards compatibility and remain close to the
existing behavior.

In the future, we can modify this behavior (e.g., via a command line
switch) to quote the arguments instead, so that all bytes are preserved
for non-utf8 data encoded as strings.

A more radical change (which seems like the right thing to do) is to
change the gRPC fields that are not actually strings to bytes, and let
the clients do the decoding.

Reported-by: Гаврилов Иван Сергеевич [email protected]
Signed-off-by: Kornilios Kourtis [email protected]

Ensure that protobuf strings are valid utf-8

[tpapagian]

LGTM, thanks! (assuming tests are green)