Re: [PATCH bpf-next 1/5] selftests/bpf: sockmap_redir: Simplify try_recv()

[Jakub Sitnicki <jakub@cloudflare.com>]

On Tue, Sep 09, 2025 at 11:51 AM +02, Jakub Sitnicki wrote:
> On Fri, Sep 05, 2025 at 01:11 PM +02, Michal Luczaj wrote:
>> try_recv() was meant to support both @expect_success cases, but all the
>> callers use @expect_success=false anyway. Drop the unused logic and fold in
>> MSG_DONTWAIT. Adapt callers.
>>
>> Subtle change here: recv() return value of 0 will also be considered (an
>> unexpected) success.
>>
>> Signed-off-by: Michal Luczaj <mhal@rbox.co>
>> ---
>>  .../selftests/bpf/prog_tests/sockmap_redir.c       | 25 +++++++++-------------
>>  1 file changed, 10 insertions(+), 15 deletions(-)
>>
>> diff --git a/tools/testing/selftests/bpf/prog_tests/sockmap_redir.c b/tools/testing/selftests/bpf/prog_tests/sockmap_redir.c
>> index 9c461d93113db20de65ac353f92dfdbe32ffbd3b..c1bf1076e8152b7d83c3e07e2dce746b5a39cf7e 100644
>> --- a/tools/testing/selftests/bpf/prog_tests/sockmap_redir.c
>> +++ b/tools/testing/selftests/bpf/prog_tests/sockmap_redir.c
>> @@ -144,17 +144,14 @@ static void get_redir_params(struct redir_spec *redir,
>>  		*redirect_flags = 0;
>>  }
>>  
>> -static void try_recv(const char *prefix, int fd, int flags, bool expect_success)
>> +static void fail_recv(const char *prefix, int fd, int more_flags)
>>  {
>>  	ssize_t n;
>>  	char buf;
>>  
>> -	errno = 0;
>> -	n = recv(fd, &buf, 1, flags);
>> -	if (n < 0 && expect_success)
>> -		FAIL_ERRNO("%s: unexpected failure: retval=%zd", prefix, n);
>> -	if (!n && !expect_success)
>> -		FAIL("%s: expected failure: retval=%zd", prefix, n);
>> +	n = recv(fd, &buf, 1, MSG_DONTWAIT | more_flags);
>> +	if (n >= 0)
>> +		FAIL("%s: unexpected success: retval=%zd", prefix, n);
>>  }
>
> This bit, which you highlighted in the description, I don't get.
>
> If we're expecting to receive exactly one byte, why treat a short read
> as a succcess? Why not make it a strict "n != 1" check?
>
> [...]

Nevermind. It makes sense now. We do want to report a failure for 0-len
msg recv as well. You're effectively checking if the rcv queue is empty.

I'd add MSG_PEEK, to signal that we're _just checking_ if the socket is
readable, and turn the check into the below to succeed only when
queue is empty:

        (n != -1 || (errno != EAGAIN && errno != EWOULDBLOCK))

It's a minor thing. Leaving it up to you. Either way:

Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>