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Wed, 10 Sep 2025 02:43:21 -0700 (PDT) From: Jakub Sitnicki To: Michal Luczaj Cc: Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Martin KaFai Lau , Eduard Zingerman , Song Liu , Yonghong Song , John Fastabend , KP Singh , Stanislav Fomichev , Hao Luo , Jiri Olsa , Mykola Lysenko , Shuah Khan , bpf@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH bpf-next 1/5] selftests/bpf: sockmap_redir: Simplify try_recv() In-Reply-To: <35912d55-eb6e-403a-9a7a-05cae551ccf3@rbox.co> (Michal Luczaj's message of "Tue, 9 Sep 2025 23:25:16 +0200") References: <20250905-redir-test-pass-drop-v1-0-9d9e43ff40df@rbox.co> <20250905-redir-test-pass-drop-v1-1-9d9e43ff40df@rbox.co> <87ikhs54z2.fsf@cloudflare.com> <87bjnk53uo.fsf@cloudflare.com> <35912d55-eb6e-403a-9a7a-05cae551ccf3@rbox.co> Date: Wed, 10 Sep 2025 11:43:20 +0200 Message-ID: <877by663t3.fsf@cloudflare.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain On Tue, Sep 09, 2025 at 11:25 PM +02, Michal Luczaj wrote: > On 9/9/25 12:15, Jakub Sitnicki wrote: >> 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 >>>> --- >>>> .../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)) > > Well, looks like adding MSG_PEEK exposed a bug in the test. I'll fix that. The gift that keeps on giving xD Other alternatives that should also work, but who knows: - select/poll/epoll readability check - ioctl(SIOCINQ) but no way to tell if 0-len msg is pending