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Wed, 15 Apr 2026 06:11:19 -0700 (PDT) Received: from leonardi-redhat ([176.206.19.176]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-488f0ebd0e1sm20362525e9.28.2026.04.15.06.11.18 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 15 Apr 2026 06:11:19 -0700 (PDT) Date: Wed, 15 Apr 2026 15:11:16 +0200 From: Luigi Leonardi To: Stefano Garzarella Cc: Stefan Hajnoczi , "Michael S. Tsirkin" , Jason Wang , Xuan Zhuo , Eugenio =?utf-8?B?UMOpcmV6?= , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Arseniy Krasnov , kvm@vger.kernel.org, virtualization@lists.linux.dev, netdev@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH net v3 2/3] vsock/test: fix MSG_PEEK handling in recv_buf() Message-ID: References: <20260414-fix_peek-v3-0-e7daead49f83@redhat.com> <20260414-fix_peek-v3-2-e7daead49f83@redhat.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; charset=utf-8; format=flowed Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Wed, Apr 15, 2026 at 01:54:43PM +0200, Stefano Garzarella wrote: >On Wed, Apr 15, 2026 at 01:31:11PM +0200, Stefano Garzarella wrote: >>On Tue, Apr 14, 2026 at 06:10:22PM +0200, Luigi Leonardi wrote: >>>`recv_buf` does not handle the MSG_PEEK flag correctly: it keeps calling >>>`recv` until all requested bytes are available or an error occurs. >>> >>>The problem is how it calculates the amount of bytes read: MSG_PEEK >>>doesn't consume any bytes, will re-read the same bytes from the buffer >>>head, so, summing the return value every time is wrong. >>> >>>Moreover, MSG_PEEK doesn't consume the bytes in the buffer, so if the >>>requested amount is more than the bytes available, the loop will never >>>terminate, because `recv` will never return EOF. For this reason we need >>>to compare the amount of read bytes with the number of bytes expected. >>> >>>Add a check, and if the MSG_PEEK flag is present, update the counter of >>>read bytes differently, and break if we read the expected amount. >> >>nit: "..., update the counter for bytes read only after all expected >>bytes have been read and break out of the loop; otherwise, try again >>after a short delay to avoid consuming too many CPU cycles." >> >>> >>>This allows us to simplify the `test_stream_credit_update_test`, by >>>reusing `recv_buf`, like some other tests already do. >>> >>>This also fixes callers that pass MSG_PEEK to recv_buf(). >> >>nit: this is implicit from the first part of the description. >> >>> >>>Suggested-by: Stefano Garzarella >>>Signed-off-by: Luigi Leonardi >>>--- >>>tools/testing/vsock/util.c | 15 +++++++++++++++ >>>tools/testing/vsock/vsock_test.c | 13 +------------ >>>2 files changed, 16 insertions(+), 12 deletions(-) >>> >>>diff --git a/tools/testing/vsock/util.c b/tools/testing/vsock/util.c >>>index 1fe1338c79cd..2c9ee3210090 100644 >>>--- a/tools/testing/vsock/util.c >>>+++ b/tools/testing/vsock/util.c >>>@@ -381,7 +381,13 @@ void send_buf(int fd, const void *buf, size_t len, int flags, >>> } >>>} >>> >>>+#define RECV_PEEK_RETRY_USEC 10 >> >>10 usec IMO are a bit low, it could be the same order of the >>syscalls involved in the loop, I'd go to some milliseconds like we >>do for SEND_SLEEP_USEC. >> >>>+ >>>/* Receive bytes in a buffer and check the return value. >>>+ * >>>+ * MSG_PEEK note: MSG_PEEK doesn't consume bytes from the buffer, so partial >>>+ * reads cannot be summed. Instead, the function retries until recv() returns >>>+ * exactly expected_ret bytes in a single call. >> >>I'd replace with something like this: >> >> * When MSG_PEEK is set, recv() is retried until it returns exactly >> * expected_ret bytes. The function returns on error, EOF, or timeout >> * as usual. >> >>Thanks, >>Stefano >> >>>* >>>* expected_ret: >>>* <0 Negative errno (for testing errors) >>>@@ -403,6 +409,15 @@ void recv_buf(int fd, void *buf, size_t len, int flags, ssize_t expected_ret) >>> if (ret <= 0) >>> break; >>> >>>+ if (flags & MSG_PEEK) { >>>+ if (ret == expected_ret) { > >On second thought, I think it would be more appropriate to check for >`ret >= expected_ret` here, because all subsequent recv() will >definitely return more bytes, so there’s no point in continuing the >loop... and anyway, we’ll check the result later, so just that change >should be fine. > >And of course I'd update the comment on top in this way: > > * When MSG_PEEK is set, recv() is retried until it returns at least > * expected_ret bytes. The function returns on error, EOF, or timeout > * as usual. > >Thanks, >Stefano > Good idea, will do. Thanks! Luigi