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 <[email protected]>
Signed-off-by: Luigi Leonardi <[email protected]>
---
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) {
+ nread = ret;
+ break;
+ }
+ timeout_usleep(RECV_PEEK_RETRY_USEC);
+ continue;
+ }
+
nread += ret;
} while (nread < len);
timeout_end();
diff --git a/tools/testing/vsock/vsock_test.c b/tools/testing/vsock/vsock_test.c
index 5bd20ccd9335..bdb0754965df 100644
--- a/tools/testing/vsock/vsock_test.c
+++ b/tools/testing/vsock/vsock_test.c
@@ -1500,18 +1500,7 @@ static void test_stream_credit_update_test(const struct
test_opts *opts,
}
/* Wait until there will be 128KB of data in rx queue. */
- while (1) {
- ssize_t res;
-
- res = recv(fd, buf, buf_size, MSG_PEEK);
- if (res == buf_size)
- break;
-
- if (res <= 0) {
- fprintf(stderr, "unexpected 'recv()' return: %zi\n",
res);
- exit(EXIT_FAILURE);
- }
- }
+ recv_buf(fd, buf, buf_size, MSG_PEEK, buf_size);
/* There is 128KB of data in the socket's rx queue, dequeue first
* 64KB, credit update is sent if 'low_rx_bytes_test' == true.
--
2.53.0
