Issuing a read(2) on a cuse device that has data to returns works as expected. When switching to readv(2) with eight iovec-s, the readv call never returns.

Logging from an instrumented cuse device shows the first read is passed the correct buffer address and size. The data appears to be transferred. All subsequent read requires processed inside the cuse device have a correct buffer address but the length is zero.

Code snippet follows:

#include <unistd.h>
#include <sys/uio.h>

int okay;
char chunk[8];
struct iovec list[8] =
{
    {chunk + 0, 1},
    {chunk + 1, 1},
    {chunk + 2, 1},
    {chunk + 3, 1},
    {chunk + 4, 1},
    {chunk + 5, 1},
    {chunk + 6, 1},
    {chunk + 7, 1}
};

int handle = open(/dev/cuse123", O_RDONLY);

do
{
    okay = readv(handle, list, 8);
    if (okay < 0) break;
    write(2, chunk, okay);
}
while (true);
close(handle);

Switching back to a simple read(2) call works as expected.

The underlying issue is likely inside the cuse kernel module but this is just a guess.