[Bug 293489] readv(2) on cuse device never returns

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=293489

            Bug ID: 293489
           Summary: readv(2) on cuse device never returns
           Product: Base System
           Version: 15.0-RELEASE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: bin
          Assignee: bugs@FreeBSD.org
          Reporter: unitrunker@unitrunker.net

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.

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