tls: Preserve sk_err across recvmsg() when data has been copied

tls: Preserve sk_err across recvmsg() when data has been copied

The sk_err check in tls_rx_rec_wait() consumes the error via
sock_error(), which clears sk_err atomically. When the caller
(tls_sw_recvmsg, tls_sw_splice_read, or tls_sw_read_sock) already
has bytes copied to userspace, it returns those bytes and discards
the error from this call. sk_err is now zero on the socket, so the
next read syscall observes only RCV_SHUTDOWN and reports a clean
EOF instead of the actual error (typically -ECONNRESET).

The race is reachable when tls_read_flush_backlog()'s periodic
sk_flush_backlog() triggers tcp_reset() in the middle of a
multi-record read.

Pass a has_copied flag to tls_rx_rec_wait(). When has_copied is
false, consume sk_err via sock_error() as before. When has_copied
is true, report the error from READ_ONCE() but leave sk_err set:
the caller returns the byte count and discards the err from this
call, and the next read syscall surfaces the preserved sk_err. This
mirrors the tcp_recvmsg() preserve-and-surface pattern.

The decrypt-abort path is unaffected: tls_err_abort() raises
sk_err to EBADMSG after tls_rx_rec_wait() returns, and nothing
on the caller's return path consumes it, so the EBADMSG surfaces
on the next read.

tls_sw_splice_read() passes has_copied=false: it processes
one record per call, so no bytes have been copied within the
function when tls_rx_rec_wait() runs. A reset that arrives
between iterations of splice_direct_to_actor() (the sendfile()
path) is still consumed by sock_error() in the later call, and the
outer loop returns the prior iterations' byte count and drops the
error. tcp_splice_read() exhibits the same pattern at the iteration
boundary; addressing it belongs at the splice_direct_to_actor()
layer and is out of scope here.

Fixes: c46b01839f7a ("tls: rx: periodically flush socket backlog")
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Link: https://patch.msgid.link/20260513125825.205189-1-cel@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>