kern: tty: fix ttyinq_read_uio assertion

It's clear from later context that `rlen` was always expected to include
`flen`, as we'll trim `flen` bytes from the end of the read.  Relax our
initial assertion to only require the total size less trimmed bytes to
lie within the out buffer size.

While we're here, I note that if we have to read more than one block and
we're trimming from the end then we'll do the wrong thing and omit
`flen` bytes from every block, rather than just the end.  Add an
assertion to make sure we're not doing that, but the only caller that
specifies a non-zero `flen` today will only really be doing so if rlen
is entirely within a single buffer.

Reviewed by:	cy, imp

(cherry picked from commit 09a43b8790bdeb97fbecd3ea767c2f599eb4a4d3)

1 files changed, 10 insertions, 1 deletions

diff --git a/sys/kern/tty_inq.c b/sys/kern/tty_inq.c
index daf3bde77712..0bf7c2fa5b5e 100644
--- a/sys/kern/tty_inq.c
+++ b/sys/kern/tty_inq.c
@@ -165,7 +165,8 @@ ttyinq_read_uio(struct ttyinq *ti, struct tty *tp, struct uio *uio,
     size_t rlen, size_t flen)
 {
 
-	MPASS(rlen <= uio->uio_resid);
+	/* rlen includes flen, flen bytes will be trimmed from the end. */
+	MPASS(rlen - flen <= uio->uio_resid);
 
 	while (rlen > 0) {
 		int error;
@@ -193,6 +194,14 @@ ttyinq_read_uio(struct ttyinq *ti, struct tty *tp, struct uio *uio,
 		rlen -= clen;
 
 		/*
+		 * Caller shouldn't request that we trim anything if we might be
+		 * reading across blocks.  We could handle it, but today we do
+		 * not.
+		 */
+		if (flen > 0)
+			MPASS(rlen == 0);
+
+		/*
 		 * We can prevent buffering in some cases:
 		 * - We need to read the block until the end.
 		 * - We don't need to read the block until the end, but