From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755599AbcAXWEN (ORCPT ); Sun, 24 Jan 2016 17:04:13 -0500 Received: from youngberry.canonical.com ([91.189.89.112]:60774 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755404AbcAXWDn (ORCPT ); Sun, 24 Jan 2016 17:03:43 -0500 From: Luis Henriques To: linux-kernel@vger.kernel.org, stable@vger.kernel.org, kernel-team@lists.ubuntu.com Cc: Peter Hurley , Greg Kroah-Hartman , Luis Henriques Subject: [PATCH 3.16.y-ckt 062/128] n_tty: Fix poll() after buffer-limited eof push read Date: Sun, 24 Jan 2016 22:00:17 +0000 Message-Id: <1453672883-2708-63-git-send-email-luis.henriques@canonical.com> In-Reply-To: <1453672883-2708-1-git-send-email-luis.henriques@canonical.com> References: <1453672883-2708-1-git-send-email-luis.henriques@canonical.com> X-Extended-Stable: 3.16 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 3.16.7-ckt23 -stable review patch. If anyone has any objections, please let me know. ---8<------------------------------------------------------------ From: Peter Hurley commit ac8f3bf8832a405cc6e4dccb1d26d5cb2994d234 upstream. commit 40d5e0905a03 ("n_tty: Fix EOF push handling") fixed EOF push for reads. However, that approach still allows a condition mismatch between poll() and read(), where poll() returns POLLIN but read() blocks. This state can happen when a previous read() returned because the user buffer was full and the next character was an EOF not at the beginning of the line. While the next read() will properly identify the condition and advance the read buffer tail without improperly indicating an EOF file condition (ie., read() will not mistakenly return 0), poll() will mistakenly indicate POLLIN. Although a possible solution would be to peek at the input buffer in n_tty_poll(), the better solution in this patch is to eat the EOF during the previous read() (ie., fix the problem by eliminating the condition). The current canon line buffer copy limits the scan for next end-of-line to the smaller of either, a. the remaining user buffer size b. completed lines in the input buffer When the remaining user buffer size is exactly one less than the end-of-line marked by EOF push, the EOF is not scanned nor skipped but left for subsequent reads. In the example below, the scan index 'eol' has stopped at the EOF because it is past the scan limit of 5 (not because it has found the next set bit in read_flags) user buffer [*nr = 5] _ _ _ _ _ read_flags 0 0 0 0 0 1 input buffer h e l l o [EOF] ^ ^ / / tail eol result: found = 0, tail += 5, *nr += 5 Instead, allow the scan to peek ahead 1 byte (while still limiting the scan to completed lines in the input buffer). For the example above, result: found = 1, tail += 6, *nr += 5 Because the scan limit is now bumped +1 byte, when the scan is completed, the tail advance and the user buffer copy limit is re-clamped to *nr when EOF is _not_ found. Fixes: 40d5e0905a03 ("n_tty: Fix EOF push handling") Signed-off-by: Peter Hurley Signed-off-by: Greg Kroah-Hartman [ luis: backported to 3.16: adjusted context ] Signed-off-by: Luis Henriques --- drivers/tty/n_tty.c | 22 +++++++++------------- 1 file changed, 9 insertions(+), 13 deletions(-) diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c index 29de4bfe5c70..d239fdba29d6 100644 --- a/drivers/tty/n_tty.c +++ b/drivers/tty/n_tty.c @@ -2048,13 +2048,13 @@ static int canon_copy_from_read_buf(struct tty_struct *tty, size_t eol; size_t tail; int ret, found = 0; - bool eof_push = 0; /* N.B. avoid overrun if nr == 0 */ - n = min(*nr, read_cnt(ldata)); - if (!n) + if (!*nr) return 0; + n = min(*nr + 1, read_cnt(ldata)); + tail = ldata->read_tail & (N_TTY_BUF_SIZE - 1); size = min_t(size_t, tail + n, N_TTY_BUF_SIZE); @@ -2075,12 +2075,11 @@ static int canon_copy_from_read_buf(struct tty_struct *tty, n = eol - tail; if (n > 4096) n += 4096; - n += found; - c = n; + c = n + found; - if (found && !ldata->push && read_buf(ldata, eol) == __DISABLED_CHAR) { - n--; - eof_push = !n && ldata->read_tail != ldata->line_start; + if (!found || read_buf(ldata, eol) != __DISABLED_CHAR) { + c = min(*nr, c); + n = c; } n_tty_trace("%s: eol:%zu found:%d n:%zu c:%zu size:%zu more:%zu\n", @@ -2111,7 +2110,7 @@ static int canon_copy_from_read_buf(struct tty_struct *tty, ldata->push = 0; tty_audit_push(tty); } - return eof_push ? -EAGAIN : 0; + return 0; } extern ssize_t redirected_tty_write(struct file *, const char __user *, @@ -2299,10 +2298,7 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file, if (ldata->icanon && !L_EXTPROC(tty)) { retval = canon_copy_from_read_buf(tty, &b, &nr); - if (retval == -EAGAIN) { - retval = 0; - continue; - } else if (retval) + if (retval) break; } else { int uncopied;