On 02/18/2016 03:25 AM, Michael Kerrisk (man-pages) wrote:

Hello Peter,

On 02/17/2016 05:37 PM, Peter Hurley wrote:

quoted

Hi Michael,

On 02/17/2016 02:09 AM, Michael Kerrisk (man-pages) wrote:

quoted

On 02/16/2016 11:30 PM, Peter Hurley wrote:

quoted

On 02/15/2016 07:28 AM, Michael Kerrisk (man-pages) wrote:

quoted

On 02/15/2016 02:26 PM, Dr. Tobias Quathamer wrote:

quoted

See https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/drivers/tty/n_tty.c#n1673
See https://bugs.debian.org/797479
---
 man3/termios.3 | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/man3/termios.3 b/man3/termios.3
index 7d738d4..3f57607 100644
--- a/man3/termios.3
+++ b/man3/termios.3

@@ -728,11 +728,20 @@ requested fewer bytes than are available in the current line of input,
 then only as many bytes as requested are read,
 and the remaining characters will be available for a future
 .BR read (2).
+.IP * 2
+The maximum line length is 4096 chars (including the line termination
+char); lines longer than 4096 chars are truncated. After 4095 chars,
+input data is still processed but not stored. Overflow processing
+ensures the tty can always receive more input until at least one
+line can be read.
 .PP
 In noncanonical mode input is available immediately (without
 the user having to type a line-delimiter character),
 no input processing is performed,
 and line editing is disabled.
+The read buffer will only accept 4095 chars; this provides the
+necessary space for a newline char if the input mode is switched
+to canonical.
 The settings of MIN
 .RI ( c_cc[VMIN] )
 and TIME

Thanks for crafting this. I've applied, and tweaked a little to clarify
some details:

       * The maximum line length is 4096 chars (including the  terminating
         newline  character);  lines longer than 4096 chars are truncated.
         After 4095 characters, input data up (but not including) any ter‐
         minating  newline  is  discarded.  This ensures that the terminal
         can always receive more input until at  least  one  line  can  be
         read.

Hmm. Neither description is accurate of the observable behavior from userspace.
For example, it's entirely possible to retrieve > 4096 bytes in non-canonical
mode, at least since 3.12

Note that the text I quoted applies just to canonical mode:

       In canonical mode:

       [...]

       * The maximum line length is 4096 chars (including the  terminat‐
         ing  newline character); lines longer than 4096 chars are trun‐
         cated.  After 4095 characters, input data up (but  not  includ‐
         ing)  any  terminating newline is discarded.  This ensures that
         the terminal can always receive more input until at  least  one
         line can be read.

So, does that seem okay?

See below.

quoted

quoted

And input processing continues on the input, even past 4096 bytes.
Line editing, ISIG, ECHOxx processing still occurs.

Yes, but only in noncanonical mode, right?

No. Input processing continues in canonical mode, including echoing.
Only when bytes are to actually be stored for userspace to later read is
the input data discarded if the buffer is full (less the 1 byte required to
store the line termination).

Every input byte received in canonical mode is always processed normally
and only discarded if the buffer is full. IOW, a Ctrl+u (line kill) received
as the 6000th byte received w/o line termination will still cause all previous
data to be flushed and the line restarted when new input arrives.

Ahh -- got it. I wasn't thinking about that stuff at all!

quoted

Similarly, all data is still echoed regardless of how long the line is.

quoted

quoted

It is true that it is not possible to retrieve > 4096 char line in canonical
mode, and I don't see that ever changing via read() because userspace may
assume it has received a terminated line in 4096-byte read buffer.

Yep. So we all seem to agree.

The way I read your edits above is that all new input data is simply discarded
until line termination, which is not the case.

Yes, because I wasn't even considering your point :-). Thanks for the
clarifications.

quoted

Dr. Quathamer's first diff hunk is accurate wrt canonical mode. However,
I think the 'Overflow processing ...' is overly specific; the tty core contains
many elements which prevent terminal lockup.

Okay -- so how's this text?

       In canonical mode:
       [...]
       * The maximum line length is 4096 chars (including the terminating
         newline character); lines longer than 4096 chars are  truncated.
         After  4095  characters,  input processing (e.g., ISIG and ECHO*
         processing) continues, but any input data after 4095  characters
         up  to (but not including) any terminating newline is discarded.
         This ensures that the terminal can  always  receive  more  input
         until at least one line can be read.

Yeah, this seems fine.

quoted

quoted

quoted

However, the noncanonical mode input buffer size may change in the near
future.

Can you say some more about that?

At very high line rates and with ptys, the size of the read buffer becomes a
bottleneck, while at the same time, an already-committed page of memory is
going unused because of the page order allocation used by vm area allocator
(where N_TTY data is stored).

The change is non-trivial however, since the maximum read size returned
by canonical mode cannot > 4096 bytes (because of the userspace assumptions
I referred to earlier).

quoted

And what changed in 3.13 with respect to noncanonical mode, by the way?

In 3.12, the entire tty input path was parallelized.

So while data is being read by userspace, input processing continues
concurrently, adding new data for userspace to read (the actual
construct used is a lockless circular buffer). In non-canonical modes,
the userspace copy is always performed twice -- first, from beginning of
unread data (head) to the end of buffer and then, from the beginning of
buffer to last unread data (tail).

Since new data may be added to the circular buffer in parallel after the
first user copy has been performed, it's possible to retrieve up to 8192
bytes (assuming the user copy buffer is that large).

Thanks for the info. Do you think this warrants any changes in the man page?

I don't think so. The goal of most changes in tty is to improve reliability
or performance w/o affecting userspace-observable behavior.

Regards,
Peter Hurley
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