busybox

From: "Doug Graham" <dgraham@nortel.com>

Date: 2009-01-22 07:20

​

Hello,

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Busybox's telnetd does not disable local (client-side) flow control

properly.  It does not put the pty into packet mode and then notify the

client whenever flow control is disabled by an application running under

its control.  The result is that ^S/^Q are not passed through to the

application, which is painful when the application is an emacs variant.

​

I suppose that support for this might be considered bloat, but the

included patch only adds about 200 bytes of text to x86 busybox and 300

bytes to mipsel busybox.  Please consider applying.

​

=============================

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NB: the patch doesn't work as-is because we now have iac_safe_write()

which quotes IACs on output.

​

=============================

Docs:

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The following ioctl(2) calls apply only to pseudo terminals:

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TIOCSTOP Stops output to a terminal (e.g. like typing ^S). Takes no parameter.

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TIOCSTART Restarts output (stopped by TIOCSTOP or by typing ^S). Takes no parameter.

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TIOCPKT         Enable/disable packet mode. When applied to the master side of a pseudo terminal, each

subsequent read(2) from the terminal will return data written on the slave part of the pseudo terminal preceded by a

zero byte (symbolically defined as TIOCPKT_DATA), or a single byte reflecting control status information.

In the latter case, the byte is an inclusive-or of zero or more of the bits:

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TIOCPKT_FLUSHREAD     whenever the read queue for the terminal is flushed.

TIOCPKT_FLUSHWRITE    whenever the write queue for the terminal is flushed.

TIOCPKT_STOP    whenever output to the terminal is stopped a la ^S.

TIOCPKT_START   whenever output to the terminal is restarted.

TIOCPKT_DOSTOP  whenever t_stopc is ^S and t_startc is ^Q.

TIOCPKT_NOSTOP  whenever the start and stop characters are not ^S/^Q.

​

While this mode is in use, the presence of control status information to be read from the master side may be detected

by a select(2) for exceptional conditions.

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This mode is used by rlogin(1) and rlogind(8) to implement a remote-echoed, locally ^S/^Q flow-controlled remote login

with proper back-flushing of output; it can be used by other similar programs.

​

TIOCUCNTL       Enable/disable a mode that allows a small number of simple user ioctl(2) commands to be passed through

the pseudo-terminal, using a protocol similar to that of TIOCPKT. The TIOCUCNTL and TIOCPKT modes are mutually

exclusive. This mode is enabled from the master side of a pseudo terminal. Each subsequent read(2) from the master side

will return data written on the slave part of the pseudo terminal preceded by a zero byte, or a single byte reflecting a

user control operation on the slave side. A user control command consists of a special ioctl(2) operation with no data;

the command is given as UIOCCMD (n), where n is a number in the range 1-255. The operation value n will be received as

a single byte on the next read(2) from the master side. The ioctl(2) UIOCCMD (0) is a no-op that may be used to probe

for the existence of this facility. As with TIOCPKT mode, command operations may be detected with a select(2) for

exceptional conditions.

​

--- busybox-1.13.2/networking/telnetd.c 2009/01/21 20:02:39 1.1

+++ busybox-1.13.2/networking/telnetd.c 2009/01/22 00:35:28

@@ -38,6 +38,9 @@

    int sockfd_read, sockfd_write, ptyfd;

    int shell_pid;