pty: using tcgetattr() to get attributes before re-setting it

To fix this error:
tst_test.c:1015: INFO: Timeout per run is 0h 50m 00s pty02.c:42: BROK: tcsetattr() failed: EINVAL

POSIX.1 General description:
Changes the attributes associated with a terminal. New attributes are specified with a termios control structure. Programs should always issue a tcgetattr() first, modify the desired fields, and then issue a tcsetattr(). tcsetattr() should never be issued using a termios structure that was not obtained using tcgetattr(). tcsetattr() should use only a termios structure that was obtained by tcgetattr().

Signed-off-by Li Wang<[email protected]>

Cc Eric Biggers<[email protected]>

Cc Cyril Hrubis<[email protected]>

---
  testcases/kernel/pty/pty02.c | 7 ++++++-
  1 file changed, 6 insertions(+), 1 deletion(-)

testcases/kernel/pty/pty02.c +6 -1

index fd3d26b..0cedca8 100644
@@ -31,13 +31,18 @@
 
  static void do_test(void)
  {
-	struct termios io = { .c_lflag = EXTPROC | ICANON };
+	struct termios io;
  	int ptmx, pts;
  	char c = 'A';
  	int nbytes;
 
  	ptmx = SAFE_OPEN("/dev/ptmx", O_WRONLY);
 
+	if (tcgetattr(ptmx,&io) != 0)
+		tst_brk(TBROK | TERRNO, "tcgetattr() failed");
+
+	io.c_lflag = EXTPROC | ICANON;
+
  	if (tcsetattr(ptmx, TCSANOW,&io) != 0)
  		tst_brk(TBROK | TERRNO, "tcsetattr() failed");
 

Hi Li,

We found other issues by running pty02:
1) The undefined EXTPROC flag led to complier error on RHEL5/6.
2) Based on the fix, reading pts blocked on RHEL6 because we didn't write newline('\n') to ptmx,
but it worked well on RHEL7. I am not sure which kernel patch changed
this behavior.
According canonical mode description:
In canonical mode:
* Input is made available line by line. An input line is available
when one of the line
delimiters is typed (NL, EOL, EOL2; or EOF at the start of
line). Except in the case of EOF,
the line delimiter is included in the buffer returned by read(2).
3) Based on the fix,
​​
tcsetattr(3) cannot detect invalid EXTPROC flag on RHEL5/6.

Hi!
I'm looking into these patches now, so if I get it right we should:

* Add EXTPROC into lapi
* Get the termios value, or it with the requested flags, then apply tcsetattr()
* Exit the test with TCONF if we get EINVAL from tcsetattr()

Would that work for both of you?

Quoted 3 lines · Cyril Hrubis

Hi!
I'm looking into these patches now, so if I get it right we should:

Thanks for taking care of this.

Quoted 8 lines

* Add EXTPROC into lapi
* Get the termios value, or it with the requested flags, then apply tcsetattr()
* Exit the test with TCONF if we get EINVAL from tcsetattr()

Would that work for both of you?

I think that's right.

But maybe we have to handle the ICANON nonuniform behavior too.

As YangXiao mentioned that, reading pts will be blocked on rhel6 forever if we don't give
newline('\n') to ptmx, but this issue not exist on rhel7.