[LTP] pty09: Cap RLIMIT_NOFILE-based slave PTY opens

[linuxtestproject.agent@gmail.com]

Hi Runli,

On Thu, 09 Jul 2026 15:49:23 +0800, Runli wrote:
> pty09: Cap RLIMIT_NOFILE-based slave PTY opens

>  	max_pid_num = limit.rlim_cur - count;
> +	if (max_pid_num > MAX_SLAVE_OPENS)
> +		max_pid_num = MAX_SLAVE_OPENS;
>  
>  	tst_res(TINFO, "Available number of pids: %u", max_pid_num);

After the cap fires, `max_pid_num` holds 4096 -- the test's self-imposed
limit -- but the TINFO label still says "Available number", which implies
the system's available FD count.

On a system where RLIMIT_NOFILE - open_fds is, say, 1048000, the output
would read "Available number of pids: 4096", which could mislead anyone
comparing it against the system limit.

Consider printing the uncapped available count and a separate note when
the cap applies, e.g.:

  tst_res(TINFO, "Available number of pids: %u (capped at %d)",
          max_pid_num, MAX_SLAVE_OPENS);

or split the TINFO into two calls -- one for the real available count
(computed before the cap), one confirming the cap was applied.

Verdict - Needs revision

Pre-existing issues:

1. `count_avail_pid()` and the variable `max_pid_num` both say "pid"
   but the code tracks file-descriptor slots, not PIDs.

2. In run(): `tst_res(TPASS, "pty has been opened %d times", max_pid_num)`
   uses %d for an unsigned int argument.

3. In run(): `int slavefd[max_pid_num]` is a VLA. If max_pid_num == 0
   (all FD slots occupied) the zero-length VLA is undefined behaviour.

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