[LTP] thp04: Add ptrace mode and fix tracee stop synchronization

[linuxtestproject.agent@gmail.com]

Hi Jan,

On July 09, 2026, Jan Polensky wrote:
> thp04: Add ptrace mode and fix tracee stop synchronization

> +	effective_mode = user_mode;
> +
> +	/* Probe direct /proc/self/mem writes only if not explicitly set to ptrace */
> +	if (!explicit_mode || user_mode == PROC_MEM_ALWAYS) {
> +		...
> +		effective_mode = PROC_MEM_PTRACE;
> +	}
> +
> +	/* Ptrace mode: tracee will be spawned in run() for each iteration */
> +	if (effective_mode == PROC_MEM_PTRACE)
> +		return;
> +
> +	tst_brk(TCONF, "Writes to /proc/self/mem disabled in kernel policy");

The final tst_brk() appears unreachable. Every code path through setup()
either returns or calls tst_brk() before reaching this line:

- Probe succeeds (PROC_MEM_ALWAYS path): early return inside the block.
- Probe fails with non-EIO: tst_brk(TBROK) inside the block.
- Probe fails with EIO and explicit_mode && user_mode==PROC_MEM_ALWAYS:
  tst_brk(TCONF) inside the block.
- Probe fails with EIO otherwise: effective_mode is set to PROC_MEM_PTRACE
  inside the block, then the "if (PROC_MEM_PTRACE) return" fires.
- Explicit ptrace mode (block skipped): effective_mode==PROC_MEM_PTRACE
  was set before the block, so "if (PROC_MEM_PTRACE) return" fires.
- Explicit never mode: tst_brk(TCONF, "...on kernel cmdline") fires
  inside the params[0].found block; execution does not continue.

Is there a case this was intended to handle that is now missing?

> +	for (i = 0; i < PTRACE_TEST_ITERATIONS; i++) {
> +		...
> +		/* Wait for tracee to stop itself with raise(SIGSTOP) */
> +		SAFE_WAITPID(tracee_pid, &status, 0);
> +
> +		if (!WIFSTOPPED(status) || WSTOPSIG(status) != SIGSTOP)
> +			tst_brk(TBROK, "Tracee did not stop with SIGSTOP: %s",
> +				tst_strstatus(status));
> +	}

When the tracee detects page pollution it calls:
  tst_res(TFAIL, "Huge zero page was polluted");
  child_cleanup();
  exit(1);

The SAFE_WAITPID above then returns with WIFEXITED(status)=true, which
triggers tst_brk(TBROK, "Tracee did not stop with SIGSTOP: exited, status=1").

The test output becomes:
  TFAIL: Huge zero page was polluted
  TBROK: Tracee did not stop with SIGSTOP: exited, status=1

The TFAIL is propagated correctly via the LTP result mechanism.
However, the TBROK message "Tracee did not stop with SIGSTOP" describes
an infrastructure anomaly rather than the actual failure (page pollution).
Would it be clearer to check WIFEXITED explicitly and omit the TBROK when
the tracee has already reported a result?

Verdict - Needs revision

Pre-existing issues:

thread_run_always(), line visible in context (unchanged from original):
  /* Wait for dirty page handling before next madvise() */
  usleep(10);
This is sleep-based synchronization. It was present before this patch and
is not introduced here, but worth noting for a follow-up cleanup.

---
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Regards,
LTP AI Reviewer