[PATCH v4 06/15] tools subcmd: Robust fallback and existence checks for process reaping

[Ian Rogers]

Update check_if_command_finished() to check if cmd->pid <= 0 to avoid
reaping process hazards when PID has been set to -1 on startup error.

Transition the procfs status fopen() fallback from active waitpid() calls
to robust kill(pid, 0) existence and ESRCH detection logic. This allows
zombie processes to remain until explicitly reaped by finish_command(),
eradicating the risk of infinite loops induced by kernel PID reuse across
unrelated system daemons.

Assisted-by: Gemini-CLI:Google Gemini 3
Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
---
 tools/lib/subcmd/run-command.c | 50 ++++++++++++++++++++++++++++++++--
 1 file changed, 47 insertions(+), 3 deletions(-)

diff --git a/tools/lib/subcmd/run-command.c b/tools/lib/subcmd/run-command.c
index b7510f83209a..a305935d2bfb 100644
--- a/tools/lib/subcmd/run-command.c
+++ b/tools/lib/subcmd/run-command.c
@@ -229,20 +229,64 @@ static int wait_or_whine(struct child_process *cmd, bool block)
 
 int check_if_command_finished(struct child_process *cmd)
 {
-#ifdef __linux__
 	char filename[6 + MAX_STRLEN_TYPE(typeof(cmd->pid)) + 7 + 1];
 	char status_line[256];
 	FILE *status_file;
 
+	if (cmd->finished)
+		return 1;
+	if (cmd->pid <= 0)
+		return 1;
+
 	/*
 	 * Check by reading /proc/<pid>/status as calling waitpid causes
 	 * stdout/stderr to be closed and data lost.
 	 */
 	sprintf(filename, "/proc/%u/status", cmd->pid);
+#ifdef __linux__
 	status_file = fopen(filename, "r");
 	if (status_file == NULL) {
-		/* Open failed assume finish_command was called. */
-		return true;
+		/*
+		 * fopen() can fail with ENOENT if the process has been reaped.
+		 * It can also fail with EMFILE/ENFILE if RLIMIT_NOFILE is reached,
+		 * or with EINTR/ENOMEM. Use kill(pid, 0) as a robust fallback
+		 * to distinguish between active processes and dead ones without
+		 * consuming file descriptors.
+		 */
+		if (errno == ENOENT)
+			return 1;
+		if (kill(cmd->pid, 0) < 0 && errno == ESRCH) {
+			waiting = waitpid(cmd->pid, &status, WNOHANG);
+			if (waiting == cmd->pid) {
+				int result;
+				int code;
+
+				cmd->finished = 1;
+				if (WIFSIGNALED(status)) {
+					result = -ERR_RUN_COMMAND_WAITPID_SIGNAL;
+				} else if (!WIFEXITED(status)) {
+					result = -ERR_RUN_COMMAND_WAITPID_NOEXIT;
+				} else {
+					code = WEXITSTATUS(status);
+					switch (code) {
+					case 127:
+						result = -ERR_RUN_COMMAND_EXEC;
+						break;
+					case 0:
+						result = 0;
+						break;
+					default:
+						result = -code;
+						break;
+					}
+				}
+				cmd->finish_result = result;
+				return 1;
+			}
+			if (waiting < 0 && errno == ECHILD)
+				return 1;
+		}
+		return 0;
 	}
 	while (fgets(status_line, sizeof(status_line), status_file) != NULL) {
 		char *p;
-- 
2.54.0.823.g6e5bcc1fc9-goog