362159593

Denys Vlasenko

2025-10-08

SUCCESS Duration: 148s

Subject

nsenter,unshare: don't use xvfork_parent_waits_and_exits(), it SEGVs on ppc64le

No binary (build failed)

Architecture: x86_64-linux-gnu Compiler: gcc (Ubuntu 11.4.0-1ubuntu1~22.04.3) 11.4.0

gcc -Wp,-MD,applets/.usage.d -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer      -Iinclude -Iinclude -o applets/usage applets/usage.c

(log yok)

libbb/xfuncs_printf.c+9 -0

@@ -696,6 +696,14 @@ pid_t FAST_FUNC xfork(void)
	}
	#endif

+	#if 0
+	/* DO NOT DO THIS. This can't be a function.
+	* It works on some arches (x86) but fails on others (ppc64le: SEGV).
+	* The reason is: the child returns from this function
+	* and likely pops up the stack in an arch-dependent way.
+	* When child eventually exits or execs, parent "reappear"
+	* in the now-unwound stack (!) and the behavior is undefined.
+	*/
	void FAST_FUNC xvfork_parent_waits_and_exits(void)
	{
	pid_t pid;
@@ -711,6 +719,7 @@ void FAST_FUNC xvfork_parent_waits_and_exits(void)
	}
	/* Child continues */
	}
+	#endif

	// Useful when we do know that pid is valid, and we just want to wait
	// for it to exit. Not existing pid is fatal. waitpid() status is not returned.

util-linux/nsenter.c+8 -1

@@ -251,7 +251,14 @@ int nsenter_main(int argc UNUSED_PARAM, char **argv)
	* explicitly requested by the user not to.
	*/
	if (!(opts & OPT_nofork) && (opts & OPT_pid)) {
-	xvfork_parent_waits_and_exits();
+	pid_t pid = xvfork();
+	if (pid > 0) {
+	/* Parent */
+	int exit_status = wait_for_exitstatus(pid);
+	if (WIFSIGNALED(exit_status))
+	kill_myself_with_sig(WTERMSIG(exit_status));
+	return WEXITSTATUS(exit_status);
+	}
	/* Child continues */
	}

util-linux/unshare.c+8 -1

@@ -333,7 +333,14 @@ int unshare_main(int argc UNUSED_PARAM, char **argv)
	* that'll become PID 1 in this new namespace.
	*/
	if (opts & OPT_fork) {
-	xvfork_parent_waits_and_exits();
+	pid_t pid = xvfork();
+	if (pid > 0) {
+	/* Parent */
+	int exit_status = wait_for_exitstatus(pid);
+	if (WIFSIGNALED(exit_status))
+	kill_myself_with_sig(WTERMSIG(exit_status));
+	return WEXITSTATUS(exit_status);
+	}
	/* Child continues */
	}