ext/pcntl: pcntl_exec() using execveat whenever possible instead.

Allows at least to avoid original process file descriptor to leak
in the new process image.
Then with Linux 6.14, we can add another layer of security to check
beforehand if the executable can be actually safely executed.

Author: devnexen

ext/pcntl/config.m4

@@ -10,7 +10,7 @@ if test "$PHP_PCNTL" != "no"; then
   done
 
   AC_CHECK_FUNCS(m4_normalize([
-    forkx
+    execveat
     getcpuid
     getpriority
     pidfd_open

ext/pcntl/pcntl.c

@@ -39,6 +39,11 @@
 #include <sys/resource.h>
 #endif
 
+#if defined(HAVE_EXECVEAT)
+#include <fcntl.h>
+#include <unistd.h>
+#endif
+
 #ifdef HAVE_WAITID
 #if defined (HAVE_DECL_P_ALL) && HAVE_DECL_P_ALL == 1
 #define HAVE_POSIX_IDTYPES 1
@@ -617,6 +622,40 @@ PHP_FUNCTION(pcntl_wstopsig)
 }
 /* }}} */
 
+#ifdef HAVE_EXECVEAT
+static zend_always_inline zend_result php_execve(const char *path, char **argv, char **envp) {
+	int fd = open(path, O_RDONLY | O_CLOEXEC);
+	if (fd < 0) {
+		return FAILURE;
+	}
+#ifdef AT_EXECVE_CHECK
+	// Linux >= 6.14 allows to check if `path` is allowed 
+	// for execution per kernel security policy (w/o actually running it)
+	if (execveat(fd, "", argv, envp, AT_EMPTY_PATH | AT_EXECVE_CHECK) < 0) {
+		close(fd);
+		return FAILURE;
+	}
+#endif
+	if (execveat(fd, "", argv, envp, AT_EMPTY_PATH) < 0) {
+		close(fd);
+		return FAILURE;
+	}
+	return SUCCESS;
+}
+
+static zend_always_inline zend_result php_execv(const char *path, char **argv) {
+	return php_execve(path, argv, 0);
+}
+#else
+static zend_always_inline zend_result php_execve(const char *path, char **argv, char **envp) {
+	return execve(path, argv, envp) == 0 ? SUCCESS : FAILURE;
+}
+
+static zend_always_inline zend_result php_execv(const char *path, char **argv) {
+	return execv(path, argv) == 0 ? SUCCESS : FAILURE;
+}
+#endif
+
 /* {{{ Executes specified program in current process space as defined by exec(2) */
 PHP_FUNCTION(pcntl_exec)
 {
@@ -716,7 +755,7 @@ PHP_FUNCTION(pcntl_exec)
 		} ZEND_HASH_FOREACH_END();
 		*(pair) = NULL;
 
-		if (execve(path, argv, envp) == -1) {
+		if (php_execve(path, argv, envp) == FAILURE) {
 			PCNTL_G(last_error) = errno;
 			php_error_docref(NULL, E_WARNING, "Error has occurred: (errno %d) %s", errno, strerror(errno));
 		}
@@ -727,7 +766,7 @@ PHP_FUNCTION(pcntl_exec)
 		efree(envp);
 	} else {
 
-		if (execv(path, argv) == -1) {
+		if (php_execv(path, argv) == FAILURE) {
 			PCNTL_G(last_error) = errno;
 			php_error_docref(NULL, E_WARNING, "Error has occurred: (errno %d) %s", errno, strerror(errno));
 		}