pidref: add explicit concept of "remote" PidRef

This PidRef just track some data, but cannot be used for any active
operation.

Background: for https://github.com/systemd/systemd/pull/34703 it makes
sense to track explicitly if some PidRef is not a local one, so that we
never attempt to for example "kill a remote process" and thus
acccidentally hit the wrong process (i.e. a local one by the same PID).

diff --git a/src/basic/pidref.c b/src/basic/pidref.c
index 1defe863e2b3e6ade74f2a3abafb371ac6bae1fe..b13cc96d6a1f59d0f0afab9b7c254fc93291c9de 100644 (file)
--- a/src/basic/pidref.c
+++ b/src/basic/pidref.c
@@ -40,6 +40,9 @@ int pidref_acquire_pidfd_id(PidRef *pidref) {
         if (!pidref_is_set(pidref))
                 return -ESRCH;
 
+        if (pidref_is_remote(pidref))
+                return -EREMOTE;
+
         if (pidref->fd < 0)
                 return -ENOMEDIUM;
 
@@ -64,23 +67,36 @@ int pidref_acquire_pidfd_id(PidRef *pidref) {
 
 bool pidref_equal(PidRef *a, PidRef *b) {
 
-        if (pidref_is_set(a)) {
-                if (!pidref_is_set(b))
+        if (!pidref_is_set(a))
+                return !pidref_is_set(b);
+
+        if (!pidref_is_set(b))
+                return false;
+
+        if (a->pid != b->pid)
+                return false;
+
+        if (pidref_is_remote(a)) {
+                /* If one is remote and the other isn't, they are not the same */
+                if (!pidref_is_remote(b))
                         return false;
 
-                if (a->pid != b->pid)
+                /* If both are remote, compare fd IDs if we have both, otherwise don't bother, and cut things short */
+                if (a->fd_id == 0 || b->fd_id == 0)
+                        return true;
+        } else {
+                if (pidref_is_remote(b))
                         return false;
 
-                /* Try to compare pidfds using their inode numbers. This way we can ensure that we don't
-                 * spuriously consider two PidRefs equal if the pid has been reused once. Note that we
-                 * ignore all errors here, not only EOPNOTSUPP, as fstat() might fail due to many reasons. */
+                /* Try to compare pidfds using their inode numbers. This way we can ensure that we
+                 * don't spuriously consider two PidRefs equal if the pid has been reused once. Note
+                 * that we ignore all errors here, not only EOPNOTSUPP, as fstat() might fail due to
+                 * many reasons. */
                 if (pidref_acquire_pidfd_id(a) < 0 || pidref_acquire_pidfd_id(b) < 0)
                         return true;
-
-                return a->fd_id == b->fd_id;
         }
 
-        return !pidref_is_set(b);
+        return a->fd_id == b->fd_id;
 }
 
 int pidref_set_pid(PidRef *pidref, pid_t pid) {
@@ -240,7 +256,9 @@ int pidref_copy(const PidRef *pidref, PidRef *dest) {
         assert(dest);
 
         if (pidref) {
-                if (pidref->fd >= 0) {
+                if (pidref_is_remote(pidref)) /* Propagate remote flag */
+                        dup_fd = -EREMOTE;
+                else if (pidref->fd >= 0) {
                         dup_fd = fcntl(pidref->fd, F_DUPFD_CLOEXEC, 3);
                         if (dup_fd < 0) {
                                 if (!ERRNO_IS_RESOURCE(errno))
@@ -311,6 +329,9 @@ int pidref_kill(const PidRef *pidref, int sig) {
         if (!pidref)
                 return -ESRCH;
 
+        if (pidref_is_remote(pidref))
+                return -EREMOTE;
+
         if (pidref->fd >= 0)
                 return RET_NERRNO(pidfd_send_signal(pidref->fd, sig, NULL, 0));
 
@@ -338,6 +359,9 @@ int pidref_sigqueue(const PidRef *pidref, int sig, int value) {
         if (!pidref)
                 return -ESRCH;
 
+        if (pidref_is_remote(pidref))
+                return -EREMOTE;
+
         if (pidref->fd >= 0) {
                 siginfo_t si;
 
@@ -370,6 +394,9 @@ int pidref_verify(const PidRef *pidref) {
         if (!pidref_is_set(pidref))
                 return -ESRCH;
 
+        if (pidref_is_remote(pidref))
+                return -EREMOTE;
+
         if (pidref->pid == 1)
                 return 1; /* PID 1 can never go away, hence never be recycled to a different process → return 1 */
 
@@ -387,6 +414,9 @@ bool pidref_is_self(const PidRef *pidref) {
         if (!pidref)
                 return false;
 
+        if (pidref_is_remote(pidref))
+                return false;
+
         return pidref->pid == getpid_cached();
 }
 
@@ -396,6 +426,9 @@ int pidref_wait(const PidRef *pidref, siginfo_t *ret, int options) {
         if (!pidref_is_set(pidref))
                 return -ESRCH;
 
+        if (pidref_is_remote(pidref))
+                return -EREMOTE;
+
         if (pidref->pid == 1 || pidref->pid == getpid_cached())
                 return -ECHILD;
 

diff --git a/src/basic/pidref.h b/src/basic/pidref.h
index 8647f4bd23bf489c473a2d4f91fe5633ba22c3e4..494bc9d8f199cd884962d1ef38588f609bae118d 100644 (file)
--- a/src/basic/pidref.h
+++ b/src/basic/pidref.h
@@ -6,10 +6,35 @@ typedef struct PidRef PidRef;
 #include "macro.h"
 #include "process-util.h"
 
-/* An embeddable structure carrying a reference to a process. Supposed to be used when tracking processes continuously. */
+/* An embeddable structure carrying a reference to a process. Supposed to be used when tracking processes
+ * continuously. This combines a PID, a modern Linux pidfd and the 64bit inode number of the pidfd into one
+ * structure. Note that depending on kernel support the pidfd might not be initialized, and if it is
+ * initialized then fd_id might still not be initialized (because the concept was added to the kernel much
+ * later than pidfds themselves).
+ *
+ * There are three special states a PidRef can be in:
+ *
+ * 1. It can be *unset*. Use pidref_is_set() to detect this case. Most operations attempted on such a PidRef
+ *    will fail with -ESRCH. Use PIDREF_NULL for initializing a PidRef in this state.
+ *
+ * 2. It can be marked as *automatic*. This is a special state indicating that a process reference is
+ *    supposed to be derived automatically from the current context. This is used by the Varlink/JSON
+ *    dispatcher as indication that a PidRef shall be derived from the connection peer, but might be
+ *    otherwise used too. When marked *automatic* the PidRef will also be considered *unset*, hence most
+ *    operations will fail with -ESRCH, as above.
+ *
+ * 3. It can be marked as *remote*. This is useful when deserializing a PidRef structure from an IPC message
+ *    or similar, and it has been determined that the given PID definitely doesn't refer to a local
+ *    process. In this case the PidRef logic will refrain from trying to acquire a pidfd for the
+ *    process. Moreover, most operations will fail with -EREMOTE. Only PidRef structures that are not marked
+ *    *unset* can be marked *remote*.
+ */
 struct PidRef {
-        pid_t pid;      /* always valid */
-        int fd;         /* only valid if pidfd are available in the kernel, and we manage to get an fd */
+        pid_t pid;      /* > 0 if the PidRef is set, otherwise set to PID_AUTOMATIC if automatic mode is
+                         * desired, or 0 otherwise. */
+        int fd;         /* only valid if pidfd are available in the kernel, and we manage to get an fd. If we
+                         * know that the PID is not from the local machine we set this to -EREMOTE, otherwise
+                         * we use -EBADF as indicator the fd is invalid. */
         uint64_t fd_id; /* the inode number of pidfd. only useful in kernel 6.9+ where pidfds live in
                            their own pidfs and each process comes with a unique inode number */
 };
@@ -31,6 +56,12 @@ static inline bool pidref_is_set(const PidRef *pidref) {
 
 bool pidref_is_automatic(const PidRef *pidref);
 
+static inline bool pidref_is_remote(const PidRef *pidref) {
+        /* If the fd is set to -EREMOTE we assume PidRef does not refer to a local PID, but on another
+         * machine (and we just got the PidRef initialized due to deserialization of some RPC message) */
+        return pidref_is_set(pidref) && pidref->fd == -EREMOTE;
+}
+
 int pidref_acquire_pidfd_id(PidRef *pidref);
 bool pidref_equal(PidRef *a, PidRef *b);
 

diff --git a/src/test/test-pidref.c b/src/test/test-pidref.c
index 7298b36596805f29d2ebcee362db82bddaf98f88..5535e98ab07b424b35ba825e0d5707ca2cf7973d 100644 (file)
--- a/src/test/test-pidref.c
+++ b/src/test/test-pidref.c
@@ -240,4 +240,26 @@ TEST(pidref_is_automatic) {
         assert_se(!pid_is_valid(PID_AUTOMATIC));
 }
 
+TEST(pidref_is_remote) {
+        assert_se(!pidref_is_remote(NULL));
+        assert_se(!pidref_is_remote(&PIDREF_NULL));
+        assert_se(!pidref_is_remote(&PIDREF_MAKE_FROM_PID(1)));
+        assert_se(!pidref_is_remote(&PIDREF_MAKE_FROM_PID(getpid_cached())));
+        assert_se(!pidref_is_remote(&PIDREF_AUTOMATIC));
+
+        static const PidRef p = {
+                .pid = 1,
+                .fd = -EREMOTE,
+                .fd_id = 4711,
+        };
+
+        assert_se(pidref_is_set(&p));
+        assert_se(pidref_is_remote(&p));
+        assert_se(!pidref_is_automatic(&p));
+        assert_se(pidref_kill(&p, SIGTERM) == -EREMOTE);
+        assert_se(pidref_kill_and_sigcont(&p, SIGTERM) == -EREMOTE);
+        assert_se(pidref_wait_for_terminate(&p, /* ret= */ NULL) == -EREMOTE);
+        assert_se(pidref_verify(&p) == -EREMOTE);
+}
+
 DEFINE_TEST_MAIN(LOG_DEBUG);