util-lib: rework rename_process() to be able to make use of PR_SET_MM_ARG_START

PR_SET_MM_ARG_START allows us to relatively cleanly implement process renaming.
However, it's only available with privileges. Hence, let's try to make use of
it, and if we can't fall back to the traditional way of overriding argv[0].

This removes size restrictions on the process name shown in argv[] at least for
privileged processes.

2 files changed, 90 insertions, 14 deletions

diff --git a/src/basic/process-util.c b/src/basic/process-util.c
index 1f4c2e4e43..d5e7edb589 100644
--- a/src/basic/process-util.c
+++ b/src/basic/process-util.c
@@ -27,6 +27,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sys/mman.h>
 #include <sys/personality.h>
 #include <sys/prctl.h>
 #include <sys/types.h>
@@ -274,27 +275,100 @@ int get_process_cmdline(pid_t pid, size_t max_length, bool comm_fallback, char *
         return 0;
 }
 
-void rename_process(const char name[8]) {
-        assert(name);
+int rename_process(const char name[]) {
+        static size_t mm_size = 0;
+        static char *mm = NULL;
+        bool truncated = false;
+        size_t l;
+
+        /* This is a like a poor man's setproctitle(). It changes the comm field, argv[0], and also the glibc's
+         * internally used name of the process. For the first one a limit of 16 chars applies; to the second one in
+         * many cases one of 10 (i.e. length of "/sbin/init") — however if we have CAP_SYS_RESOURCES it is unbounded;
+         * to the third one 7 (i.e. the length of "systemd". If you pass a longer string it will likely be
+         * truncated.
+         *
+         * Returns 0 if a name was set but truncated, > 0 if it was set but not truncated. */
+
+        if (isempty(name))
+                return -EINVAL; /* let's not confuse users unnecessarily with an empty name */
 
-        /* This is a like a poor man's setproctitle(). It changes the
-         * comm field, argv[0], and also the glibc's internally used
-         * name of the process. For the first one a limit of 16 chars
-         * applies, to the second one usually one of 10 (i.e. length
-         * of "/sbin/init"), to the third one one of 7 (i.e. length of
-         * "systemd"). If you pass a longer string it will be
-         * truncated */
+        l = strlen(name);
 
+        /* First step, change the comm field. */
         (void) prctl(PR_SET_NAME, name);
+        if (l > 15) /* Linux process names can be 15 chars at max */
+                truncated = true;
+
+        /* Second step, change glibc's ID of the process name. */
+        if (program_invocation_name) {
+                size_t k;
+
+                k = strlen(program_invocation_name);
+                strncpy(program_invocation_name, name, k);
+                if (l > k)
+                        truncated = true;
+        }
+
+        /* Third step, completely replace the argv[] array the kernel maintains for us. This requires privileges, but
+         * has the advantage that the argv[] array is exactly what we want it to be, and not filled up with zeros at
+         * the end. This is the best option for changing /proc/self/cmdline.*/
+        if (mm_size < l+1) {
+                size_t nn_size;
+                char *nn;
+
+                /* Let's not bother with this if we don't have euid == 0. Strictly speaking if people do weird stuff
+                 * with capabilities this could work even for euid != 0, but our own code generally doesn't do that,
+                 * hence let's use this as quick bypass check, to avoid calling mmap() if PR_SET_MM_ARG_START fails
+                 * with EPERM later on anyway. After all geteuid() is dead cheap to call, but mmap() is not. */
+                if (geteuid() != 0) {
+                        log_debug("Skipping PR_SET_MM_ARG_START, as we don't have privileges.");
+                        goto use_saved_argv;
+                }
+
+                nn_size = PAGE_ALIGN(l+1);
+                nn = mmap(NULL, nn_size, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
+                if (nn == MAP_FAILED) {
+                        log_debug_errno(errno, "mmap() failed: %m");
+                        goto use_saved_argv;
+                }
+
+                strncpy(nn, name, nn_size);
+
+                /* Now, let's tell the kernel about this new memory */
+                if (prctl(PR_SET_MM, PR_SET_MM_ARG_START, (unsigned long) nn, 0, 0) < 0) {
+                        log_debug_errno(errno, "PR_SET_MM_ARG_START failed, proceeding without: %m");
+                        (void) munmap(nn, nn_size);
+                        goto use_saved_argv;
+                }
+
+                /* And update the end pointer to the new end, too. If this fails, we don't really know what to do, it's
+                 * pretty unlikely that we can rollback, hence we'll just accept the failure, and continue. */
+                if (prctl(PR_SET_MM, PR_SET_MM_ARG_END, (unsigned long) nn + l + 1, 0, 0) < 0)
+                        log_debug_errno(errno, "PR_SET_MM_ARG_END failed, proceeding without: %m");
 
-        if (program_invocation_name)
-                strncpy(program_invocation_name, name, strlen(program_invocation_name));
+                if (mm)
+                        (void) munmap(mm, mm_size);
+
+                mm = nn;
+                mm_size = nn_size;
+        } else
+                strncpy(mm, name, mm_size);
+
+use_saved_argv:
+        /* Fourth step: in all cases we'll also update the original argv[], so that our own code gets it right too if
+         * it still looks here */
 
         if (saved_argc > 0) {
                 int i;
 
-                if (saved_argv[0])
-                        strncpy(saved_argv[0], name, strlen(saved_argv[0]));
+                if (saved_argv[0]) {
+                        size_t k;
+
+                        k = strlen(saved_argv[0]);
+                        strncpy(saved_argv[0], name, k);
+                        if (l > k)
+                                truncated = true;
+                }
 
                 for (i = 1; i < saved_argc; i++) {
                         if (!saved_argv[i])
@@ -303,6 +377,8 @@ void rename_process(const char name[8]) {
                         memzero(saved_argv[i], strlen(saved_argv[i]));
                 }
         }
+
+        return !truncated;
 }
 
 int is_kernel_thread(pid_t pid) {
diff --git a/src/basic/process-util.h b/src/basic/process-util.h
index 89dfeb4d6a..d378901399 100644
--- a/src/basic/process-util.h
+++ b/src/basic/process-util.h
@@ -64,7 +64,7 @@ void sigkill_waitp(pid_t *pid);
 
 int kill_and_sigcont(pid_t pid, int sig);
 
-void rename_process(const char name[8]);
+int rename_process(const char name[]);
 int is_kernel_thread(pid_t pid);
 
 int getenv_for_pid(pid_t pid, const char *field, char **_value);