basic/process-util: limit command line lengths to _SC_ARG_MAX

This affects systemd-journald and systemd-coredump.

Example entry:
$ journalctl -o export -n1 'MESSAGE=Something logged'
__CURSOR=s=976542d120c649f494471be317829ef9;i=34e;b=4871e4c474574ce4a462dfe3f1c37f06;m=c7d0c37dd2;t=57c4ac58f3b98;x=67598e942bd23dc0
__REALTIME_TIMESTAMP=1544035467475864
__MONOTONIC_TIMESTAMP=858200964562
_BOOT_ID=4871e4c474574ce4a462dfe3f1c37f06
PRIORITY=6
_UID=1000
_GID=1000
_CAP_EFFECTIVE=0
_SELINUX_CONTEXT=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
_AUDIT_SESSION=1
_AUDIT_LOGINUID=1000
_SYSTEMD_OWNER_UID=1000
_SYSTEMD_UNIT=user@1000.service
_SYSTEMD_SLICE=user-1000.slice
_SYSTEMD_USER_SLICE=-.slice
_SYSTEMD_INVOCATION_ID=1c4a469986d448719cb0f9141a10810e
_MACHINE_ID=08a5690a2eed47cf92ac0a5d2e3cf6b0
_HOSTNAME=krowka
_TRANSPORT=syslog
SYSLOG_FACILITY=17
SYSLOG_IDENTIFIER=syslog-caller
MESSAGE=Something logged
_COMM=poc
_EXE=/home/zbyszek/src/systemd-work3/poc
_SYSTEMD_CGROUP=/user.slice/user-1000.slice/user@1000.service/gnome-terminal-server.service
_SYSTEMD_USER_UNIT=gnome-terminal-server.service
SYSLOG_PID=4108
SYSLOG_TIMESTAMP=Dec  5 19:44:27
_PID=4108
_CMDLINE=./poc AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA>
_SOURCE_REALTIME_TIMESTAMP=1544035467475848

$ journalctl -o export -n1 'MESSAGE=Something logged' --output-fields=_CMDLINE|wc
      6    2053 2097410

2MB might be hard for some clients to use meaningfully, but OTOH, it is
important to log the full commandline sometimes. For example, when the program
is crashing, the exact argument list is useful.

1 files changed, 25 insertions, 48 deletions

diff --git a/src/basic/process-util.c b/src/basic/process-util.c
index 0a4f917cb..a20f1e3cc 100644
--- a/src/basic/process-util.c
+++ b/src/basic/process-util.c
@@ -128,6 +128,13 @@ int get_process_cmdline(pid_t pid, size_t max_length, bool comm_fallback, char *
 
         (void) __fsetlocking(f, FSETLOCKING_BYCALLER);
 
+        if (max_length == 0) {
+                /* This is supposed to be a safety guard against runaway command lines. */
+                long l = sysconf(_SC_ARG_MAX);
+                assert(l > 0);
+                max_length = l;
+        }
+
         if (max_length == 1) {
 
                 /* If there's only room for one byte, return the empty string */
@@ -138,32 +145,6 @@ int get_process_cmdline(pid_t pid, size_t max_length, bool comm_fallback, char *
                 *line = ans;
                 return 0;
 
-        } else if (max_length == 0) {
-                size_t len = 0, allocated = 0;
-
-                while ((c = getc(f)) != EOF) {
-
-                        if (!GREEDY_REALLOC(ans, allocated, len+3)) {
-                                free(ans);
-                                return -ENOMEM;
-                        }
-
-                        if (isprint(c)) {
-                                if (space) {
-                                        ans[len++] = ' ';
-                                        space = false;
-                                }
-
-                                ans[len++] = c;
-                        } else if (len > 0)
-                                space = true;
-               }
-
-                if (len > 0)
-                        ans[len] = '\0';
-                else
-                        ans = mfree(ans);
-
         } else {
                 bool dotdotdot = false;
                 size_t left;
@@ -235,34 +216,30 @@ int get_process_cmdline(pid_t pid, size_t max_length, bool comm_fallback, char *
                 if (h < 0)
                         return h;
 
-                if (max_length == 0)
-                        ans = strjoin("[", t, "]");
-                else {
-                        size_t l;
-
-                        l = strlen(t);
+                size_t l = strlen(t);
 
-                        if (l + 3 <= max_length)
-                                ans = strjoin("[", t, "]");
-                        else if (max_length <= 6) {
+                if (l + 3 <= max_length) {
+                        ans = strjoin("[", t, "]");
+                        if (!ans)
+                                return -ENOMEM;
 
-                                ans = new(char, max_length);
-                                if (!ans)
-                                        return -ENOMEM;
+                } else if (max_length <= 6) {
+                        ans = new(char, max_length);
+                        if (!ans)
+                                return -ENOMEM;
 
-                                memcpy(ans, "[...]", max_length-1);
-                                ans[max_length-1] = 0;
-                        } else {
-                                t[max_length - 6] = 0;
+                        memcpy(ans, "[...]", max_length-1);
+                        ans[max_length-1] = 0;
+                } else {
+                        t[max_length - 6] = 0;
 
-                                /* Chop off final spaces */
-                                delete_trailing_chars(t, WHITESPACE);
+                        /* Chop off final spaces */
+                        delete_trailing_chars(t, WHITESPACE);
 
-                                ans = strjoin("[", t, "...]");
-                        }
+                        ans = strjoin("[", t, "...]");
+                        if (!ans)
+                                return -ENOMEM;
                 }
-                if (!ans)
-                        return -ENOMEM;
         }
 
         *line = ans;