process-util: dont allocate max length to read /proc/PID/cmdline

Alternative title: Replace get_process_cmdline()'s fopen()/fread() with
read_full_virtual_file().

When RLIMIT_STACK is set to infinity:infinity, _SC_ARG_MAX will
return 4611686018427387903 (depending on the system, but definitely
something larger than most systems have). It's impractical to allocate this
in one go when most cmdlines are much shorter than that.

Instead use read_full_virtual_file() which seems to increase the buffer
depending on the size of the contents.

src/basic/process-util.c

@@ -124,14 +124,10 @@ int get_process_comm(pid_t pid, char **ret) {
 }
 
 int get_process_cmdline(pid_t pid, size_t max_columns, ProcessCmdlineFlags flags, char **line) {
-        _cleanup_fclose_ FILE *f = NULL;
         _cleanup_free_ char *t = NULL, *ans = NULL;
         const char *p;
-        int r;
         size_t k;
-
-        /* This is supposed to be a safety guard against runaway command lines. */
-        size_t max_length = sc_arg_max();
+        int r;
 
         assert(line);
         assert(pid >= 0);
@@ -147,36 +143,18 @@ int get_process_cmdline(pid_t pid, size_t max_columns, ProcessCmdlineFlags flags
          * comm_fallback is false). Returns 0 and sets *line otherwise. */
 
         p = procfs_file_alloca(pid, "cmdline");
-        r = fopen_unlocked(p, "re", &f);
+        r = read_full_virtual_file(p, &t, &k);
         if (r == -ENOENT)
                 return -ESRCH;
         if (r < 0)
                 return r;
 
-        /* We assume that each four-byte character uses one or two columns. If we ever check for combining
-         * characters, this assumption will need to be adjusted. */
-        if ((size_t) 4 * max_columns + 1 < max_columns)
-                max_length = MIN(max_length, (size_t) 4 * max_columns + 1);
-
-        t = new(char, max_length);
-        if (!t)
-                return -ENOMEM;
-
-        k = fread(t, 1, max_length, f);
         if (k > 0) {
                 /* Arguments are separated by NULs. Let's replace those with spaces. */
                 for (size_t i = 0; i < k - 1; i++)
                         if (t[i] == '\0')
                                 t[i] = ' ';
-
-                t[k] = '\0'; /* Normally, t[k] is already NUL, so this is just a guard in case of short read */
         } else {
-                /* We only treat getting nothing as an error. We *could* also get an error after reading some
-                 * data, but we ignore that case, as such an error is rather unlikely and we prefer to get
-                 * some data rather than none. */
-                if (ferror(f))
-                        return -errno;
-
                 if (!(flags & PROCESS_CMDLINE_COMM_FALLBACK))
                         return -ENOENT;
 
@@ -187,7 +165,7 @@ int get_process_cmdline(pid_t pid, size_t max_columns, ProcessCmdlineFlags flags
                 if (r < 0)
                         return r;
 
-                mfree(t);
+                free(t);
                 t = strjoin("[", t2, "]");
                 if (!t)
                         return -ENOMEM;

src/test/test-process-util.c

@@ -236,6 +236,11 @@ static void test_get_process_cmdline_harder(void) {
                 return;
         }
 
+        /* Set RLIMIT_STACK to infinity to test we don't try to allocate unncessarily large values to read
+         * the cmdline. */
+        if (setrlimit(RLIMIT_STACK, &RLIMIT_MAKE_CONST(RLIM_INFINITY)) < 0)
+                log_warning("Testing without RLIMIT_STACK=infinity");
+
         assert_se(unlink(path) >= 0);
 
         assert_se(prctl(PR_SET_NAME, "testa") >= 0);