config: don't pass null strings to *printf()

musl and glibc's *printf() convert NULL strings to "(null)" [0][1], but
this is undefined behaviour.

context.value has to be set to two backspaces, so that the extra colon
is removed. context.key is set to one backspace, so that the extra dot
is removed. context.section is now set to "main" by default, so it is
never NULL.

[0]: https://git.musl-libc.org/cgit/musl/tree/src/stdio/vfprintf.c#n593
[1]: https://sourceware.org/git/?p=glibc.git;a=blob;f=stdio-common/vfprintf-internal.c;hb=HEAD#l1011

config.c

@@ -209,10 +209,19 @@ log_contextual(struct context *ctx, enum log_class log_class,
     char *formatted_msg = xvasprintf(fmt, va);
     va_end(va);
 
+    bool print_dot = ctx->key != NULL;
+    bool print_colon = ctx->value != NULL;
+
+    if (!print_dot)
+        ctx->key = "";
+
+    if (!print_colon)
+        ctx->value = "";
+
     log_and_notify(
-        ctx->conf, log_class, file, lineno, "%s:%d: [%s].%s: %s: %s",
-        ctx->path, ctx->lineno, ctx->section, ctx->key, ctx->value,
-        formatted_msg);
+        ctx->conf, log_class, file, lineno, "%s:%d: [%s]%s%s%s%s: %s",
+        ctx->path, ctx->lineno, ctx->section, print_dot ? "." : "",
+        ctx->key, print_colon ? ": " : "", ctx->value, formatted_msg);
     free(formatted_msg);
 }
 
@@ -2321,31 +2330,41 @@ parse_section_tweak(struct context *ctx)
 static bool
 parse_key_value(char *kv, const char **section, const char **key, const char **value)
 {
+    bool section_is_needed = section != NULL;
+
     /*strip leading whitespace*/
     while (*kv && isspace(*kv))
         ++kv;
 
-    if (section != NULL)
-        *section = NULL;
+    if (section_is_needed)
+        *section = "main";
+
+    if (kv[0] == '=')
+        return false;
+
     *key = kv;
     *value = NULL;
 
     size_t kvlen = strlen(kv);
     for (size_t i = 0; i < kvlen; ++i) {
-        if (kv[i] == '.') {
-            if (section != NULL && *section == NULL) {
-                *section = kv;
-                kv[i] = '\0';
-                *key = &kv[i + 1];
-            }
-        } else if (kv[i] == '=') {
-            if (section != NULL && *section == NULL)
-                *section = "main";
+        if (kv[i] == '.' && section_is_needed) {
+            section_is_needed = false;
+            *section = kv;
             kv[i] = '\0';
+            if (i == kvlen - 1 || kv[i + 1] == '=') {
+                *key = NULL;
+                return false;
+            }
+            *key = &kv[i + 1];
+        } else if (kv[i] == '=') {
+            kv[i] = '\0';
+            if (i == kvlen - 1)
+                return false;
             *value = &kv[i + 1];
             break;
         }
     }
+
     if (*value == NULL)
         return false;
 
@@ -2499,22 +2518,37 @@ parse_config_file(FILE *f, struct config *conf, const char *path, bool errors_ar
 
         /* Check for new section */
         if (key_value[0] == '[') {
+            key_value++;
+
+            if (key_value[0] == ']') {
+                LOG_CONTEXTUAL_ERR("empty section name");
+                error_or_continue();
+            }
+
+            context.section = key_value;
             char *end = strchr(key_value, ']');
+
             if (end == NULL) {
                 LOG_CONTEXTUAL_ERR("syntax error: no closing ']'");
                 error_or_continue();
             }
 
-            *end = '\0';
+            end[0] = '\0';
 
-            section = str_to_section(&key_value[1]);
+            if (end[1] != '\0') {
+                LOG_CONTEXTUAL_ERR("section declaration contains trailing "
+                                   "characters");
+                error_or_continue();
+            }
+
+            section = str_to_section(key_value);
             if (section == SECTION_COUNT) {
-                LOG_CONTEXTUAL_ERR("invalid section name: %s", &key_value[1]);
+                LOG_CONTEXTUAL_ERR("invalid section name: %s", key_value);
                 error_or_continue();
             }
 
             free(section_name);
-            section_name = xstrdup(&key_value[1]);
+            section_name = xstrdup(key_value);
             context.section = section_name;
 
             /* Process next line */
@@ -2527,7 +2561,8 @@ parse_config_file(FILE *f, struct config *conf, const char *path, bool errors_ar
         }
 
         if (!parse_key_value(key_value, NULL, &context.key, &context.value)) {
-            LOG_CONTEXTUAL_ERR("syntax error: key/value pair has no value");
+            LOG_CONTEXTUAL_ERR("syntax error: key/value pair has no %s",
+                               context.key == NULL ? "key" : "value");
             if (errors_are_fatal)
                 goto err;
             break;
@@ -2977,7 +3012,15 @@ config_override_apply(struct config *conf, config_override_t *overrides,
         if (!parse_key_value(
                 it->item, &context.section, &context.key, &context.value))
         {
-            LOG_CONTEXTUAL_ERR("syntax error: key/value pair has no value");
+            LOG_CONTEXTUAL_ERR("syntax error: key/value pair has no %s",
+                               context.key == NULL ? "key" : "value");
+            if (errors_are_fatal)
+                return false;
+            continue;
+        }
+
+        if (context.section[0] == '\0') {
+            LOG_CONTEXTUAL_ERR("empty section name");
             if (errors_are_fatal)
                 return false;
             continue;