syslinux/extlinux: check if config file is empty

For some reason (may be a user manual intervention or some issue on previous
updates) the syslinux/extlinux config file may be empty and in that case cbm will
segfault when reading the content's buffer.

Signed-off-by: Leandro Dorileo <[email protected]>

src/bootloaders/extlinux.c

@@ -194,7 +194,7 @@ static bool extlinux_set_default_kernel(const BootManager *manager, const Kernel
         }
 
         /* If the file is the same, don't write it again or sync */
-        if (file_get_text(config_path, &old_conf)) {
+        if (cbm_file_has_content(config_path) && file_get_text(config_path, &old_conf)) {
                 if (streq(old_conf, writer->buffer)) {
                         return true;
                 }

src/bootloaders/syslinux.c

@@ -223,7 +223,7 @@ static bool syslinux_set_default_kernel(const BootManager *manager, const Kernel
         }
 
         /* If the file is the same, don't write it again or sync */
-        if (file_get_text(config_path, &old_conf)) {
+        if (cbm_file_has_content(config_path) && file_get_text(config_path, &old_conf)) {
                 if (streq(old_conf, writer->buffer)) {
                         return true;
                 }

src/lib/files.c

@@ -169,6 +169,32 @@ static bool get_parent_disk_devno(char *path, dev_t *diskdevno)
         return true;
 }
 
+bool cbm_file_has_content(char *path)
+{
+        int fd = -1;
+        struct stat st = { 0 };
+        ssize_t length = -1;
+
+        if (!nc_file_exists(path)) {
+                return false;
+        }
+
+        fd = open(path, O_RDONLY);
+        if (fd < 0) {
+                return false;
+        }
+
+        if (fstat(fd, &st) != 0) {
+                close(fd);
+                return false;
+        }
+
+        length = st.st_size;
+        close(fd);
+
+        return length != 0;
+}
+
 char *get_parent_disk(char *path)
 {
         dev_t devt;

src/lib/files.h

@@ -180,6 +180,11 @@ bool cbm_path_check(const char *path, const char *resolved);
  */
 bool cbm_is_dir_empty(const char *path);
 
+/**
+ * Check if @path exists and if it's empty.
+ */
+bool cbm_file_has_content(char *path);
+
 /**
  * Ensure a stack pointer vs a heap pointer, to save on copies
  */