pextlib1.0: re-copy to SIP workaround if pextlib changed

When we modify the copying algorithm for the trace mode SIP workaround,
re-copy the files if the mtime didn't change since they were initially
copied, but their ctime is older than that of the Pextlib.dylib (or
darwintrace.dylib) that contains the copying code.

This allows us to modify the copying algorithm, which will then cause
all previously copied files to be considered outdated.

src/pextlib1.0/sip_copy_proc.c

@@ -46,6 +46,8 @@
 #include <sys/uio.h>
 #include <unistd.h>
 
+#include <mach-o/dyld.h>
+
 #include <config.h>
 
 #ifdef HAVE_COPYFILE
@@ -334,6 +336,46 @@ static int resign(const char *filepath) {
     return result;
 }
 
+static struct timespec determine_pextlib_mtime() {
+    const char *needle_darwintrace = "/darwintrace1.0/darwintrace.dylib";
+    size_t needle_darwintrace_len = strlen(needle_darwintrace);
+    const char *needle_pextlib = "/pextlib1.0/Pextlib.dylib";
+    size_t needle_pextlib_len = strlen(needle_pextlib);
+
+    const char *lib_path = NULL;
+
+    for (uint32_t idx = 0; idx < _dyld_image_count(); ++idx) {
+        const char *image_name = _dyld_get_image_name(idx);
+        size_t image_name_len = strlen(image_name);
+
+        if (image_name_len > needle_darwintrace_len) {
+            if (strcmp(image_name + image_name_len - needle_darwintrace_len, needle_darwintrace) == 0) {
+                lib_path = image_name;
+                break;
+            }
+        }
+        if (image_name_len > needle_pextlib_len) {
+            if (strcmp(image_name + image_name_len - needle_pextlib_len, needle_pextlib) == 0) {
+                lib_path = image_name;
+                break;
+            }
+        }
+    }
+
+    if (lib_path == NULL) {
+        fprintf(stderr, "Failed to find Pextlib.dylib or darwintrace.dylib path in _dyld_get_image_name()\n");
+        abort();
+    }
+
+    struct stat st;
+    if (-1 == stat(lib_path, &st)) {
+        fprintf(stderr, "stat(%s) failed: %s", lib_path, strerror(errno));
+        abort();
+    }
+
+    return st.st_mtimespec;
+}
+
 static char *lazy_copy(const char *path, struct stat *in_st) {
     char *retval = NULL;
     uid_t euid = geteuid();
@@ -381,12 +423,27 @@ static char *lazy_copy(const char *path, struct stat *in_st) {
         goto lazy_copy_out;
     }
 
-    // check whether copying is needed; it isn't if the file exists and the
-    // modification times match
+    /* Check whether copying is needed; it isn't if the file exists and the
+     * modification times match. As an exception to this rule, also copy if the
+     * ctime of the target file is older than the ctime of the Pextlib.dylib
+     * that contains this code. This exception allows updating the copy
+     * mechanism and triggers new copies of files if it possibly changed. */
+    static struct timespec pextlib_mtimespec = {0};
+    if (pextlib_mtimespec.tv_sec == 0 && pextlib_mtimespec.tv_nsec == 0) {
+        pextlib_mtimespec = determine_pextlib_mtime();
+    }
+
     struct stat out_st = {0};
     if (-1 != stat(target_path, &out_st)
         && in_st->st_mtimespec.tv_sec == out_st.st_mtimespec.tv_sec
-        && in_st->st_mtimespec.tv_nsec == out_st.st_mtimespec.tv_nsec) {
+        && in_st->st_mtimespec.tv_nsec == out_st.st_mtimespec.tv_nsec
+        && (
+            out_st.st_ctimespec.tv_sec > pextlib_mtimespec.tv_sec
+            || (
+                out_st.st_ctimespec.tv_sec == pextlib_mtimespec.tv_sec
+                && out_st.st_ctimespec.tv_nsec > pextlib_mtimespec.tv_nsec
+            )
+        )) {
         // copying not needed
         retval = target_path;
         goto lazy_copy_out;