Switch to internal API and add fallback user for fstatat()

- Add `fstatat_with_fallback_user` function to handle UIDs in `my_fstatat.c`
- Add `#include <stdbool.h>` and `bool retried` to `proc_parse.c`
- Add `retry_o_path` goto and `if (!retried)` check to `proc_parse.c`
- Use `fstatat_with_fallback_user

[criu/my_fstatat.c]
- Add `fstatat_with_fallback_user` function
- Set the UID based on the `CURRENT_TASK_USER` environment variable
- Rollback to the original UID after the call to `fstatat`
[criu/proc_parse.c]
- Add `#include <stdbool.h>`
- Add `bool retried`
- Add `retry_o_path` goto
- Add `if (!retried)` check
- Add `flags = O_PATH`
[criu/files-reg.c]
- Include `my_fstatat.c`
- Use `fstatat_with_fallback_user` instead of `fstatat`

criu/files-reg.c

@@ -44,6 +44,8 @@
 #include "files-reg.h"
 #include "plugin.h"
 
+#include "my_fstatat.c"
+
 int setfsuid(uid_t fsuid);
 int setfsgid(gid_t fsuid);
 
@@ -1206,7 +1208,7 @@ static int check_path_remap(struct fd_link *link, const struct fd_parms *parms,
 	if (mntns_root < 0)
 		return -1;
 
-	ret = fstatat(mntns_root, rpath, &pst, 0);
+	ret = fstatat_with_fallback_user(mntns_root, rpath, &pst, 0);
 	if (ret < 0) {
 		/*
 		 * Linked file, but path is not accessible (unless any

criu/my_fstatat.c

+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <pwd.h>
+#include <grp.h>
+#include <fcntl.h>
+#include <sys/stat.h>
+
+static int fstatat_with_fallback_user (int __fd, const char *__restrict __file,
+		    struct stat *__restrict __buf, int __flag) {
+	int rst;
+	rst = fstatat(__fd, __file, __buf, __flag);
+	if (rst >= 0) {
+		return rst;
+	}
+
+	char *current_task_user = getenv("CURRENT_TASK_USER");
+    if (current_task_user == NULL) {
+        fprintf(stderr, "Error: no env CURRENT_TASK_USER '%s'\n", current_task_user);
+		return rst;
+    }
+
+    struct passwd *pw = getpwnam(current_task_user);
+    if (pw == NULL) {
+        fprintf(stderr, "Error: no such user '%s'\n", current_task_user);
+		return rst;
+    }
+    uid_t uid = pw->pw_uid;
+    printf("UID for user '%s' is %d\n", current_task_user, uid);
+
+	seteuid(uid);
+
+	rst = fstatat(__fd, __file, __buf, __flag);
+
+   if (seteuid(getuid()) == -1) {
+        fprintf(stderr, "Error: rollback to original uid failed'\n");
+    }
+	return rst;
+}

criu/proc_parse.c

+#include <stdbool.h>
 #include <stdio.h>
 #include <unistd.h>
 #include <fcntl.h>
@@ -352,6 +353,7 @@ static int vma_get_mapfile(const char *fname, struct vma_area *vma, DIR *mfd,
 {
 	char path[32];
 	int flags;
+	bool retried;
 
 	/* Figure out if it's file mapping */
 	snprintf(path, sizeof(path), "%"PRIx64"-%"PRIx64, vma->e->start, vma->e->end);
@@ -403,6 +405,7 @@ static int vma_get_mapfile(const char *fname, struct vma_area *vma, DIR *mfd,
 		 */
 		flags = O_RDONLY;
 
+retry_o_path:
 	*vm_file_fd = openat(dirfd(mfd), path, flags);
 	if (*vm_file_fd < 0) {
 		if (errno == ENOENT)
@@ -437,6 +440,12 @@ static int vma_get_mapfile(const char *fname, struct vma_area *vma, DIR *mfd,
 			return vma_get_mapfile_user(fname, vma, vfi, vm_file_fd, path);
 
 		pr_perror("Can't open map_files");
+
+        if (!retried) {
+			flags = O_PATH;
+     	   	retried = true;
+     	   	goto retry_o_path;
+        }
 		return -1;
 	}