# libkernel_sys technical debt

### Functional

- [fchdir](../../../core/libs/prx/libkernel_sys/Export.cpp) (libkernel_sys) changes the guest working directory to the descriptor's directory: on Windows through the path recorded when the descriptor was opened, on Linux through the `/proc/self/fd` link target. It returns `EBADF` for a descriptor that is not open and `ENOTDIR` for one that is not a directory. FreeBSD `change_dir`'s search-permission check (`EACCES`) is not modelled, and a directory renamed after it was opened is still reached through the recorded path. Console behaviour is unverified
- [openat](../../../core/libs/prx/libkernel_sys/Export.cpp) (libkernel_sys) passes `AT_FDCWD` (-100) and absolute paths to `open`. Any other relative path is joined to the descriptor's host directory, found the same way as for `fchdir`, so guest path aliases and case-insensitive name matching do not apply to it, and a `..` component is not kept inside the guest root. It returns `EBADF` for a descriptor that is not open and `ENOTDIR` for one that is not a directory. As with `open`, `O_NOFOLLOW` and `O_NONBLOCK` are ignored. PPSA99003 opens directory entries this way. Console behaviour is unverified
- [fchmodat](../../../core/libs/prx/libkernel_sys/Export.cpp) (libkernel_sys) passes `AT_FDCWD` (-100) and absolute paths to `chmod`; any other relative path is joined to the descriptor's host directory the same way as for `openat`, so guest path aliases and case-insensitive name matching do not apply to it and a `..` component is not kept inside the guest root. A flag other than `0` or `AT_SYMLINK_NOFOLLOW` (0x200) returns `EINVAL`; the mode is masked to `07777`. With `AT_SYMLINK_NOFOLLOW` a symbolic link throws, since the host cannot change a link's own mode, while a regular file is changed either way. The mode maps to the host file mode, which on Windows is only the CRT's read-only bit. PPSA99621 calls it through `std::filesystem::permissions`. Console behaviour is unverified
- [pathconf](../../../core/libs/prx/libkernel_sys/Export.cpp) (libkernel_sys) looks the path up as FreeBSD 11's `kern_pathconf` does (links followed, `ENAMETOOLONG` for a path of 1024 bytes or more or a component over 255) and returns the constants of FreeBSD 11's UFS `ufs_pathconf` for files and directories alike: `_PC_LINK_MAX` 32767, `_PC_NAME_MAX` 255, `_PC_PATH_MAX` and `_PC_SYMLINK_MAX` 1024, `_PC_PIPE_BUF` 512, `_PC_CHOWN_RESTRICTED` and `_PC_NO_TRUNC` 1, `_PC_FILESIZEBITS` 64, `_PC_REC_MAX_XFER_SIZE` -1, `_PC_PRIO_IO` and `_PC_SYNC_IO` 0. Whether the console's filesystems report the same is not verified. It throws for the names whose UFS value depends on the mount (`_PC_ALLOC_SIZE_MIN`, `_PC_REC_INCR_XFER_SIZE`, `_PC_REC_MIN_XFER_SIZE`, `_PC_MIN_HOLE_SIZE`), the kernel options (`_PC_ACL_EXTENDED`, `_PC_ACL_PATH_MAX`, `_PC_ACL_NFS4`, `_PC_MAC_PRESENT`) or the page size (`_PC_REC_XFER_ALIGN`), and on which UFS and `vop_stdpathconf` disagree (`_PC_MAX_CANON`, `_PC_MAX_INPUT`, `_PC_VDISABLE`, `_PC_ASYNC_IO`); other names fail with `EINVAL`
- [mkdirat](../../../core/libs/prx/libkernel_sys/Export.cpp) (libkernel_sys) passes `AT_FDCWD` (-100) and absolute paths to `mkdir`. Any other relative path is joined to the descriptor's host directory, found the same way as for `fchdir`, so guest path aliases and case-insensitive name matching do not apply to it, and a `..` component is not kept inside the guest root. It returns `EBADF` for a descriptor that is not open and `ENOTDIR` for one that is not a directory. On Windows, `mode` is ignored. Console behaviour is unverified
- [readlink](../../../core/libs/prx/libkernel_sys/Export.cpp) (libkernel_sys) on Windows reads a symbolic link's target from its reparse data and returns it with `/` separators, only when the target is relative, as `symlink` creates them. A symbolic link with an absolute target and a junction throw, as a host path has no guest form. Any other reparse point throws too, where FreeBSD reports `EINVAL` for a file that is not a link
- [readlinkat](../../../core/libs/prx/libkernel_sys/Export.cpp) (libkernel_sys) passes `AT_FDCWD` (-100) and absolute paths to `readlink`. Any other relative path is joined to the descriptor's host directory, found the same way as for `fchdir`, so guest path aliases and case-insensitive name matching do not apply to it, and a `..` component is not kept inside the guest root. It returns `EBADF` for a descriptor that is not open and `ENOTDIR` for one that is not a directory. Console behaviour is unverified
- [unlinkat](../../../core/libs/prx/libkernel_sys/Export.cpp) (libkernel_sys) passes `AT_FDCWD` (-100) and absolute paths to `rmdir` (with `AT_REMOVEDIR`, 0x800) or `unlink`. Any other relative path is joined to the descriptor's host directory, found the same way as for `fchdir`, so guest path aliases and case-insensitive name matching do not apply to it, and a `..` component is not kept inside the guest root. It returns `EBADF` for a descriptor that is not open and `ENOTDIR` for one that is not a directory. Removing a directory without `AT_REMOVEDIR` fails with `EPERM`, removing `.` or `..` with `AT_REMOVEDIR` fails with `EINVAL`, and flags other than `AT_REMOVEDIR` fail with `EINVAL`. Console behaviour is unverified
- [renameat](../../../core/libs/prx/libkernel_sys/Export.cpp) (libkernel_sys) passes `AT_FDCWD` (-100) and absolute paths to `rename`. Any other relative path is joined to its descriptor's host directory, found the same way as for `fchdir`, so guest path aliases and case-insensitive name matching do not apply to relative descriptors, and a `..` component is not kept inside the guest root. It returns `EBADF` for a descriptor that is not open and `ENOTDIR` for one that is not a directory. Console behaviour is unverified
- [fork](../../../core/libs/prx/libkernel_sys/Export.cpp) (libkernel_sys) throws: creating a child process is not supported. PPSA19111 imports it and fails to load without it. Its engine calls `fork` in `Posix_Daemonize`, only with `-daemonize`, and in `Platform_ShellExecute` before `execvp("xdg-open")`, which the main menu's Previews button reaches, so pressing Previews ends in this throw. [setsid](../../../core/libs/prx/libkernel_sys/Export.cpp) fails with `EPERM`, as FreeBSD's does for a process group leader, since the guest is taken to lead its own process group (see `kill`). [umask](../../../core/libs/prx/libkernel_sys/Export.cpp) keeps the guest's mask (initially 022, masked to 07777) and returns the previous one, Linux named shared-memory creation applies it, but ordinary files are still created with the host process's mask
- [chroot](../../../core/libs/prx/libkernel_sys/Export.cpp) (libkernel_sys) follows FreeBSD 11.4's [`sys_chroot`](https://github.com/freebsd/freebsd-src/blob/8c639945a44588f6cfdddfb057bb5aad88735da0/sys/kern/vfs_syscalls.c#L776-L807) and [`pwd_chroot`](https://github.com/freebsd/freebsd-src/blob/8c639945a44588f6cfdddfb057bb5aad88735da0/sys/kern/kern_descrip.c#L3059-L3085) for a root caller: `EFAULT` for a null path, `ENOENT` for an empty or missing one, `ENAMETOOLONG` for a path of 1024 bytes or more or a component over 255, and `ENOTDIR` for a path that is not a directory or passes through a file. A directory that is the guest root (`/`, `/..`, or `.` after `fchdir` to a descriptor of `/`) succeeds and changes nothing, as the root stays the same directory; the guest is taken never to have changed its root, so open directory descriptors do not make it fail with `EPERM` (`kern.chroot_allow_open_directories` 1). Any other directory throws, naming the path: the guest path resolver has a fixed root and mount aliases (`/app0`, `/savedata0`) that do not move with it. On the console a title starts in a sandbox root, where `pwd_chroot` refuses with `EPERM` while a directory is open; whether that holds for titles, and whether a title holds `PRIV_VFS_CHROOT`, are unverified. Porpoise (PPSA99764) weak-imports it and calls it only as `fchdir` to its saved descriptor of `/` then `chroot(".")`, to put its sandbox root back after a jailbreak daemon replaced it ([`put_root_back`](https://github.com/elripalda/Porpoise-Dolphin-Emulator-for-PS5/blob/2ed1ab5a06995aa198f7bccd4da0f90c3fc29d39/src/porpoise_jailbreak.cpp#L303-L331))