The 6.17 kernel added[1] syscalls for getting/setting certain file flags
and attributes. It's meant to be a more extensible replacement for these
ioctl's:
- `FS_IOC_GETFLAGS`/`FS_IOC_SETFLAGS`.
- `FS_IOC_FSGETXATTR`/`FS_IOC_FSSETXATTR`.
The definitions of these calls are as follows:
```zig
const file_attr = extern struct {
/// Extended flags that apply to this file. (get/set).
xflags: u64,
/// Preferred extent allocation size, in bytes. (get/set).
extsize: u32,
/// One of:
/// - The number of data extents in this file.
/// - If `FS_IOC_FSGETXATTRA` is set, the number of extended attribute events in the file.
/// (get)
nextents: u32,
/// Project Identifier (get/set).
projid: u32,
/// Preferred extent allocation size for CoW operations, in bytes (get/set).
cowextsize: u32,
};
// size=@sizeOf(file_attr)
fn file_getattr(dirfd: fd_t, path: [*:0], fattr: *file_attr, size: usize, at_flags: u32) {}
fn file_setattr(dirfd: fd_t, path: [*:0], fattr: *file_attr, size: usize, at_flags: u32) {}
```
Users need to set/check `xflags` with the `FS_XFLAG` flags defined in
`linux/fs.h`. `ioctl_xfs_fsgetxattr(2)` has more information about the
type of information one can retrieve.
[1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=57fcb7d930d8f00f383e995aeebdcd2b416a187a
Maintaining the POSIX `stat` bits for Zig is a pain. The order and
bit-length of members differ between all architectures, and int types
can be signed or unsigned. The libcs deal with this by introducing the
own version of `struct stat` and copying the kernel structure members to
it. In the case of glibc, they did it twice thanks to the largefile
transition!
In practice, the project needs to maintain three versions of `struct
stat`:
- What the kernel defines.
- What musl wants for `struct stat`.
- What glibc wants for `struct stat64`. Make sure to use `fstatat64`!
This isn't as simple as running `zig translate-c`. In #21440 I had to:
- Compile toolchains for each arch+glibc/musl combo.
- Create a test `fstat` program with/without `FILE_OFFSET_BITS=64`.
- Dump the value for `struct stat`.
- Stare at `std.os.linux`/`std.c` and cry.
- Add some missing padding.
The fact that so many target checks in the `linux` and `posix` tests
exist is most likely due to writing to padding bits and failing later.
The solution to this madness is `statx(2)`:
- It takes a single structure that is the same for all arches AND libcs.
- It uses a custom timestamp format, but it is 64-bit ready.
- It gives the same info as `fstatat(2)` and more!
- Unlike `fstatat(2)`, you can request a subset of the info required
based on passing a mask.
It's so good that modern Linux arches (e.g. riscv) don't even implement
`stat`, with the libcs using a generic `struct stat` and copying from
`struct statx`.
Therefore, this commit rips out all the `stat` bits from `std.os.linux`
and `std.c`. `std.posix.Stat` is now `void`, and calling
`std.posix.*stat` is an compile-time error. A wrapper around `statx` has
been added to `std.os.linux`, and callers have been upgraded to use it.
Tests have also been updated to use `statx` where possible.
While I was here, I converted the mask and file attributes to be packed
struct bitfields. A nice side effect is checking that you actually
recieved the members you asked for via `Statx.mask`, which I have used
by adding `assert`s at specific callsites.
MIPS I has load hazards so we need to insert nops in a few places. This is not a
problem for MIPS II and later.
While doing this, I also touched up all the inline asm to use ABI register
aliases and a consistent formatting convention. Also fixed a few places that
didn't properly check if the syscall return value should be negated.
This is very likely full of wrong stuff. It's effectively just a copy of the
x86_64 file - needed because the former stopped using usize/isize. To be clear,
this is no more broken than the old situation was; this just makes the
brokenness explicit.
This is very likely full of wrong stuff. It's effectively just a copy of the
mips64 file - needed because the former stopped using usize/isize. To be clear,
this is no more broken than the old situation was; this just makes the
brokenness explicit.
Andrew Kelley