Detect when statx isn't available on older versions of Docker.

In older versions of Docker, `statx` can fail with `EPERM` meaning it
isn't available. Recognize this, and translate it into `ENOSYS`. Also,
have `statx` remember this and fail quickly in future calls.

This is more emulation than rustix usually likes to do, but in this
case, this is the behavior seems likely to be what most users will
want, and we might as well put the code in one place.

Fixes #352.

Author: sunfishcode

src/fs/statx.rs

@@ -1,15 +1,19 @@
 //! Linux `statx`.
 
+use crate::fd::{AsFd, BorrowedFd};
+use crate::ffi::CStr;
 use crate::fs::AtFlags;
 use crate::{imp, io, path};
-use imp::fd::AsFd;
+use core::sync::atomic::{AtomicU8, Ordering};
 
 pub use imp::fs::types::{Statx, StatxFlags, StatxTimestamp};
 
 /// `statx(dirfd, path, flags, mask, statxbuf)`
 ///
-/// This isn't available on Linux before 4.11; it returns `ENOSYS` in that
-/// case.
+/// This function returns [`io::Errno::NOSYS`] if `statx` is not available on
+/// the platform, such as Linux before 4.11. This also includes older Docker
+/// versions where the actual syscall fails with different error codes; Rustix
+/// handles this and translates them into `NOSYS`.
 ///
 /// # References
 ///  - [Linux]
@@ -22,5 +26,66 @@ pub fn statx<P: path::Arg, Fd: AsFd>(
     flags: AtFlags,
     mask: StatxFlags,
 ) -> io::Result<Statx> {
-    path.into_with_c_str(|path| imp::fs::syscalls::statx(dirfd.as_fd(), path, flags, mask))
+    path.into_with_c_str(|path| _statx(dirfd.as_fd(), path, flags, mask))
+}
+
+// Linux kernel prior to 4.11 old versions of Docker don't support `statx`. We
+// store the availability in a global to avoid unnecessary syscalls.
+//
+// 0: Unknown
+// 1: Not available
+// 2: Available
+static STATX_STATE: AtomicU8 = AtomicU8::new(0);
+
+#[inline]
+fn _statx(
+    dirfd: BorrowedFd<'_>,
+    path: &CStr,
+    flags: AtFlags,
+    mask: StatxFlags,
+) -> io::Result<Statx> {
+    match STATX_STATE.load(Ordering::Relaxed) {
+        0 => statx_init(dirfd, path, flags, mask),
+        1 => Err(io::Errno::NOSYS),
+        _ => imp::fs::syscalls::statx(dirfd, path, flags, mask),
+    }
+}
+
+/// The first `statx` call. We don't know if `statx` is available yet.
+fn statx_init(
+    dirfd: BorrowedFd<'_>,
+    path: &CStr,
+    flags: AtFlags,
+    mask: StatxFlags,
+) -> io::Result<Statx> {
+    match imp::fs::syscalls::statx(dirfd, path, flags, mask) {
+        Err(io::Errno::NOSYS) => statx_error_nosys(),
+        Err(io::Errno::PERM) => statx_error_perm(),
+        result => {
+            STATX_STATE.store(2, Ordering::Relaxed);
+            result
+        }
+    }
+}
+
+/// The first `statx` call failed with `NOSYS` (or something we're treating
+/// like `NOSYS`).
+#[cold]
+fn statx_error_nosys() -> io::Result<Statx> {
+    STATX_STATE.store(1, Ordering::Relaxed);
+    Err(io::Errno::NOSYS)
+}
+
+/// The first `statx` call failed with `PERM`.
+#[cold]
+fn statx_error_perm() -> io::Result<Statx> {
+    // Some old versions of Docker have `statx` fail with `PERM` when it isn't
+    // recognized. Check whether `statx` really is available, and if so, fail
+    // with `PERM`, and if not, treat it like `NOSYS`.
+    if imp::fs::syscalls::is_statx_available() {
+        STATX_STATE.store(2, Ordering::Relaxed);
+        Err(io::Errno::PERM)
+    } else {
+        statx_error_nosys()
+    }
 }

src/imp/libc/fs/syscalls.rs

@@ -89,6 +89,8 @@ use crate::fs::SealFlags;
 )))]
 // not implemented in libc for netbsd yet
 use crate::fs::StatFs;
+#[cfg(any(target_os = "android", target_os = "linux"))]
+use crate::fs::{cwd, RenameFlags, ResolveFlags, Statx, StatxFlags};
 #[cfg(not(any(
     target_os = "ios",
     target_os = "macos",
@@ -97,8 +99,6 @@ use crate::fs::StatFs;
 )))]
 use crate::fs::{Dev, FileType};
 use crate::fs::{FdFlags, Mode, OFlags, Stat, Timestamps};
-#[cfg(any(target_os = "android", target_os = "linux"))]
-use crate::fs::{RenameFlags, ResolveFlags, Statx, StatxFlags};
 use crate::io::{self, OwnedFd, SeekFrom};
 #[cfg(not(target_os = "wasi"))]
 use crate::process::{Gid, Uid};
@@ -116,18 +116,15 @@ use core::convert::TryInto;
 ))]
 use core::mem::size_of;
 use core::mem::MaybeUninit;
+#[cfg(any(target_os = "android", target_os = "linux"))]
+use core::ptr::null;
 #[cfg(any(
     target_os = "android",
     target_os = "ios",
     target_os = "linux",
     target_os = "macos"
 ))]
 use core::ptr::null_mut;
-#[cfg(all(
-    any(target_os = "android", target_os = "linux"),
-    any(target_pointer_width = "32", target_arch = "mips64")
-))]
-use core::sync::atomic::{AtomicBool, Ordering::Relaxed};
 #[cfg(any(target_os = "ios", target_os = "macos"))]
 use {
     super::super::conv::nonnegative_ret,
@@ -351,14 +348,11 @@ pub(crate) fn statat(dirfd: BorrowedFd<'_>, path: &CStr, flags: AtFlags) -> io::
         any(target_pointer_width = "32", target_arch = "mips64")
     ))]
     {
-        if !NO_STATX.load(Relaxed) {
-            match statx(dirfd, path, flags, StatxFlags::BASIC_STATS) {
-                Ok(x) => return statx_to_stat(x),
-                Err(io::Errno::NOSYS) => NO_STATX.store(true, Relaxed),
-                Err(e) => return Err(e),
-            }
+        match statx(dirfd, path, flags, StatxFlags::BASIC_STATS) {
+            Ok(x) => return statx_to_stat(x),
+            Err(io::Errno::NOSYS) => statat_old(dirfd, path, flags),
+            Err(e) => return Err(e),
         }
-        statat_old(dirfd, path, flags)
     }
 
     // Main version: libc is y2038 safe. Or, the platform is not y2038 safe and
@@ -904,13 +898,6 @@ pub(crate) fn flock(fd: BorrowedFd<'_>, operation: FlockOperation) -> io::Result
     unsafe { ret(c::flock(borrowed_fd(fd), operation as c::c_int)) }
 }
 
-/// `statx` was introduced in Linux 4.11.
-#[cfg(all(
-    any(target_os = "android", target_os = "linux"),
-    any(target_pointer_width = "32", target_arch = "mips64")
-))]
-static NO_STATX: AtomicBool = AtomicBool::new(false);
-
 pub(crate) fn fstat(fd: BorrowedFd<'_>) -> io::Result<Stat> {
     // 32-bit and mips64 Linux: `struct stat64` is not y2038 compatible; use
     // `statx`.
@@ -919,14 +906,11 @@ pub(crate) fn fstat(fd: BorrowedFd<'_>) -> io::Result<Stat> {
         any(target_pointer_width = "32", target_arch = "mips64")
     ))]
     {
-        if !NO_STATX.load(Relaxed) {
-            match statx(fd, cstr!(""), AtFlags::EMPTY_PATH, StatxFlags::BASIC_STATS) {
-                Ok(x) => return statx_to_stat(x),
-                Err(io::Errno::NOSYS) => NO_STATX.store(true, Relaxed),
-                Err(e) => return Err(e),
-            }
+        match statx(fd, cstr!(""), AtFlags::EMPTY_PATH, StatxFlags::BASIC_STATS) {
+            Ok(x) => return statx_to_stat(x),
+            Err(io::Errno::NOSYS) => fstat_old(fd),
+            Err(e) => return Err(e),
         }
-        fstat_old(fd)
     }
 
     // Main version: libc is y2038 safe. Or, the platform is not y2038 safe and
@@ -1398,12 +1382,9 @@ fn stat64_to_stat(s64: c::stat64) -> io::Result<Stat> {
 
 #[cfg(any(target_os = "android", target_os = "linux"))]
 #[allow(non_upper_case_globals)]
-pub(crate) fn statx(
-    dirfd: BorrowedFd<'_>,
-    path: &CStr,
-    flags: AtFlags,
-    mask: StatxFlags,
-) -> io::Result<Statx> {
+mod sys {
+    use super::{c, BorrowedFd, Statx};
+
     #[cfg(all(target_os = "android", target_arch = "arm"))]
     const SYS_statx: c::c_long = 397;
     #[cfg(all(target_os = "android", target_arch = "x86"))]
@@ -1414,18 +1395,27 @@ pub(crate) fn statx(
     const SYS_statx: c::c_long = 332;
 
     weak_or_syscall! {
-        fn statx(
+        pub(super) fn statx(
             pirfd: BorrowedFd<'_>,
             path: *const c::c_char,
             flags: c::c_int,
             mask: c::c_uint,
             buf: *mut Statx
         ) via SYS_statx -> c::c_int
     }
+}
 
+#[cfg(any(target_os = "android", target_os = "linux"))]
+#[allow(non_upper_case_globals)]
+pub(crate) fn statx(
+    dirfd: BorrowedFd<'_>,
+    path: &CStr,
+    flags: AtFlags,
+    mask: StatxFlags,
+) -> io::Result<Statx> {
     let mut statx_buf = MaybeUninit::<Statx>::uninit();
     unsafe {
-        ret(statx(
+        ret(sys::statx(
             dirfd,
             c_str(path),
             flags.bits(),
@@ -1436,6 +1426,19 @@ pub(crate) fn statx(
     }
 }
 
+#[cfg(any(target_os = "android", target_os = "linux"))]
+#[inline]
+pub(crate) fn is_statx_available() -> bool {
+    unsafe {
+        // Call `statx` with null pointers so that if it fails for any reason
+        // other than `EFAULT`, we know it's not supported.
+        matches!(
+            ret(sys::statx(cwd(), null(), 0, 0, null_mut())),
+            Err(io::Errno::FAULT)
+        )
+    }
+}
+
 #[cfg(any(target_os = "ios", target_os = "macos"))]
 pub(crate) unsafe fn fcopyfile(
     from: BorrowedFd<'_>,