Auto merge of rust-lang#85953 - inquisitivecrystal:weak-linkat-in-fs-hardlink, r=joshtriplett

Fix linker error

Currently, `fs::hard_link` determines whether platforms have `linkat` based on the OS, and uses `link` if they don't. However, this heuristic does not work well if a platform provides `linkat` on newer versions but not on older ones. On old MacOS, this currently causes a linking error.

This commit fixes `fs::hard_link` by telling it to use `weak!` on macOS. This means that, on  that operating system, we now check for `linkat` at runtime and use `link` if it is not available.

Fixes rust-lang#80804.

`@rustbot` label T-libs-impl

Author: bors

library/std/src/fs.rs

@@ -1736,8 +1736,11 @@ pub fn copy<P: AsRef<Path>, Q: AsRef<Path>>(from: P, to: Q) -> io::Result<u64> {
 ///
 /// # Platform-specific behavior
 ///
-/// This function currently corresponds to the `linkat` function with no flags
-/// on Unix and the `CreateHardLink` function on Windows.
+/// This function currently corresponds the `CreateHardLink` function on Windows.
+/// On most Unix systems, it corresponds to the `linkat` function with no flags.
+/// On Android, VxWorks, and Redox, it instead corresponds to the `link` function.
+/// On MacOS, it uses the `linkat` function if it is available, but on very old
+/// systems where `linkat` is not available, `link` is selected at runtime instead.
 /// Note that, this [may change in the future][changes].
 ///
 /// [changes]: io#platform-specific-behavior

library/std/src/fs/tests.rs

@@ -19,6 +19,10 @@ use crate::os::unix::fs::symlink as symlink_junction;
 use crate::os::windows::fs::{symlink_dir, symlink_file};
 #[cfg(windows)]
 use crate::sys::fs::symlink_junction;
+#[cfg(target_os = "macos")]
+use crate::sys::weak::weak;
+#[cfg(target_os = "macos")]
+use libc::{c_char, c_int};
 
 macro_rules! check {
     ($e:expr) => {
@@ -79,6 +83,17 @@ pub fn got_symlink_permission(tmpdir: &TempDir) -> bool {
     }
 }
 
+#[cfg(target_os = "macos")]
+fn able_to_not_follow_symlinks_while_hard_linking() -> bool {
+    weak!(fn linkat(c_int, *const c_char, c_int, *const c_char, c_int) -> c_int);
+    linkat.get().is_some()
+}
+
+#[cfg(not(target_os = "macos"))]
+fn able_to_not_follow_symlinks_while_hard_linking() -> bool {
+    return true;
+}
+
 #[test]
 fn file_test_io_smoke_test() {
     let message = "it's alright. have a good time";
@@ -1347,6 +1362,9 @@ fn symlink_hard_link() {
     if !got_symlink_permission(&tmpdir) {
         return;
     };
+    if !able_to_not_follow_symlinks_while_hard_linking() {
+        return;
+    }
 
     // Create "file", a file.
     check!(fs::File::create(tmpdir.join("file")));

library/std/src/sys/unix/android.rs

@@ -21,7 +21,7 @@
 use libc::{c_int, c_void, sighandler_t, size_t, ssize_t};
 use libc::{ftruncate, pread, pwrite};
 
-use super::{cvt, cvt_r};
+use super::{cvt, cvt_r, weak::weak};
 use crate::io;
 
 // The `log2` and `log2f` functions apparently appeared in android-18, or at

library/std/src/sys/unix/fs.rs

@@ -12,8 +12,23 @@ use crate::sys::time::SystemTime;
 use crate::sys::{cvt, cvt_r};
 use crate::sys_common::{AsInner, FromInner};
 
+#[cfg(any(
+    all(target_os = "linux", target_env = "gnu"),
+    target_os = "macos",
+    target_os = "ios",
+))]
+use crate::sys::weak::syscall;
+#[cfg(target_os = "macos")]
+use crate::sys::weak::weak;
+
 use libc::{c_int, mode_t};
 
+#[cfg(any(
+    target_os = "macos",
+    target_os = "ios",
+    all(target_os = "linux", target_env = "gnu")
+))]
+use libc::c_char;
 #[cfg(any(target_os = "linux", target_os = "emscripten", target_os = "android"))]
 use libc::dirfd;
 #[cfg(any(target_os = "linux", target_os = "emscripten"))]
@@ -92,7 +107,7 @@ cfg_has_statx! {{
     // Default `stat64` contains no creation time.
     unsafe fn try_statx(
         fd: c_int,
-        path: *const libc::c_char,
+        path: *const c_char,
         flags: i32,
         mask: u32,
     ) -> Option<io::Result<FileAttr>> {
@@ -107,7 +122,7 @@ cfg_has_statx! {{
         syscall! {
             fn statx(
                 fd: c_int,
-                pathname: *const libc::c_char,
+                pathname: *const c_char,
                 flags: c_int,
                 mask: libc::c_uint,
                 statxbuf: *mut libc::statx
@@ -756,7 +771,7 @@ impl File {
         cfg_has_statx! {
             if let Some(ret) = unsafe { try_statx(
                 fd,
-                b"\0" as *const _ as *const libc::c_char,
+                b"\0" as *const _ as *const c_char,
                 libc::AT_EMPTY_PATH | libc::AT_STATX_SYNC_AS_STAT,
                 libc::STATX_ALL,
             ) } {
@@ -1087,15 +1102,28 @@ pub fn link(original: &Path, link: &Path) -> io::Result<()> {
     let link = cstr(link)?;
     cfg_if::cfg_if! {
         if #[cfg(any(target_os = "vxworks", target_os = "redox", target_os = "android"))] {
-            // VxWorks, Redox, and old versions of Android lack `linkat`, so use
-            // `link` instead. POSIX leaves it implementation-defined whether
-            // `link` follows symlinks, so rely on the `symlink_hard_link` test
-            // in library/std/src/fs/tests.rs to check the behavior.
+            // VxWorks and Redox lack `linkat`, so use `link` instead. POSIX leaves
+            // it implementation-defined whether `link` follows symlinks, so rely on the
+            // `symlink_hard_link` test in library/std/src/fs/tests.rs to check the behavior.
+            // Android has `linkat` on newer versions, but we happen to know `link`
+            // always has the correct behavior, so it's here as well.
             cvt(unsafe { libc::link(original.as_ptr(), link.as_ptr()) })?;
+        } else if #[cfg(target_os = "macos")] {
+            // On MacOS, older versions (<=10.9) lack support for linkat while newer
+            // versions have it. We want to use linkat if it is available, so we use weak!
+            // to check. `linkat` is preferable to `link` ecause it gives us a flag to
+            // specify how symlinks should be handled. We pass 0 as the flags argument,
+            // meaning it shouldn't follow symlinks.
+            weak!(fn linkat(c_int, *const c_char, c_int, *const c_char, c_int) -> c_int);
+
+            if let Some(f) = linkat.get() {
+                cvt(unsafe { f(libc::AT_FDCWD, original.as_ptr(), libc::AT_FDCWD, link.as_ptr(), 0) })?;
+            } else {
+                cvt(unsafe { libc::link(original.as_ptr(), link.as_ptr()) })?;
+            };
         } else {
-            // Use `linkat` with `AT_FDCWD` instead of `link` as `linkat` gives
-            // us a flag to specify how symlinks should be handled. Pass 0 as
-            // the flags argument, meaning don't follow symlinks.
+            // Where we can, use `linkat` instead of `link`; see the comment above
+            // this one for details on why.
             cvt(unsafe { libc::linkat(libc::AT_FDCWD, original.as_ptr(), libc::AT_FDCWD, link.as_ptr(), 0) })?;
         }
     }
@@ -1278,7 +1306,7 @@ pub fn copy(from: &Path, to: &Path) -> io::Result<u64> {
         fn fclonefileat(
             srcfd: libc::c_int,
             dst_dirfd: libc::c_int,
-            dst: *const libc::c_char,
+            dst: *const c_char,
             flags: libc::c_int
         ) -> libc::c_int
     }

library/std/src/sys/unix/kernel_copy.rs

@@ -61,6 +61,7 @@ use crate::process::{ChildStderr, ChildStdin, ChildStdout};
 use crate::ptr;
 use crate::sync::atomic::{AtomicBool, AtomicU8, Ordering};
 use crate::sys::cvt;
+use crate::sys::weak::syscall;
 use libc::{EBADF, EINVAL, ENOSYS, EOPNOTSUPP, EOVERFLOW, EPERM, EXDEV};
 
 #[cfg(test)]

library/std/src/sys/unix/os.rs

@@ -23,6 +23,9 @@ use crate::sys::memchr;
 use crate::sys_common::rwlock::{StaticRWLock, StaticRWLockReadGuard};
 use crate::vec;
 
+#[cfg(all(target_env = "gnu", not(target_os = "vxworks")))]
+use crate::sys::weak::weak;
+
 use libc::{c_char, c_int, c_void};
 
 const TMPBUF_SZ: usize = 128;

library/std/src/sys/unix/process/process_unix.rs

@@ -9,6 +9,14 @@ use crate::sys;
 use crate::sys::cvt;
 use crate::sys::process::process_common::*;
 
+#[cfg(any(
+    target_os = "macos",
+    target_os = "freebsd",
+    all(target_os = "linux", target_env = "gnu"),
+    all(target_os = "linux", target_env = "musl"),
+))]
+use crate::sys::weak::weak;
+
 #[cfg(target_os = "vxworks")]
 use libc::RTP_ID as pid_t;
 

library/std/src/sys/unix/rand.rs

@@ -25,6 +25,9 @@ mod imp {
     use crate::fs::File;
     use crate::io::Read;
 
+    #[cfg(any(target_os = "linux", target_os = "android"))]
+    use crate::sys::weak::syscall;
+
     #[cfg(any(target_os = "linux", target_os = "android"))]
     fn getrandom(buf: &mut [u8]) -> libc::ssize_t {
         // A weak symbol allows interposition, e.g. for perf measurements that want to
@@ -108,6 +111,7 @@ mod imp {
     use crate::fs::File;
     use crate::io::Read;
     use crate::sys::os::errno;
+    use crate::sys::weak::weak;
     use libc::{c_int, c_void, size_t};
 
     fn getentropy_fill_bytes(v: &mut [u8]) -> bool {

library/std/src/sys/unix/thread.rs

@@ -7,6 +7,8 @@ use crate::ptr;
 use crate::sys::{os, stack_overflow};
 use crate::time::Duration;
 
+#[cfg(any(target_os = "linux", target_os = "solaris", target_os = "illumos"))]
+use crate::sys::weak::weak;
 #[cfg(not(any(target_os = "l4re", target_os = "vxworks")))]
 pub const DEFAULT_MIN_STACK_SIZE: usize = 2 * 1024 * 1024;
 #[cfg(target_os = "l4re")]

library/std/src/sys/unix/weak.rs

@@ -26,8 +26,11 @@ use crate::marker;
 use crate::mem;
 use crate::sync::atomic::{self, AtomicUsize, Ordering};
 
-macro_rules! weak {
+// Temporary null documentation to work around #57569 until the fix is beta
+#[cfg_attr(bootstrap, doc = "")]
+pub(crate) macro weak {
     (fn $name:ident($($t:ty),*) -> $ret:ty) => (
+        #[allow(non_upper_case_globals)]
         static $name: crate::sys::weak::Weak<unsafe extern "C" fn($($t),*) -> $ret> =
             crate::sys::weak::Weak::new(concat!(stringify!($name), '\0'));
     )
@@ -100,8 +103,10 @@ unsafe fn fetch(name: &str) -> usize {
     libc::dlsym(libc::RTLD_DEFAULT, name.as_ptr()) as usize
 }
 
+// Temporary null documentation to work around #57569 until the fix is beta
+#[cfg_attr(bootstrap, doc = "")]
 #[cfg(not(any(target_os = "linux", target_os = "android")))]
-macro_rules! syscall {
+pub(crate) macro syscall {
     (fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
         unsafe fn $name($($arg_name: $t),*) -> $ret {
             use super::os;
@@ -118,10 +123,12 @@ macro_rules! syscall {
     )
 }
 
+#[cfg_attr(bootstrap, doc = "")]
 #[cfg(any(target_os = "linux", target_os = "android"))]
-macro_rules! syscall {
+pub(crate) macro syscall {
     (fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
         unsafe fn $name($($arg_name:$t),*) -> $ret {
+            use weak;
             // This looks like a hack, but concat_idents only accepts idents
             // (not paths).
             use libc::*;