From 91a9f83dd1d73cfd451f81306361df3fafad84a5 Mon Sep 17 00:00:00 2001 From: Dan Gohman Date: Fri, 16 Oct 2020 09:09:20 -0700 Subject: [PATCH] Define `fs::hard_link` to not follow symlinks. POSIX leaves it implementation-defined whether `link` follows symlinks. In practice, for example, on Linux it does not and on FreeBSD it does. So, switch to `linkat`, so that we can pick a behavior rather than depending on OS defaults. Pick the option to not follow symlinks. This is somewhat arbitrary, but seems the less surprising choice because hard linking is a very low-level feature which requires the source and destination to be on the same mounted filesystem, and following a symbolic link could end up in a different mounted filesystem. --- library/std/src/fs.rs | 7 +++-- library/std/src/fs/tests.rs | 51 ++++++++++++++++++++++++++++++++++ library/std/src/sys/unix/fs.rs | 5 +++- 3 files changed, 60 insertions(+), 3 deletions(-) diff --git a/library/std/src/fs.rs b/library/std/src/fs.rs index 161bfe3795c2..c611bf4d74a4 100644 --- a/library/std/src/fs.rs +++ b/library/std/src/fs.rs @@ -1701,10 +1701,13 @@ pub fn copy, Q: AsRef>(from: P, to: Q) -> io::Result { /// The `dst` path will be a link pointing to the `src` path. Note that systems /// often require these two paths to both be located on the same filesystem. /// +/// If `src` names a symbolic link, it is not followed. The created hard link +/// points to the symbolic link itself. +/// /// # Platform-specific behavior /// -/// This function currently corresponds to the `link` function on Unix -/// and the `CreateHardLink` function on Windows. +/// This function currently corresponds to the `linkat` function with no flags +/// on Unix and the `CreateHardLink` function on Windows. /// Note that, this [may change in the future][changes]. /// /// [changes]: io#platform-specific-behavior diff --git a/library/std/src/fs/tests.rs b/library/std/src/fs/tests.rs index 65a29076fefa..8a723d3b4ae2 100644 --- a/library/std/src/fs/tests.rs +++ b/library/std/src/fs/tests.rs @@ -1337,3 +1337,54 @@ fn metadata_access_times() { } } } + +/// Test creating hard links to symlinks. +#[test] +fn symlink_hard_link() { + let tmpdir = tmpdir(); + + // Create "file", a file. + check!(fs::File::create(tmpdir.join("file"))); + + // Create "symlink", a symlink to "file". + check!(symlink_file("file", tmpdir.join("symlink"))); + + // Create "hard_link", a hard link to "symlink". + check!(fs::hard_link(tmpdir.join("symlink"), tmpdir.join("hard_link"))); + + // "hard_link" should appear as a symlink. + assert!(check!(fs::symlink_metadata(tmpdir.join("hard_link"))).file_type().is_symlink()); + + // We sould be able to open "file" via any of the above names. + let _ = check!(fs::File::open(tmpdir.join("file"))); + assert!(fs::File::open(tmpdir.join("file.renamed")).is_err()); + let _ = check!(fs::File::open(tmpdir.join("symlink"))); + let _ = check!(fs::File::open(tmpdir.join("hard_link"))); + + // Rename "file" to "file.renamed". + check!(fs::rename(tmpdir.join("file"), tmpdir.join("file.renamed"))); + + // Now, the symlink and the hard link should be dangling. + assert!(fs::File::open(tmpdir.join("file")).is_err()); + let _ = check!(fs::File::open(tmpdir.join("file.renamed"))); + assert!(fs::File::open(tmpdir.join("symlink")).is_err()); + assert!(fs::File::open(tmpdir.join("hard_link")).is_err()); + + // The symlink and the hard link should both still point to "file". + assert!(fs::read_link(tmpdir.join("file")).is_err()); + assert!(fs::read_link(tmpdir.join("file.renamed")).is_err()); + assert_eq!(check!(fs::read_link(tmpdir.join("symlink"))), Path::new("file")); + assert_eq!(check!(fs::read_link(tmpdir.join("hard_link"))), Path::new("file")); + + // Remove "file.renamed". + check!(fs::remove_file(tmpdir.join("file.renamed"))); + + // Now, we can't open the file by any name. + assert!(fs::File::open(tmpdir.join("file")).is_err()); + assert!(fs::File::open(tmpdir.join("file.renamed")).is_err()); + assert!(fs::File::open(tmpdir.join("symlink")).is_err()); + assert!(fs::File::open(tmpdir.join("hard_link")).is_err()); + + // "hard_link" should still appear as a symlink. + assert!(check!(fs::symlink_metadata(tmpdir.join("hard_link"))).file_type().is_symlink()); +} diff --git a/library/std/src/sys/unix/fs.rs b/library/std/src/sys/unix/fs.rs index 819e8ef18415..88693e4786c0 100644 --- a/library/std/src/sys/unix/fs.rs +++ b/library/std/src/sys/unix/fs.rs @@ -1067,7 +1067,10 @@ pub fn symlink(src: &Path, dst: &Path) -> io::Result<()> { pub fn link(src: &Path, dst: &Path) -> io::Result<()> { let src = cstr(src)?; let dst = cstr(dst)?; - cvt(unsafe { libc::link(src.as_ptr(), dst.as_ptr()) })?; + // Use `linkat` with `AT_FDCWD` instead of `link` as `link` leaves it + // implmentation-defined whether it follows symlinks. Pass 0 as the + // `linkat` flags argument so that we don't follow symlinks. + cvt(unsafe { libc::linkat(libc::AT_FDCWD, src.as_ptr(), libc::AT_FDCWD, dst.as_ptr(), 0) })?; Ok(()) }