Use RESOLVE_NO_SYMLINKS on Linux when following symlinks is disabled

This addresses a bug where a forward slash is the last element of a
symlink directory path and `open_dir_nofollow` is used.

If a symlink is created to a directory without specifying an absolute
path, then the path behind the symlink does not error when opened
with `open_dir_nofollow`.

Take these two symlinks:

```
> ls -l /home/ylk/test-dir-symlink*
lrwxrwxrwx 1 ylk ylk 24 Feb 19 21:15 /home/ylk/test-dir-symlink ->
/local/home/ylk/test-dir/
lrwxrwxrwx 1 ylk ylk 8 Feb 19 21:27 /home/ylk/test-dir-symlink2 ->
test-dir/
```

Without this patch, the following behavior is observed when using
open_dir_nofollow:

test-dir-symlink/ -> error
test-dir-symlink -> error
test-dir-symlink2 -> error
test-dir-symlink2/ -> **no error**

Author: gitstashpop

cap-primitives/src/rustix/linux/fs/open_impl.rs

@@ -9,6 +9,7 @@
 
 #[cfg(racy_asserts)]
 use crate::fs::is_same_file;
+use crate::fs::FollowSymlinks;
 use crate::fs::{manually, OpenOptions};
 use std::path::Path;
 use std::{fs, io};
@@ -82,13 +83,11 @@ pub(crate) fn open_beneath(
         // times, because there's no limit on how often this can happen. The actual
         // number here is currently an arbitrarily chosen guess.
         for _ in 0..4 {
-            match openat2(
-                start,
-                path_c_str,
-                oflags,
-                mode,
-                ResolveFlags::BENEATH | ResolveFlags::NO_MAGICLINKS,
-            ) {
+            let mut resolve_flags = ResolveFlags::BENEATH | ResolveFlags::NO_MAGICLINKS;
+            if options.follow == FollowSymlinks::No {
+                resolve_flags |= ResolveFlags::NO_SYMLINKS;
+            }
+            match openat2(start, path_c_str, oflags, mode, resolve_flags) {
                 Ok(file) => {
                     let file = fs::File::from_into_fd(file);
 

tests/symlinks.rs

@@ -258,19 +258,19 @@ fn open_dir_nofollow() {
             let name = format!("{}{}", symlink_dir, suffix);
             check!(tmpdir.open_dir(&name));
             // On Windows, a trailing dot is stripped early.
-            if cfg!(not(windows)) || suffix != &"/." {
-                check!(tmpdir.open_dir_nofollow(&name));
-            } else {
+            if (cfg!(windows) && suffix == &"/.") || cfg!(target_os = "linux") {
                 assert!(tmpdir.open_dir_nofollow(&name).is_err());
+            } else {
+                check!(tmpdir.open_dir_nofollow(&name));
             }
             for dir_name in &["dir", "symlink_dir"] {
                 let name = format!("{}/../{}", dir_name, name);
                 check!(tmpdir.open_dir(&name));
                 // On Windows, a trailing dot is stripped early.
-                if cfg!(not(windows)) || suffix != &"/." {
-                    check!(tmpdir.open_dir_nofollow(&name));
-                } else {
+                if (cfg!(windows) && suffix == &"/.") || cfg!(target_os = "linux") {
                     assert!(tmpdir.open_dir_nofollow(&name).is_err());
+                } else {
+                    check!(tmpdir.open_dir_nofollow(&name));
                 }
             }
         }
@@ -296,7 +296,11 @@ fn open_dir_nofollow() {
             #[cfg(not(windows))]
             {
                 check!(tmpdir.open_dir(&name));
-                check!(tmpdir.open_dir_nofollow(&name));
+                if cfg!(target_os = "linux") {
+                    assert!(tmpdir.open_dir_nofollow(&name).is_err());
+                } else {
+                    check!(tmpdir.open_dir_nofollow(&name));
+                }
             }
             for dir_name in &["dir", "symlink_dir"] {
                 let name = format!("{}/../{}", dir_name, name);
@@ -308,7 +312,11 @@ fn open_dir_nofollow() {
                 #[cfg(not(windows))]
                 {
                     check!(tmpdir.open_dir(&name));
-                    check!(tmpdir.open_dir_nofollow(&name));
+                    if cfg!(target_os = "linux") {
+                        assert!(tmpdir.open_dir_nofollow(&name).is_err());
+                    } else {
+                        check!(tmpdir.open_dir_nofollow(&name));
+                    }
                 }
             }
         }