df: use statfs fallback when mount table is unavailable

Author: ChrisDryden

src/uu/df/src/df.rs

@@ -16,7 +16,7 @@ use uucore::error::{UError, UResult, USimpleError, get_exit_code};
 use uucore::fsext::{MountInfo, read_fs_list};
 use uucore::parser::parse_size::ParseSizeError;
 use uucore::translate;
-use uucore::{format_usage, show};
+use uucore::{format_usage, show, show_warning};
 
 use clap::{Arg, ArgAction, ArgMatches, Command, parser::ValueSource};
 
@@ -111,6 +111,13 @@ impl Default for Options {
     }
 }
 
+impl Options {
+    /// Whether -a, -l, -t, or -x options require the mount table.
+    fn requires_mount_table(&self) -> bool {
+        self.show_all_fs || self.show_local_fs || self.include.is_some() || self.exclude.is_some()
+    }
+}
+
 #[derive(Debug, Error)]
 enum OptionsError {
     // TODO This needs to vary based on whether `--block-size`
@@ -358,14 +365,37 @@ where
     P: AsRef<Path>,
 {
     // The list of all mounted filesystems.
-    let mounts: Vec<MountInfo> = read_fs_list()?;
+    let mounts_result = read_fs_list();
+
+    let (mounts, use_fallback) = match mounts_result {
+        Ok(m) => (m, false),
+        Err(e) => {
+            if opt.requires_mount_table() {
+                return Err(e);
+            }
+            show_warning!(
+                "{}",
+                translate!("df-error-cannot-read-table-of-mounted-filesystems")
+            );
+            (vec![], true)
+        }
+    };
 
     let mut result = vec![];
 
     // Convert each path into a `Filesystem`, which contains
     // both the mount information and usage information.
     for path in paths {
-        match Filesystem::from_path(&mounts, path) {
+        #[cfg(unix)]
+        let fs_result = if use_fallback {
+            Filesystem::from_path_direct(path)
+        } else {
+            Filesystem::from_path(&mounts, path)
+        };
+        #[cfg(not(unix))]
+        let fs_result = Filesystem::from_path(&mounts, path);
+
+        match fs_result {
             Ok(fs) => {
                 if is_included(&fs.mount_info, opt) {
                     result.push(fs);

src/uu/df/src/filesystem.rs

@@ -8,10 +8,15 @@
 //! filesystem mounted at a particular directory. It also includes
 //! information on amount of space available and amount of space used.
 // spell-checker:ignore canonicalized
+use std::io;
+use std::path::PathBuf;
 use std::{ffi::OsString, path::Path};
 
 #[cfg(unix)]
-use uucore::fsext::statfs;
+use std::os::unix::fs::MetadataExt;
+
+#[cfg(unix)]
+use uucore::fsext::{FsMeta, pretty_fstype, statfs};
 use uucore::fsext::{FsUsage, MountInfo};
 
 /// Summary representation of a filesystem.
@@ -61,6 +66,31 @@ fn is_over_mounted(mounts: &[MountInfo], mount: &MountInfo) -> bool {
     }
 }
 
+/// Find mount point by walking up the directory tree until device ID changes.
+#[cfg(unix)]
+pub(crate) fn find_mount_point<P: AsRef<Path>>(path: P) -> io::Result<PathBuf> {
+    let mut current = path.as_ref().canonicalize()?;
+    let current_dev = current.metadata()?.dev();
+
+    loop {
+        let parent = match current.parent() {
+            Some(p) if !p.as_os_str().is_empty() => p,
+            _ => return Ok(current),
+        };
+
+        let parent_dev = parent.metadata()?.dev();
+        if parent_dev != current_dev {
+            return Ok(current);
+        }
+
+        if parent == current {
+            return Ok(current);
+        }
+
+        current = parent.to_path_buf();
+    }
+}
+
 /// Find the mount info that best matches a given filesystem path.
 ///
 /// This function returns the element of `mounts` on which `path` is
@@ -195,6 +225,43 @@ impl Filesystem {
         #[cfg(not(windows))]
         return result.and_then(|mount_info| Self::from_mount(mounts, mount_info, Some(file)));
     }
+
+    /// Fallback using statfs when mount table is unavailable.
+    #[cfg(unix)]
+    pub(crate) fn from_path_direct<P>(path: P) -> Result<Self, FsError>
+    where
+        P: AsRef<Path>,
+    {
+        let file = path.as_ref().as_os_str().to_owned();
+
+        let canonical_path = path
+            .as_ref()
+            .canonicalize()
+            .map_err(|_| FsError::InvalidPath)?;
+
+        let stat_result = statfs(canonical_path.as_os_str()).map_err(|_| FsError::MountMissing)?;
+        let mount_dir = find_mount_point(&canonical_path).map_err(|_| FsError::MountMissing)?;
+        let fs_type = pretty_fstype(stat_result.fs_type()).into_owned();
+
+        let mount_info = MountInfo {
+            dev_id: String::new(),
+            dev_name: "-".to_string(),
+            fs_type,
+            mount_dir: mount_dir.into_os_string(),
+            mount_option: String::new(),
+            mount_root: OsString::new(),
+            remote: false,
+            dummy: false,
+        };
+
+        let usage = FsUsage::new(stat_result);
+
+        Ok(Self {
+            file: Some(file),
+            mount_info,
+            usage,
+        })
+    }
 }
 
 #[cfg(test)]

tests/by-util/test_df.rs

@@ -15,6 +15,7 @@ use std::collections::HashSet;
 #[cfg(not(any(target_os = "freebsd", target_os = "windows")))]
 use uutests::at_and_ucmd;
 use uutests::new_ucmd;
+use uutests::util::TestScenario;
 
 #[test]
 fn test_invalid_arg() {
@@ -1091,3 +1092,62 @@ fn test_df_hides_binfmt_misc_by_default() {
     }
     // If binfmt_misc is not mounted, skip the test silently
 }
+
+/// Run df inside a mount namespace where /proc is masked with tmpfs.
+/// Returns (success, stdout, stderr).
+#[cfg(target_os = "linux")]
+fn run_df_with_masked_proc(args: &str) -> Option<(bool, String, String)> {
+    use std::process::Command;
+
+    // Check if user namespaces are available
+    if !Command::new("unshare")
+        .args(["-rm", "true"])
+        .status()
+        .is_ok_and(|s| s.success())
+    {
+        return None;
+    }
+
+    let df_path = TestScenario::new("df").bin_path.clone();
+    let output = Command::new("unshare")
+        .args(["-rm", "sh", "-c"])
+        .arg(format!(
+            "mount -t tmpfs tmpfs /proc && {} df {args}",
+            df_path.display()
+        ))
+        .output()
+        .ok()?;
+
+    Some((
+        output.status.success(),
+        String::from_utf8_lossy(&output.stdout).to_string(),
+        String::from_utf8_lossy(&output.stderr).to_string(),
+    ))
+}
+
+/// Test df fallback when /proc is masked - should work with path, fail without or with filters.
+#[test]
+#[cfg(target_os = "linux")]
+fn test_df_masked_proc_fallback() {
+    if let Some((ok, stdout, stderr)) = run_df_with_masked_proc(".") {
+        assert!(ok, "df . should succeed: {stderr}");
+        assert!(stderr.contains("cannot read table of mounted file systems"));
+        assert!(stdout.contains("Filesystem"));
+    }
+
+    if let Some((ok, _, _)) = run_df_with_masked_proc("") {
+        assert!(!ok, "df without args should fail when /proc is masked");
+    }
+
+    for args in ["-a .", "-l .", "-t ext4 .", "-x tmpfs ."] {
+        if let Some((ok, _, _)) = run_df_with_masked_proc(args) {
+            assert!(!ok, "df {args} should fail when /proc is masked");
+        }
+    }
+
+    for args in ["-i .", "-T .", "--total ."] {
+        if let Some((ok, _, stderr)) = run_df_with_masked_proc(args) {
+            assert!(ok, "df {args} should succeed: {stderr}");
+        }
+    }
+}