env: detect the lowerdir overlay mount options at runtime

Replace the "legacy-mount-options" feature flag with runtime
detection of this feature. We assume that systems with kernel
version v6.8 and newer are capable of using the "lowerdir+"
mount option.

Related-to: #968
Related-to: #1164

Author: davvid

crates/spfs/Cargo.toml

@@ -18,11 +18,6 @@ default = []
 # If enabled, will create the "local" repository in a subdirectory
 # of the standard storage root, named "ci/pipeline_${CI_PIPELINE_ID}".
 gitlab-ci-local-repo-isolation = []
-# Use "mount" commands that are compatible with older centos7-era kernels that
-# do not support the "lowerdir+=" overlayfs options. "legacy-mount-options"
-# can run into path length limits when mounting many layers.
-# https://github.com/spkenv/spk/issues/968
-legacy-mount-options = []
 sentry = ["dep:sentry"]
 server = ["hyper/server", "tokio-util/codec", "tokio-util/io-util"]
 "protobuf-src" = ["dep:protobuf-src"]

crates/spfs/src/env.rs

@@ -1036,13 +1036,16 @@ const OVERLAY_ARGS_INDEX: &str = "index";
 const OVERLAY_ARGS_INDEX_ON: &str = "index=on";
 const OVERLAY_ARGS_METACOPY: &str = "metacopy";
 const OVERLAY_ARGS_METACOPY_ON: &str = "metacopy=on";
+pub(crate) const OVERLAY_ARGS_LOWERDIR_APPEND: &str = "lowerdir+";
 
 /// A struct for holding the options that will be included
 /// in the overlayfs mount command when mounting an environment.
 #[derive(Default)]
 pub(crate) struct OverlayMountOptions {
     /// Specifies that the overlay file system is mounted as read-only
     pub read_only: bool,
+    /// The lowerdir+ mount option will be used to append layers when true.
+    pub lowerdir_append: bool,
     /// When true, inodes are indexed in the mount so that
     /// files which share the same inode (hardlinks) are broken
     /// in the final mount and changes to one file don't affect
@@ -1073,11 +1076,22 @@ impl OverlayMountOptions {
     fn new(rt: &runtime::Runtime) -> Self {
         Self {
             read_only: !rt.status.editable,
+            lowerdir_append: true,
             break_hardlinks: true,
             metadata_copy_up: true,
         }
     }
 
+    /// Update state variables to match the features supported by the current overlay version.
+    fn query(mut self) -> Self {
+        let params = runtime::overlayfs::overlayfs_available_options();
+        if self.lowerdir_append && !params.contains(OVERLAY_ARGS_LOWERDIR_APPEND) {
+            self.lowerdir_append = false;
+        }
+
+        self
+    }
+
     /// Return the options that should be included in the mount request.
     pub fn to_options(&self) -> Vec<&'static str> {
         let params = runtime::overlayfs::overlayfs_available_options();
@@ -1114,7 +1128,7 @@ pub(crate) fn get_overlay_args<P: AsRef<Path>>(
     // Allocate a large buffer up front to avoid resizing/copying.
     let mut args = String::with_capacity(4096);
 
-    let mount_options = OverlayMountOptions::new(rt);
+    let mount_options = OverlayMountOptions::new(rt).query();
     for option in mount_options.to_options() {
         args.push_str(option);
         args.push(',');
@@ -1126,19 +1140,19 @@ pub(crate) fn get_overlay_args<P: AsRef<Path>>(
     // the rightmost on the command line is the bottom layer, and the
     // leftmost is on the top). For more details see:
     // https://docs.kernel.org/filesystems/overlayfs.html#multiple-lower-layers
-    if cfg!(feature = "legacy-mount-options") {
-        args.push_str("lowerdir=");
+    if mount_options.lowerdir_append {
         for path in layer_dirs.iter().rev() {
+            args.push_str("lowerdir+=");
             args.push_str(&path.as_ref().to_string_lossy());
-            args.push(':');
+            args.push(',');
         }
+        args.push_str("lowerdir+=");
     } else {
+        args.push_str("lowerdir=");
         for path in layer_dirs.iter().rev() {
-            args.push_str("lowerdir+=");
             args.push_str(&path.as_ref().to_string_lossy());
-            args.push(',');
+            args.push(':');
         }
-        args.push_str("lowerdir+=");
     }
     args.push_str(&rt.config.lower_dir.to_string_lossy());
 

crates/spfs/src/runtime/overlayfs.rs

@@ -6,6 +6,7 @@ use std::collections::HashSet;
 use std::io::{BufRead, BufReader};
 use std::os::unix::fs::MetadataExt;
 
+use crate::env::OVERLAY_ARGS_LOWERDIR_APPEND;
 use crate::{Error, Result};
 
 #[cfg(test)]
@@ -57,10 +58,36 @@ fn query_overlayfs_available_options() -> Result<HashSet<String>> {
 #[cfg(target_os = "linux")]
 fn parse_modinfo_params<R: BufRead>(reader: &mut R) -> Result<HashSet<String>> {
     let mut params = HashSet::new();
+    let mut vermagic_seen = false;
     for line in reader.lines() {
         let line = line.map_err(|err| {
             Error::String(format!("Failed to read kernel module information: {err}"))
         })?;
+
+        // The output from "modinfo overlay" looks like this:
+        // ...
+        // vermagic:       6.12.9-amd64 SMP preempt mod_unload modversions
+        // ...
+        // parm:           metacopy:Default to on or off for the metadata only copy up feature (bool)
+        //
+        // The "vermagic:" line appears before the "parm:" lines.
+
+        if !vermagic_seen {
+            let version_string = match line.strip_prefix("vermagic:") {
+                Some(remainder) => remainder.trim(),
+                None => continue,
+            };
+            vermagic_seen = true;
+            let mut parts = version_string.splitn(3, '.'); // ("6", "12", "9-...")
+            let major_version = parts.next().unwrap_or("0").parse().unwrap_or(0);
+            let minor_version = parts.next().unwrap_or("0").parse().unwrap_or(0);
+            // The "lowerdir+" option was added in Linux v6.8.
+            // https://docs.kernel.org/6.8/filesystems/overlayfs.html#multiple-lower-layers
+            if major_version >= 7 || (major_version == 6 && minor_version >= 8) {
+                params.insert(OVERLAY_ARGS_LOWERDIR_APPEND.to_string());
+            }
+            continue;
+        }
         let param = match line.strip_prefix("parm:") {
             Some(remainder) => remainder.trim(),
             None => continue,