feat(snix-build/oci): Use user's subordinate ids in oci builds.

subuid/subgids used to be hardcoded, which resulted in build failures
if those did not match the ones of the effective user.

fixes #86

Change-Id: I3b0c3e9ef710aa9e3de998891abe10fd1a893189
Reviewed-on: https://cl.snix.dev/c/snix/+/30301
Tested-by: besadii
Reviewed-by: Florian Klink <flokli@flokli.de>

snix/Cargo.lock

@@ -2574,6 +2574,18 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "nix"
+version = "0.29.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "71e2746dc3a24dd78b3cfcb7be93368c6de9963d30f43a6a73998a9cf4b17b46"
+dependencies = [
+ "bitflags 2.6.0",
+ "cfg-if",
+ "cfg_aliases",
+ "libc",
+]
+
 [[package]]
 name = "nix-compat"
 version = "0.1.0"
@@ -4147,6 +4159,7 @@ dependencies = [
  "futures",
  "itertools 0.12.1",
  "mimalloc",
+ "nix 0.29.0",
  "oci-spec",
  "prost",
  "prost-build",
@@ -5275,7 +5288,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "69fff37da548239c3bf9e64a12193d261e8b22b660991c6fd2df057c168f435f"
 dependencies = [
  "cc",
- "windows-targets 0.48.5",
+ "windows-targets 0.52.6",
 ]
 
 [[package]]

snix/Cargo.nix

@@ -8153,6 +8153,53 @@ rec {
         };
         resolvedDefaultFeatures = [ "default" "fs" ];
       };
+      "nix 0.29.0" = rec {
+        crateName = "nix";
+        version = "0.29.0";
+        edition = "2021";
+        sha256 = "0ikvn7s9r2lrfdm3mx1h7nbfjvcc6s9vxdzw7j5xfkd2qdnp9qki";
+        authors = [
+          "The nix-rust Project Developers"
+        ];
+        dependencies = [
+          {
+            name = "bitflags";
+            packageId = "bitflags 2.6.0";
+          }
+          {
+            name = "cfg-if";
+            packageId = "cfg-if";
+          }
+          {
+            name = "libc";
+            packageId = "libc";
+            features = [ "extra_traits" ];
+          }
+        ];
+        buildDependencies = [
+          {
+            name = "cfg_aliases";
+            packageId = "cfg_aliases";
+          }
+        ];
+        features = {
+          "aio" = [ "pin-utils" ];
+          "dir" = [ "fs" ];
+          "memoffset" = [ "dep:memoffset" ];
+          "mount" = [ "uio" ];
+          "mqueue" = [ "fs" ];
+          "net" = [ "socket" ];
+          "pin-utils" = [ "dep:pin-utils" ];
+          "ptrace" = [ "process" ];
+          "sched" = [ "process" ];
+          "signal" = [ "process" ];
+          "socket" = [ "memoffset" ];
+          "ucontext" = [ "signal" ];
+          "user" = [ "feature" ];
+          "zerocopy" = [ "fs" "uio" ];
+        };
+        resolvedDefaultFeatures = [ "default" "feature" "user" ];
+      };
       "nix-compat" = rec {
         crateName = "nix-compat";
         version = "0.1.0";
@@ -13483,6 +13530,11 @@ rec {
             name = "mimalloc";
             packageId = "mimalloc";
           }
+          {
+            name = "nix";
+            packageId = "nix 0.29.0";
+            features = [ "user" ];
+          }
           {
             name = "oci-spec";
             packageId = "oci-spec";
@@ -17658,7 +17710,7 @@ rec {
         dependencies = [
           {
             name = "windows-targets";
-            packageId = "windows-targets 0.48.5";
+            packageId = "windows-targets 0.52.6";
             target = { target, features }: (target."windows" or false);
           }
         ];

snix/build/Cargo.toml

@@ -25,6 +25,7 @@ bstr = "1.6.0"
 data-encoding = "2.5.0"
 futures = "0.3.30"
 oci-spec = "0.7.0"
+nix = { version = "0.29.0", features = ["user"] }
 serde_json = "1.0.111"
 snix-tracing = { path = "../tracing" }
 uuid = { version = "1.7.0", features = ["v4"] }

snix/build/src/buildservice/oci.rs

@@ -1,6 +1,5 @@
 use anyhow::Context;
 use bstr::BStr;
-use oci_spec::runtime::{LinuxIdMapping, LinuxIdMappingBuilder};
 use snix_castore::{
     blobservice::BlobService,
     directoryservice::DirectoryService,
@@ -29,11 +28,6 @@ pub struct OCIBuildService<BS, DS> {
     /// Root path in which all bundles are created in
     bundle_root: PathBuf,
 
-    /// uid mappings to set up for the workloads
-    uid_mappings: Vec<LinuxIdMapping>,
-    /// uid mappings to set up for the workloads
-    gid_mappings: Vec<LinuxIdMapping>,
-
     /// Handle to a [BlobService], used by filesystems spawned during builds.
     blob_service: BS,
     /// Handle to a [DirectoryService], used by filesystems spawned during builds.
@@ -49,40 +43,11 @@ impl<BS, DS> OCIBuildService<BS, DS> {
         // We map root inside the container to the uid/gid this is running at,
         // and allocate one for uid 1000 into the container from the range we
         // got in /etc/sub{u,g}id.
-        // TODO: actually read uid, and /etc/subuid. Maybe only when we try to build?
         // FUTUREWORK: use different uids?
         Self {
             bundle_root,
             blob_service,
             directory_service,
-            uid_mappings: vec![
-                LinuxIdMappingBuilder::default()
-                    .host_id(1000_u32)
-                    .container_id(0_u32)
-                    .size(1_u32)
-                    .build()
-                    .unwrap(),
-                LinuxIdMappingBuilder::default()
-                    .host_id(100000_u32)
-                    .container_id(1000_u32)
-                    .size(1_u32)
-                    .build()
-                    .unwrap(),
-            ],
-            gid_mappings: vec![
-                LinuxIdMappingBuilder::default()
-                    .host_id(100_u32)
-                    .container_id(0_u32)
-                    .size(1_u32)
-                    .build()
-                    .unwrap(),
-                LinuxIdMappingBuilder::default()
-                    .host_id(100000_u32)
-                    .container_id(100_u32)
-                    .size(1_u32)
-                    .build()
-                    .unwrap(),
-            ],
             concurrent_builds: tokio::sync::Semaphore::new(MAX_CONCURRENT_BUILDS),
         }
     }
@@ -108,11 +73,7 @@ where
             .context("failed to create spec")
             .map_err(std::io::Error::other)?;
 
-        let mut linux = runtime_spec.linux().clone().unwrap();
+        let linux = runtime_spec.linux().clone().unwrap();
-
-        // edit the spec, we need to setup uid/gid mappings.
-        linux.set_uid_mappings(Some(self.uid_mappings.clone()));
-        linux.set_gid_mappings(Some(self.gid_mappings.clone()));
 
         runtime_spec.set_linux(Some(linux));
 

snix/build/src/oci/mod.rs

@@ -1,5 +1,6 @@
 mod bundle;
 mod spec;
+pub(crate) mod subuid;
 
 pub(crate) use bundle::get_host_output_paths;
 pub(crate) use bundle::make_bundle;

snix/build/src/oci/spec.rs

@@ -1,12 +1,23 @@
 //! Module to create a OCI runtime spec for a given [BuildRequest].
 use crate::buildservice::{BuildConstraints, BuildRequest};
-use oci_spec::{
+use oci_spec::runtime::{
-    runtime::{Capability, LinuxNamespace, LinuxNamespaceBuilder, LinuxNamespaceType},
+    Capability, LinuxIdMappingBuilder, LinuxNamespace, LinuxNamespaceBuilder, LinuxNamespaceType,
-    OciSpecError,
 };
 use std::{collections::HashSet, path::Path};
 
-use super::scratch_name;
+use super::{
+    scratch_name,
+    subuid::{SubordinateError, SubordinateInfo},
+};
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+pub enum SpecError {
+    #[error("oci error: {0}")]
+    OciError(oci_spec::OciSpecError),
+    #[error("subordinate error: {0}")]
+    SubordinateError(SubordinateError),
+}
 
 /// For a given [BuildRequest], return an OCI runtime spec.
 ///
@@ -33,7 +44,7 @@ pub(crate) fn make_spec(
     request: &BuildRequest,
     rootless: bool,
     sandbox_shell: &str,
-) -> Result<oci_spec::runtime::Spec, oci_spec::OciSpecError> {
+) -> Result<oci_spec::runtime::Spec, SpecError> {
     let allow_network = request
         .constraints
         .contains(&BuildConstraints::NetworkAccess);
@@ -57,7 +68,8 @@ pub(crate) fn make_spec(
     }
 
     oci_spec::runtime::SpecBuilder::default()
-        .process(configure_process(
+        .process(
+            configure_process(
                 &request.command_args,
                 &request.working_dir,
                 request
@@ -72,24 +84,31 @@ pub(crate) fn make_spec(
                     })
                     .collect::<Vec<_>>(),
                 rootless,
-        )?)
+            )
+            .map_err(SpecError::OciError)?,
+        )
         .linux(configure_linux(allow_network, rootless)?)
         .root(
             oci_spec::runtime::RootBuilder::default()
                 .path("root")
                 .readonly(true)
-                .build()?,
+                .build()
+                .map_err(SpecError::OciError)?,
         )
         .hostname("localhost")
-        .mounts(configure_mounts(
+        .mounts(
+            configure_mounts(
                 rootless,
                 allow_network,
                 request.scratch_paths.iter().map(|e| e.as_path()),
                 request.inputs.iter(),
                 &request.inputs_dir,
                 ro_host_mounts,
-        )?)
+            )
+            .map_err(SpecError::OciError)?,
+        )
         .build()
+        .map_err(SpecError::OciError)
 }
 
 /// Return the Process part of the OCI Runtime spec.
@@ -162,7 +181,7 @@ fn configure_process<'a>(
 fn configure_linux(
     allow_network: bool,
     rootless: bool,
-) -> Result<oci_spec::runtime::Linux, OciSpecError> {
+) -> Result<oci_spec::runtime::Linux, SpecError> {
     let mut linux = oci_spec::runtime::Linux::default();
 
     // explicitly set namespaces, depending on allow_network.
@@ -187,7 +206,8 @@ fn configure_linux(
         namespace_types
             .into_iter()
             .map(|e| LinuxNamespaceBuilder::default().typ(e).build())
-            .collect::<Result<Vec<LinuxNamespace>, _>>()?
+            .collect::<Result<Vec<LinuxNamespace>, _>>()
+            .map_err(SpecError::OciError)?
     }));
 
     linux.set_masked_paths(Some(
@@ -217,6 +237,35 @@ fn configure_linux(
         .map(|e| e.to_string())
         .collect::<Vec<_>>(),
     ));
+    let info = SubordinateInfo::for_effective_user().map_err(SpecError::SubordinateError)?;
+    linux.set_uid_mappings(Some(vec![
+        LinuxIdMappingBuilder::default()
+            .host_id(info.uid)
+            .container_id(0_u32)
+            .size(1_u32)
+            .build()
+            .unwrap(),
+        LinuxIdMappingBuilder::default()
+            .host_id(info.subuid)
+            .container_id(1000_u32)
+            .size(1_u32)
+            .build()
+            .unwrap(),
+    ]));
+    linux.set_gid_mappings(Some(vec![
+        LinuxIdMappingBuilder::default()
+            .host_id(info.gid)
+            .container_id(0_u32)
+            .size(1_u32)
+            .build()
+            .unwrap(),
+        LinuxIdMappingBuilder::default()
+            .host_id(info.subgid)
+            .container_id(100_u32)
+            .size(1_u32)
+            .build()
+            .unwrap(),
+    ]));
 
     Ok(linux)
 }

snix/build/src/oci/subuid.rs

@@ -0,0 +1,165 @@
+use std::{
+    fs::File,
+    io::{BufRead, BufReader},
+    num::ParseIntError,
+    path::PathBuf,
+};
+
+use nix::{
+    errno::Errno,
+    unistd::{Gid, Group, Uid, User},
+};
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+pub(crate) enum SubordinateError {
+    #[error("can't determine user {0}")]
+    UidError(Errno),
+
+    #[error("user entry for {0} does not exist")]
+    NoPasswdEntry(Uid),
+
+    #[error("can't determine group {0}")]
+    GidError(Errno),
+
+    #[error("group entry for {0} does not exist")]
+    NoGroupEntry(Gid),
+
+    #[error("io error {0:?}, file {1}")]
+    IoError(std::io::Error, PathBuf),
+
+    #[error("failed to parse {0} line '{1}', error {2}")]
+    ParseError(PathBuf, String, ParseIntError),
+
+    #[error("Missing entry in {0}, for {1}({2})")]
+    MissingEntry(PathBuf, String, u32),
+}
+
+/// Represents a single (subuid,subgid) pair for a user and their group.
+///
+/// In practice there are usually many more subordinate ids than just one, but
+/// for oci builds we only need one. If we ever need more, we can improve this
+/// implementation.
+#[derive(Debug, PartialEq, Eq)]
+pub(crate) struct SubordinateInfo {
+    pub uid: u32,
+    pub gid: u32,
+    pub subuid: u32,
+    pub subgid: u32,
+}
+
+impl SubordinateInfo {
+    /// Parses /etc/subuid and /etc/subgid and returns a single [SubordinateInfo] for the effective user.
+    pub(crate) fn for_effective_user() -> Result<SubordinateInfo, SubordinateError> {
+        let (user, group) = user_info()?;
+
+        let subuid =
+            first_subordinate_id(&PathBuf::from("/etc/subuid"), user.uid.as_raw(), &user.name)?;
+        let subgid = first_subordinate_id(
+            &PathBuf::from("/etc/subgid"),
+            group.gid.as_raw(),
+            &group.name,
+        )?;
+        Ok(SubordinateInfo {
+            uid: user.uid.as_raw(),
+            gid: group.gid.as_raw(),
+            subuid,
+            subgid,
+        })
+    }
+}
+
+/// Returns user and group entries for current effective user.
+fn user_info() -> Result<(User, Group), SubordinateError> {
+    let u = Uid::effective();
+    let user = User::from_uid(u)
+        .map_err(SubordinateError::UidError)?
+        .ok_or(SubordinateError::NoPasswdEntry(u))?;
+    let g = Gid::effective();
+    let group = Group::from_gid(g)
+        .map_err(SubordinateError::GidError)?
+        .ok_or(SubordinateError::NoGroupEntry(g))?;
+    Ok((user, group))
+}
+
+fn first_subordinate_id(file: &PathBuf, id: u32, name: &str) -> Result<u32, SubordinateError> {
+    let f = File::open(file).map_err(|e| SubordinateError::IoError(e, file.clone()))?;
+    let reader = BufReader::new(f).lines();
+
+    for line in reader {
+        let line = line.map_err(|e| SubordinateError::IoError(e, file.clone()))?;
+        let line = line.trim();
+        let parts: Vec<&str> = line.split(':').collect();
+        if parts.len() == 3 && (parts[0] == name || id.to_string() == parts[0]) {
+            let subuid = parts[1]
+                .parse::<u32>()
+                .map_err(|e| SubordinateError::ParseError(file.clone(), line.into(), e))?;
+            let range = parts[2]
+                .parse::<u32>()
+                .map_err(|e| SubordinateError::ParseError(file.clone(), line.into(), e))?;
+            if range > 0 {
+                return Ok(subuid);
+            }
+        }
+    }
+
+    Err(SubordinateError::MissingEntry(
+        file.clone(),
+        name.into(),
+        id,
+    ))
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::oci::subuid::SubordinateError;
+
+    fn create_fixture<'a>(content: impl IntoIterator<Item = &'a str>) -> tempfile::NamedTempFile {
+        use std::io::Write;
+        let mut file = tempfile::NamedTempFile::new().expect("Could not create tempfile");
+        for line in content.into_iter() {
+            writeln!(file, "{}", line).expect("");
+        }
+        file
+    }
+
+    #[test]
+    fn test_parse_uid_file_with_name_should_return_first_match() {
+        let file = create_fixture(["nobody:10000:65", "root:1000:2", "0:2:2"]);
+        let id = super::first_subordinate_id(&file.path().into(), 0, "root")
+            .expect("Faild to look up subordinate id.");
+        assert_eq!(id, 1000);
+    }
+
+    #[test]
+    fn test_parse_uid_file_with_uid_should_return_first_match() {
+        let file = create_fixture(["nobody:10000:65", "0:2:2"]);
+        let id = super::first_subordinate_id(&file.path().into(), 0, "root")
+            .expect("Failed to look up subordinate id.");
+        assert_eq!(id, 2);
+    }
+
+    #[test]
+    fn test_missing() {
+        let file = create_fixture(["roots:1000:2", "1000:2:2"]);
+        let id = super::first_subordinate_id(&file.path().into(), 0, "root")
+            .expect_err("Expected not to find a matching subordinate entry.");
+        assert!(matches!(id, SubordinateError::MissingEntry(_, _, _)));
+    }
+
+    #[test]
+    fn test_parse_error() {
+        let file = create_fixture(["root:hello:2", "1000:2:2"]);
+        let id = super::first_subordinate_id(&file.path().into(), 0, "root")
+            .expect_err("Expected parsing to fail.");
+        assert!(matches!(id, SubordinateError::ParseError(_, _, _)));
+    }
+
+    #[test]
+    fn test_parse_errors_in_other_users_files_are_ignored() {
+        let file = create_fixture(["root:hello:2", "1000:2:2"]);
+        let id = super::first_subordinate_id(&file.path().into(), 1000, "user")
+            .expect("Failed to look up subordinate id.");
+        assert_eq!(id, 2);
+    }
+}