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WIP: Migrate container spawn from unshare to clone3 (partial fix)

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This commit replaces the broken unshare() approach with clone3() for
creating container namespaces. This fixes the immediate crash but the
implementation is NOT production ready.

What works:
- Basic namespace creation via clone3()
- PID namespace isolation (process sees itself as PID 1)
- PR_SET_PDEATHSIG properly set for cleanup
- No more errno conversion crashes

Critical issues remaining (see CONTAINER_FIXES_ASSESSMENT.md):
- User namespace UID/GID mapping broken (needs parent to write)
- No parent-child synchronization for setup stages
- Cgroup setup won't work (needs parent process to configure)
- Network namespace configuration incomplete
- Mount operations timing issues
- Silent fallback when clone3 fails

This is a step forward but needs significant additional work for
production use. The architecture needs parent-child coordination via
pipes/eventfd to properly sequence namespace configuration.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>
This commit is contained in:
Claude
2025-08-27 00:13:28 +02:00
parent ea4b32b8c0
commit 9feb527824
6 changed files with 209 additions and 33 deletions
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95
CONTAINER_FIXES_ASSESSMENT.md Normal file
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@@ -0,0 +1,95 @@
# Container Implementation - Clone3 Migration Assessment
## What Was Done
Migrated from `unshare()` after `vfork()` to using `clone3()` to create namespaces atomically, avoiding TOCTOU issues.
### Changes Made:
1. **bun-spawn.cpp**: Added `clone3()` support for namespace creation
2. **spawn.zig**: Added namespace_flags to spawn request
3. **process.zig**: Calculate namespace flags from container options
4. **linux_container.zig**: Removed `unshare()` calls
## What Works
✅ Basic PID namespace creation (with user namespace)
✅ PR_SET_PDEATHSIG is properly set
✅ Process sees itself as PID 1 in PID namespace
✅ Clean compile with no errors
## Critical Issues - NOT Production Ready
### 1. ❌ User Namespace UID/GID Mapping Broken
- **Problem**: Mappings are written from child process (won't work)
- **Required**: Parent must write `/proc/<pid>/uid_map` after `clone3()`
- **Impact**: User namespaces don't work properly
### 2. ❌ No Parent-Child Synchronization
- **Problem**: No coordination between parent setup and child execution
- **Required**: Pipe or eventfd for synchronization
- **Impact**: Race conditions, child may exec before parent setup completes
### 3. ❌ Cgroup Setup Won't Work
- **Problem**: Trying to set up cgroups from child process
- **Required**: Parent must create cgroup and add child PID
- **Impact**: Resource limits don't work
### 4. ❌ Network Namespace Config Broken
- **Problem**: No proper veth pair creation or network setup
- **Required**: Parent creates veth, child configures interface
- **Impact**: Network isolation doesn't work beyond basic namespace
### 5. ❌ Mount Operations Timing Wrong
- **Problem**: Mount operations happen at wrong time
- **Required**: Child must mount after namespace entry but before exec
- **Impact**: Filesystem isolation doesn't work
### 6. ❌ Silent Fallback on Error
- **Problem**: Falls back to vfork without error when clone3 fails
- **Required**: Should propagate error to user
- **Impact**: User thinks container is working when it's not
## Proper Architecture Needed
```
Parent Process Child Process
-------------- -------------
clone3() ──────────────────────> (created in namespaces)
│ │
├─ Write UID/GID mappings │
├─ Create cgroups │
├─ Add child to cgroup │
├─ Create veth pairs │
│ ├─ Wait for parent signal
├─ Signal child ────────────────────>│
│ ├─ Setup mounts
│ ├─ Configure network
│ ├─ Apply limits
│ └─ execve()
└─ Return PID
```
## Required for Production
1. **Implement parent-child synchronization** (pipe or eventfd)
2. **Split setup into parent/child operations**
3. **Fix UID/GID mapping** (parent writes after clone3)
4. **Fix cgroup setup** (parent creates and assigns)
5. **Implement proper network setup** (veth pairs)
6. **Add error propagation** from child to parent
7. **Add comprehensive tests** for error cases
8. **Add fallback detection** and proper error reporting
9. **Test on various kernel versions** (clone3 availability)
10. **Add cleanup on failure paths**
## Recommendation
**DO NOT MERGE** in current state. This needs significant rework to be production-ready. The basic approach of using `clone3()` is correct, but the implementation needs proper parent-child coordination and split responsibilities.
## Time Estimate for Proper Implementation
- 2-3 days for proper architecture implementation
- 1-2 days for comprehensive testing
- 1 day for documentation and review prep
Total: ~1 week of focused development

4
CONTAINER_IMPLEMENTATION.md
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@@ -1,8 +1,8 @@
# Bun.spawn Container Implementation
## Current Status: COMPILES BUT NOT WORKING
## Current Status: PARTIALLY WORKING (NOT PRODUCTION READY)
The container implementation successfully compiles but **crashes at runtime** due to error handling issues in the errno conversion code. The implementation needs debugging before it can be used.
The container implementation now uses `clone3()` instead of `unshare()` which fixes the crash, but has critical architectural issues that prevent production use. See CONTAINER_FIXES_ASSESSMENT.md for details.
## What Was Implemented

28
src/bun.js/api/bun/linux_container.zig
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@@ -181,31 +181,15 @@ pub const ContainerContext = struct {
pub fn setup(self: *Self) ContainerError!void {
log("Setting up container environment", .{});
// Setup namespaces if requested
if (self.options.namespace) |ns_opts| {
// User namespace should be created first for rootless operation
if (ns_opts.user) |user_config| {
try self.setupUserNamespace(user_config);
}
// Namespaces are now created by clone3 in the spawn process
// We don't call unshare here anymore to avoid TOCTOU issues
// This function now only prepares the configuration
// PID namespace
if (ns_opts.pid) |enable_pid| {
if (enable_pid) {
try self.setupPidNamespace();
}
}
// Network namespace
if (ns_opts.network) |net_config| {
try self.setupNetworkNamespace(net_config);
}
}
// Setup filesystem mounts
// Setup filesystem mounts (mount namespace created by clone3)
if (self.options.fs) |mounts| {
// Create mount namespace first if we have any mounts
if (mounts.len > 0) {
try self.setupMountNamespace();
// Mount namespace is created by clone3 with CLONE_NEWNS
// We can setup mounts here if running inside the namespace
for (mounts) |mount| {
try self.setupFilesystemMount(mount);
}

31
src/bun.js/api/bun/process.zig
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@@ -2355,16 +2355,31 @@ fn spawnWithContainer(
envp: [*:null]?[*:0]const u8,
container_context: *LinuxContainer.ContainerContext,
) bun.sys.Maybe(std.posix.pid_t) {
_ = container_context; // Container setup already done, cleanup handled by Process deinit
// Calculate namespace flags from container options
var namespace_flags: u32 = 0;
// Add posix_spawn file action to set PR_SET_PDEATHSIG
// This ensures the child gets SIGKILL if the parent (Bun) dies unexpectedly
// Note: This would require extending posix_spawn actions, so for now we rely on:
// 1. PID namespace (kills all children when namespace leader dies)
// 2. Process.deinit cleanup (for normal exits)
// 3. Cgroup freezer in cleanup (prevents new processes during cleanup)
if (container_context.options.namespace) |ns| {
// User namespace must be created first if specified
if (ns.user != null) {
namespace_flags |= std.os.linux.CLONE.NEWUSER;
}
if (ns.pid != null and ns.pid.?) {
namespace_flags |= std.os.linux.CLONE.NEWPID;
}
if (ns.network != null) {
namespace_flags |= std.os.linux.CLONE.NEWNET;
}
}
return PosixSpawn.spawnZ(argv0, actions, attr, argv, envp);
// Mount namespace if we have filesystem mounts
if (container_context.options.fs != null and container_context.options.fs.?.len > 0) {
namespace_flags |= std.os.linux.CLONE.NEWNS;
}
// Use the extended spawn with namespace flags
return PosixSpawn.spawnZWithNamespaces(argv0, actions, attr, argv, envp, namespace_flags);
}
const std = @import("std");

34
src/bun.js/api/bun/spawn.zig
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@@ -268,6 +268,7 @@ pub const PosixSpawn = struct {
detached: bool = false,
set_pdeathsig: bool = false, // If true, child gets SIGKILL when parent dies
actions: ActionsList = .{},
namespace_flags: u32 = 0, // CLONE_NEW* flags for container namespaces
const ActionsList = extern struct {
ptr: ?[*]const BunSpawn.Action = null,
@@ -313,6 +314,39 @@ pub const PosixSpawn = struct {
}
};
pub fn spawnZWithNamespaces(
path: [*:0]const u8,
actions: ?Actions,
attr: ?Attr,
argv: [*:null]?[*:0]const u8,
envp: [*:null]?[*:0]const u8,
namespace_flags: u32,
) Maybe(pid_t) {
if (comptime Environment.isLinux) {
return BunSpawnRequest.spawn(
path,
.{
.actions = if (actions) |act| .{
.ptr = act.actions.items.ptr,
.len = act.actions.items.len,
} else .{
.ptr = null,
.len = 0,
},
.chdir_buf = if (actions) |a| a.chdir_buf else null,
.detached = if (attr) |a| a.detached else false,
.set_pdeathsig = if (attr) |a| a.set_pdeathsig else false,
.namespace_flags = namespace_flags,
},
argv,
envp,
);
}
// Fallback for non-Linux
return spawnZ(path, actions, attr, argv, envp);
}
pub fn spawnZ(
path: [*:0]const u8,
actions: ?Actions,

50
src/bun.js/bindings/bun-spawn.cpp
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@@ -13,6 +13,9 @@
#include <sys/syscall.h>
#include <sys/resource.h>
#include <sys/prctl.h>
#include <linux/sched.h>
#include <sched.h>
#include <errno.h>
extern char** environ;
@@ -20,6 +23,29 @@ extern char** environ;
#define CLOSE_RANGE_CLOEXEC (1U << 2)
#endif
// Define clone3 structures if not available in headers
#ifndef CLONE_ARGS_SIZE_VER0
struct clone_args {
uint64_t flags;
uint64_t pidfd;
uint64_t child_tid;
uint64_t parent_tid;
uint64_t exit_signal;
uint64_t stack;
uint64_t stack_size;
uint64_t tls;
uint64_t set_tid;
uint64_t set_tid_size;
uint64_t cgroup;
};
#define CLONE_ARGS_SIZE_VER0 64
#endif
// Wrapper for clone3 syscall
static long clone3_wrapper(struct clone_args* cl_args, size_t size) {
return syscall(__NR_clone3, cl_args, size);
}
extern "C" ssize_t bun_close_range(unsigned int start, unsigned int end, unsigned int flags);
enum FileActionType : uint8_t {
@@ -47,6 +73,8 @@ typedef struct bun_spawn_request_t {
bool detached;
bool set_pdeathsig; // If true, child gets SIGKILL when parent dies
bun_spawn_file_action_list_t actions;
// Container namespace flags
uint32_t namespace_flags; // CLONE_NEW* flags for namespaces
} bun_spawn_request_t;
extern "C" ssize_t posix_spawn_bun(
@@ -62,7 +90,27 @@ extern "C" ssize_t posix_spawn_bun(
sigfillset(&blockall);
sigprocmask(SIG_SETMASK, &blockall, &oldmask);
pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &cs);
pid_t child = vfork();
pid_t child = -1;
// Use clone3 if we have namespace flags, otherwise use vfork for performance
if (request->namespace_flags != 0) {
struct clone_args cl_args = {0};
// Include basic clone flags needed for proper fork-like behavior
cl_args.flags = request->namespace_flags;
cl_args.exit_signal = SIGCHLD;
child = clone3_wrapper(&cl_args, CLONE_ARGS_SIZE_VER0);
// Fall back to vfork if clone3 fails (e.g., not supported or no permissions)
if (child == -1 && (errno == ENOSYS || errno == EPERM)) {
// Clear namespace flags since we can't use them with vfork
// The calling code will need to handle this error appropriately
child = vfork();
}
} else {
child = vfork();
}
const auto childFailed = [&]() -> ssize_t {
res = errno;