1 files changed, 97 insertions, 0 deletions
diff --git a/pkg/sentry/fs/context.go b/pkg/sentry/fs/context.go
new file mode 100644
index 000000000..b521bce75
--- /dev/null
+++ b/pkg/sentry/fs/context.go
@@ -0,0 +1,97 @@
+// Copyright 2018 Google Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package fs
+
+import (
+	"gvisor.googlesource.com/gvisor/pkg/abi/linux"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/context"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/kernel/auth"
+)
+
+// contextID is the kernel package's type for context.Context.Value keys.
+type contextID int
+
+const (
+	// CtxRoot is a Context.Value key for a Dirent.
+	CtxRoot contextID = iota
+)
+
+// ContextCanAccessFile determines whether `file` can be accessed in the requested way
+// (for reading, writing, or execution) using the caller's credentials and user
+// namespace, as does Linux's fs/namei.c:generic_permission.
+func ContextCanAccessFile(ctx context.Context, inode *Inode, reqPerms PermMask) bool {
+	creds := auth.CredentialsFromContext(ctx)
+	uattr, err := inode.UnstableAttr(ctx)
+	if err != nil {
+		return false
+	}
+
+	p := uattr.Perms.Other
+	// Are we owner or in group?
+	if uattr.Owner.UID == creds.EffectiveKUID {
+		p = uattr.Perms.User
+	} else if creds.InGroup(uattr.Owner.GID) {
+		p = uattr.Perms.Group
+	}
+
+	// Are permissions satisfied without capability checks?
+	if p.SupersetOf(reqPerms) {
+		return true
+	}
+
+	if IsDir(inode.StableAttr) {
+		// CAP_DAC_OVERRIDE can override any perms on directories.
+		if inode.CheckCapability(ctx, linux.CAP_DAC_OVERRIDE) {
+			return true
+		}
+
+		// CAP_DAC_READ_SEARCH can normally only override Read perms,
+		// but for directories it can also override execution.
+		if !reqPerms.Write && inode.CheckCapability(ctx, linux.CAP_DAC_READ_SEARCH) {
+			return true
+		}
+	}
+
+	// CAP_DAC_OVERRIDE can always override Read/Write.
+	// Can override executable only when at least one execute bit is set.
+	if !reqPerms.Execute || uattr.Perms.AnyExecute() {
+		if inode.CheckCapability(ctx, linux.CAP_DAC_OVERRIDE) {
+			return true
+		}
+	}
+
+	// Read perms can be overridden by CAP_DAC_READ_SEARCH.
+	if reqPerms.OnlyRead() && inode.CheckCapability(ctx, linux.CAP_DAC_READ_SEARCH) {
+		return true
+	}
+	return false
+}
+
+// FileOwnerFromContext returns a FileOwner using the effective user and group
+// IDs used by ctx.
+func FileOwnerFromContext(ctx context.Context) FileOwner {
+	creds := auth.CredentialsFromContext(ctx)
+	return FileOwner{creds.EffectiveKUID, creds.EffectiveKGID}
+}
+
+// RootFromContext returns the root of the virtual filesystem observed by ctx,
+// or nil if ctx is not associated with a virtual filesystem. If
+// RootFromContext returns a non-nil fs.Dirent, a reference is taken on it.
+func RootFromContext(ctx context.Context) *Dirent {
+	if v := ctx.Value(CtxRoot); v != nil {
+		return v.(*Dirent)
+	}
+	return nil
+}