bake: change evaluation of entitlement paths

Currently, to compare the local path used by bake against the paths allowed
by entitlements, symlinks were evaluated for path normalization so that the
local path used by build was allowed to not exist while the path allowed by
entitlement needed to exist. If the path used by the build did not exist,
then the deepest existing parent path was used instead. This was concistent
with entitlement rules as that parent path would be the actual path access
is needed.

This raised an issue with `--set` if one provides a non-existing path as
an argument, as these paths are supposed to be allowed automatically. With
the above restrictions set to allowed paths, this meant the build would fail
as it can't grant entitlement to the non-existing paths.

This changes the evaluation logic for allowing paths so that they do not
need to exist. If such a case appears, then the path is evaluated to the
last component that exists, and then the rest of the path is appended as is.

This means that for example, if `output = /tmp/out/foo/` is set in HCL
and `/tmp` is the last component that exists then invoking build with
`--allow fs.write=/tmp/out/foo` will not fail with stat error anymore
but will fail in entitlements validation as build would also need to
write `/tmp/out` that is not inside the allowed `/tmp/out/foo` path. The
same would apply to `--set` as well so that if it points to
a non-existing path, then an additional `--allow` rule is needed
providing access to writing to the last existing component of that path.
This may or may not be unexpected.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

bake/entitlements.go

@@ -19,6 +19,7 @@ import (
 	"github.com/docker/buildx/util/osutil"
 	"github.com/moby/buildkit/util/entitlements"
 	"github.com/pkg/errors"
+	"github.com/sirupsen/logrus"
 )
 
 type EntitlementKey string
@@ -443,10 +444,18 @@ func evaluatePaths(in []string) ([]string, bool, error) {
 			continue
 		}
 		v, err := filepath.Abs(p)
+		if err != nil {
+			logrus.Warnf("failed to evaluate entitlement path %q: %v", p, err)
+			continue
+		}
+		v, rest, err := evaluateToExistingPath(v)
 		if err != nil {
 			return nil, false, errors.Wrapf(err, "failed to evaluate path %q", p)
 		}
-		v, err = filepath.EvalSymlinks(v)
+		if rest != "" {
+			v = filepath.Join(v, rest)
+		}
+		v, err = osutil.GetLongPathName(v)
 		if err != nil {
 			return nil, false, errors.Wrapf(err, "failed to evaluate path %q", p)
 		}
@@ -458,7 +467,7 @@ func evaluatePaths(in []string) ([]string, bool, error) {
 func evaluateToExistingPaths(in map[string]struct{}) (map[string]struct{}, error) {
 	m := make(map[string]struct{}, len(in))
 	for p := range in {
-		v, err := evaluateToExistingPath(p)
+		v, _, err := evaluateToExistingPath(p)
 		if err != nil {
 			return nil, errors.Wrapf(err, "failed to evaluate path %q", p)
 		}
@@ -471,10 +480,10 @@ func evaluateToExistingPaths(in map[string]struct{}) (map[string]struct{}, error
 	return m, nil
 }
 
-func evaluateToExistingPath(in string) (string, error) {
+func evaluateToExistingPath(in string) (string, string, error) {
 	in, err := filepath.Abs(in)
 	if err != nil {
-		return "", err
+		return "", "", err
 	}
 
 	volLen := volumeNameLen(in)
@@ -529,29 +538,29 @@ func evaluateToExistingPath(in string) (string, error) {
 			if os.IsNotExist(err) {
 				for r := len(dest) - 1; r >= volLen; r-- {
 					if os.IsPathSeparator(dest[r]) {
-						return dest[:r], nil
+						return dest[:r], in[start:], nil
 					}
 				}
-				return vol, nil
+				return vol, in[start:], nil
 			}
-			return "", err
+			return "", "", err
 		}
 
 		if fi.Mode()&fs.ModeSymlink == 0 {
 			if !fi.Mode().IsDir() && end < len(in) {
-				return "", syscall.ENOTDIR
+				return "", "", syscall.ENOTDIR
 			}
 			continue
 		}
 
 		linksWalked++
 		if linksWalked > 255 {
-			return "", errors.New("too many symlinks")
+			return "", "", errors.New("too many symlinks")
 		}
 
 		link, err := os.Readlink(dest)
 		if err != nil {
-			return "", err
+			return "", "", err
 		}
 
 		in = link + in[end:]
@@ -584,7 +593,7 @@ func evaluateToExistingPath(in string) (string, error) {
 			end = 0
 		}
 	}
-	return filepath.Clean(dest), nil
+	return filepath.Clean(dest), "", nil
 }
 
 func volumeNameLen(s string) int {

bake/entitlements_test.go

@@ -89,7 +89,7 @@ func TestEvaluateToExistingPath(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			result, err := evaluateToExistingPath(tt.input)
+			result, _, err := evaluateToExistingPath(tt.input)
 
 			if tt.expectErr {
 				require.Error(t, err)
@@ -341,7 +341,7 @@ func TestValidateEntitlements(t *testing.T) {
 						return nil
 					}
 					// if not, then escapeLink is not allowed
-					exp, err := evaluateToExistingPath(escapeLink)
+					exp, _, err := evaluateToExistingPath(escapeLink)
 					require.NoError(t, err)
 					exp, err = filepath.EvalSymlinks(exp)
 					require.NoError(t, err)