pkg/bisect: start cause bisections from any commits

For cause bisections, don't require Kernel.Commit to be reachable from
Kernel.Branch. We can start cause bisections from any existing commit.

This should help with linux-next bisections, where no older HEADs are
ever reachable from each new `master`.

2 files changed, 29 insertions, 2 deletions

diff --git a/pkg/bisect/bisect.go b/pkg/bisect/bisect.go
index 440de5a5d..fbb8d031c 100644
--- a/pkg/bisect/bisect.go
+++ b/pkg/bisect/bisect.go
@@ -348,6 +348,22 @@ func (env *env) identifyRewrittenCommit() (string, error) {
 		return cfg.Kernel.Commit, err
 	}
 
+	if !cfg.Fix {
+		// If we're doing a cause bisection, we don't really need the commit to be
+		// reachable from cfg.Kernel.Branch.
+		// So let's try to force tag fetch and check if the commit is present in the
+		// repository.
+		env.log("fetch other tags and check if the commit is present")
+		commit, err := env.repo.CheckoutCommit(cfg.Kernel.Repo, cfg.Kernel.Commit)
+		if err != nil {
+			// Ignore the error because the command will fail if the commit is really not
+			// present in the tree.
+			env.log("fetch failed with %s", err)
+		} else if commit != nil {
+			return commit.Hash, nil
+		}
+	}
+
 	// We record the tested kernel commit when syzkaller triggers a crash. These commits can become
 	// unreachable after the crash was found, when the history of the tested kernel branch was
 	// rewritten. The commit might have been completely deleted from the branch or just changed in
diff --git a/pkg/bisect/bisect_test.go b/pkg/bisect/bisect_test.go
index c57ef7020..95849e419 100644
--- a/pkg/bisect/bisect_test.go
+++ b/pkg/bisect/bisect_test.go
@@ -173,7 +173,8 @@ func createTestRepo(t *testing.T) string {
 			}
 		}
 	}
-	// Emulate another tree, that's needed for cross-tree tests.
+	// Emulate another tree, that's needed for cross-tree tests and
+	// for cause bisections for commits not reachable from master.
 	repo.Git("checkout", "v8.0")
 	repo.Git("checkout", "-b", "v8-branch")
 	repo.CommitFileChange("850", "v8-branch")
@@ -248,7 +249,7 @@ func testBisection(t *testing.T, baseDir string, test BisectionTest) {
 
 	res, err := runImpl(cfg, r, inst)
 	checkBisectionError(test, res, err)
-	if !test.crossTree {
+	if !test.crossTree && !test.noFakeHashTest {
 		// Should be mitigated via GetCommitByTitle during bisection.
 		cfg.Kernel.Commit = fmt.Sprintf("fake-hash-for-%v-%v", cfg.Kernel.Commit, cfg.Kernel.CommitTitle)
 		res, err = runImpl(cfg, r, inst)
@@ -326,6 +327,7 @@ type BisectionTest struct {
 	baselineConfig  string
 	resultingConfig string
 	crossTree       bool
+	noFakeHashTest  bool
 
 	extraTest func(t *testing.T, res *Result)
 }
@@ -662,6 +664,15 @@ var bisectionTests = []BisectionTest{
 		crossTree:         true,
 		fixCommit:         "903",
 	},
+	{
+		name:              "cause-finds-other-branch-commit",
+		startCommit:       852,
+		startCommitBranch: "v8-branch",
+		commitLen:         1,
+		expectRep:         true,
+		introduced:        "602",
+		noFakeHashTest:    true,
+	},
 }
 
 func TestBisectionResults(t *testing.T) {