errors: remove verifier truncation heuristic, rely on ENOSPC

In order to capture full verifier errors of large programs, the caller can
choose to repeatedly retry loading the program with progressively larger log
buffers, since the important info is always at the end of the log.

In doing so, it's incredibly likely for the last character of the buffer to
be a newline, resulting in the truncated flag being set to false when it should
instead be set to true, regardless of the kernel's ENOSPC return code.

This commit removes the newline heuristic that currently aims to detect log
truncation. Instead, we now require ENOSPC to be sent by the kernel in order
for the Truncated flag to be set in the VerifierError.

The heuristic was initially introduced for parsing BTF verifier errors,
since there's a latent kernel bug that causes ENOSPC only being returned
on a successful BTF load. Unfortunately, it turns out that not all BTF
verifier errors end in a newline either (see calls to btf_verifier_log()),
making the newline heuristic ineffective for BTF logs as well.

ErrorWithLog now takes an ancillary error from a second load operation that
is used to determine truncation status. This second error will not be wrapped
into the VerifierError.

Signed-off-by: Timo Beckers <timo@isovalent.com>

btf/btf.go

@@ -737,9 +737,13 @@ func NewHandle(spec *Spec) (*Handle, error) {
 		attr.BtfLogBuf = sys.NewSlicePointer(logBuf)
 		attr.BtfLogSize = uint32(len(logBuf))
 		attr.BtfLogLevel = 1
-		// NB: The syscall will never return ENOSPC as of 5.18-rc4.
-		_, _ = sys.BtfLoad(attr)
-		return nil, internal.ErrorWithLog(err, logBuf)
+
+		// Up until at least kernel 6.0, the BTF verifier does not return ENOSPC
+		// if there are other verification errors. ENOSPC is only returned when
+		// the BTF blob is correct, a log was requested, and the provided buffer
+		// is too small.
+		_, ve := sys.BtfLoad(attr)
+		return nil, internal.ErrorWithLog(err, logBuf, errors.Is(ve, unix.ENOSPC))
 	}
 
 	return &Handle{fd, attr.BtfSize}, nil

internal/errors.go

@@ -7,34 +7,23 @@ import (
 	"strings"
 )
 
-// ErrorWithLog returns an error which includes logs from the kernel verifier.
+// ErrorWithLog wraps err in a VerifierError that includes the parsed verifier
+// log buffer.
 //
 // The default error output is a summary of the full log. The latter can be
 // accessed via VerifierError.Log or by formatting the error, see Format.
-//
-// A set of heuristics is used to determine whether the log has been truncated.
-func ErrorWithLog(err error, log []byte) *VerifierError {
+func ErrorWithLog(err error, log []byte, truncated bool) *VerifierError {
 	const whitespace = "\t\r\v\n "
 
 	// Convert verifier log C string by truncating it on the first 0 byte
 	// and trimming trailing whitespace before interpreting as a Go string.
-	truncated := false
 	if i := bytes.IndexByte(log, 0); i != -1 {
-		if i > 0 && i == len(log)-1 && log[i-1] != '\n' {
-			// The null byte is at the end of the buffer and it's not preceded
-			// by a newline character. Most likely the buffer was too short.
-			truncated = true
-		}
-
 		log = log[:i]
-	} else if len(log) > 0 {
-		// No null byte? Dodgy!
-		truncated = true
 	}
 
 	log = bytes.Trim(log, whitespace)
 	if len(log) == 0 {
-		return &VerifierError{err, nil, false}
+		return &VerifierError{err, nil, truncated}
 	}
 
 	logLines := bytes.Split(log, []byte{'\n'})

internal/errors_test.go

@@ -21,62 +21,59 @@ func TestVerifierErrorWhitespace(t *testing.T) {
 		0, 0, // trailing NUL bytes
 	)
 
-	err := ErrorWithLog(errors.New("test"), b)
+	err := ErrorWithLog(errors.New("test"), b, false)
 	qt.Assert(t, err.Error(), qt.Equals, "test: unreachable insn 28")
 
-	err = ErrorWithLog(errors.New("test"), nil)
+	err = ErrorWithLog(errors.New("test"), nil, false)
 	qt.Assert(t, err.Error(), qt.Equals, "test")
 
-	err = ErrorWithLog(errors.New("test"), []byte("\x00"))
+	err = ErrorWithLog(errors.New("test"), []byte("\x00"), false)
 	qt.Assert(t, err.Error(), qt.Equals, "test")
 
-	err = ErrorWithLog(errors.New("test"), []byte(" "))
+	err = ErrorWithLog(errors.New("test"), []byte(" "), false)
 	qt.Assert(t, err.Error(), qt.Equals, "test")
 }
 
-func TestVerifierError(t *testing.T) {
-	for _, test := range []struct {
-		name string
-		log  string
-	}{
-		{"missing null", "foo"},
-		{"missing newline before null", "foo\x00"},
-	} {
-		t.Run("truncate "+test.name, func(t *testing.T) {
-			ve := ErrorWithLog(syscall.ENOENT, []byte(test.log))
-			qt.Assert(t, ve, qt.IsNotNil, qt.Commentf("should return error"))
-			qt.Assert(t, ve.Truncated, qt.IsTrue, qt.Commentf("should be truncated"))
-		})
-	}
+func TestVerifierErrorWrapping(t *testing.T) {
+	ve := ErrorWithLog(unix.ENOENT, nil, false)
+	qt.Assert(t, ve, qt.ErrorIs, unix.ENOENT, qt.Commentf("should wrap provided error"))
+	qt.Assert(t, ve.Truncated, qt.IsFalse, qt.Commentf("verifier log should not be marked as truncated"))
+
+	ve = ErrorWithLog(unix.EINVAL, nil, true)
+	qt.Assert(t, ve, qt.ErrorIs, unix.EINVAL, qt.Commentf("should wrap provided error"))
+	qt.Assert(t, ve.Truncated, qt.IsTrue, qt.Commentf("verifier log should be marked as truncated"))
 
-	ve := ErrorWithLog(syscall.ENOENT, nil)
-	qt.Assert(t, ve, qt.IsNotNil, qt.Commentf("should return error without log or logErr"))
+	ve = ErrorWithLog(unix.EINVAL, []byte("foo"), false)
+	qt.Assert(t, ve, qt.ErrorIs, unix.EINVAL, qt.Commentf("should wrap provided error"))
+	qt.Assert(t, ve.Error(), qt.Contains, "foo", qt.Commentf("verifier log should appear in error string"))
 
+	ve = ErrorWithLog(unix.ENOSPC, []byte("foo"), true)
+	qt.Assert(t, ve, qt.ErrorIs, unix.ENOSPC, qt.Commentf("should wrap provided error"))
+	qt.Assert(t, ve.Error(), qt.Contains, "foo", qt.Commentf("verifier log should appear in error string"))
+	qt.Assert(t, ve.Truncated, qt.IsTrue, qt.Commentf("verifier log should be marked truncated"))
+}
+
+func TestVerifierErrorSummary(t *testing.T) {
+	// Suppress the last line containing 'processed ... insns'.
 	errno524 := readErrorFromFile(t, "testdata/errno524.log")
-	t.Log(errno524)
 	qt.Assert(t, errno524.Error(), qt.Contains, "JIT doesn't support bpf-to-bpf calls")
 	qt.Assert(t, errno524.Error(), qt.Not(qt.Contains), "processed 39 insns")
 
+	// Include the previous line if the current one starts with a tab.
 	invalidMember := readErrorFromFile(t, "testdata/invalid-member.log")
-	t.Log(invalidMember)
 	qt.Assert(t, invalidMember.Error(), qt.Contains, "STRUCT task_struct size=7744 vlen=218: cpus_mask type_id=109 bitfield_size=0 bits_offset=7744 Invalid member")
 
+	// Only include the last line.
 	issue43 := readErrorFromFile(t, "testdata/issue-43.log")
-	t.Log(issue43)
 	qt.Assert(t, issue43.Error(), qt.Contains, "[11] FUNC helper_func2 type_id=10 vlen != 0")
 	qt.Assert(t, issue43.Error(), qt.Not(qt.Contains), "[10] FUNC_PROTO (anon) return=3 args=(3 arg)")
 
-	truncated := readErrorFromFile(t, "testdata/truncated.log")
-	t.Log(truncated)
-	qt.Assert(t, truncated.Truncated, qt.IsTrue)
-	qt.Assert(t, truncated.Error(), qt.Contains, "str_off: 3166088: str_len: 228")
-
+	// Include instruction that caused invalid register access.
 	invalidR0 := readErrorFromFile(t, "testdata/invalid-R0.log")
-	t.Log(invalidR0)
 	qt.Assert(t, invalidR0.Error(), qt.Contains, "0: (95) exit: R0 !read_ok")
 
+	// Include symbol that doesn't match context type.
 	invalidCtx := readErrorFromFile(t, "testdata/invalid-ctx-access.log")
-	t.Log(invalidCtx)
 	qt.Assert(t, invalidCtx.Error(), qt.Contains, "func '__x64_sys_recvfrom' arg0 type FWD is not a struct: invalid bpf_context access off=0 size=8")
 }
 
@@ -120,5 +117,5 @@ func readErrorFromFile(tb testing.TB, file string) *VerifierError {
 		tb.Fatal("Read file:", err)
 	}
 
-	return ErrorWithLog(unix.EINVAL, contents)
+	return ErrorWithLog(unix.EINVAL, contents, false)
 }

prog.go

@@ -329,14 +329,17 @@ func newProgramWithOptions(spec *ProgramSpec, opts ProgramOptions, handles *hand
 		return &Program{unix.ByteSliceToString(logBuf), fd, spec.Name, "", spec.Type}, nil
 	}
 
-	// If the caller did not specify a log level,
-	// re-run with branch-level verifier logs enabled.
+	// A verifier error occurred, but the caller did not specify a log level.
+	// Re-run with branch-level verifier logs enabled to obtain more info.
+	var truncated bool
 	if !opts.LogDisabled && opts.LogLevel == 0 {
 		logBuf = make([]byte, opts.LogSize)
 		attr.LogLevel = LogLevelBranch
 		attr.LogSize = uint32(len(logBuf))
 		attr.LogBuf = sys.NewSlicePointer(logBuf)
-		_, _ = sys.ProgLoad(attr)
+
+		_, ve := sys.ProgLoad(attr)
+		truncated = errors.Is(ve, unix.ENOSPC)
 	}
 
 	switch {
@@ -361,7 +364,7 @@ func newProgramWithOptions(spec *ProgramSpec, opts ProgramOptions, handles *hand
 		}
 	}
 
-	err = internal.ErrorWithLog(err, logBuf)
+	err = internal.ErrorWithLog(err, logBuf, truncated)
 	if btfDisabled {
 		return nil, fmt.Errorf("load program: %w (kernel without BTF support)", err)
 	}