x/tools/go/analysis/passes/printf: `%#x` recursion is not recognized

[database64128]

Go version

go version go1.24.3 linux/amd64

Output of go env in your module/workspace:

AR='ar'
CC='gcc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='g++'
GCCGO='gccgo'
GO111MODULE=''
GOAMD64='v3'
GOARCH='amd64'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/home/database64128/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/home/database64128/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build676182649=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/dev/null'
GOMODCACHE='/home/database64128/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/database64128/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/lib/go'
GOSUMDB='sum.golang.org'
GOTELEMETRY='on'
GOTELEMETRYDIR='/home/database64128/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='local'
GOTOOLDIR='/usr/lib/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.24.3'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

type UnsupportedVersionError byte

func (v UnsupportedVersionError) Error() string {
	return fmt.Sprintf("unsupported version: %#x", v)
}

What did you see happen?

vet did not recognize %#x or %#X causes recursive Error call.

Change the format verb to %x and it correctly reports the issue.

What did you expect to see?

vet should recognize that %#x and %#X cause recursive Error call.

[gabyhelp]

Related Issues

• cmd/vet: doesn't handle formatting strings in variables #44063
• cmd/vet: Should warn against using %v print verb in Error() methods #33884 (closed)
• cmd/vet: Printf format %.T has unrecognized flag #30363
• x/xerrors: tests broken (via vet error) with go1.15rc1 #40536 (closed)
• go vet incorrectly recognizes format verb in string #38131 (closed)
• cmd/vet: TestVet fails with the latest tip version #33481 (closed)
• cmd/vet: false positive for user-defined verbs #36564 (closed)
• cmd/vet: exit code should be 1 in case a problem is reported, not 2 #45515
• cmd/vet: printf analysis missing support for %O #36385 (closed)
• cmd/vet: Go vet misses error when running in shell and not Makefile #21688 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[adonovan]

I think the checker is working as intended: the # flag prevents the calling of String or Error methods on the value, defeating the recursion, and the analyzer knows this. Example: https://go.dev/play/p/3ACWO2vBqAb.

Please re-open if I misunderstand.

[database64128]

@adonovan It's %#x or %#X, not %#v. They are not the same. Your example stops working once I change %#v to %#x or %#X.

I do not have the required permissions to reopen the issue.

[adonovan]

Sorry, I need to get my eyes checked. This is indeed a bug. The logic in x/tools/go/analysis/passes/printf needs to examine the interaction between v.verb and operation.Flags more carefully:

	if v.typ&argString != 0 && v.verb != 'T' && !strings.Contains(operation.Flags, "#") {
		if methodName, ok := recursiveStringer(pass, arg); ok {
			pass.ReportRangef(call, "%s format %s with arg %s causes recursive %s method call", name, operation.Text, analysisinternal.Format(pass.Fset, arg), methodName)
			return false
		}
	}

[gopherbot]

Change https://go.dev/cl/676115 mentions this issue: go/analysis/passes/printf: fix the issue where %#x recursion is not recognized

[cuishuang]

It does seem to be a bug. The %#x format does not trigger recursive call detection in go vet ./...

I have reviewed and modified the source code and added tests. Please review.