CGO/Clang Build failure

[ReillyBrogan]

We just updated to runc 1.5.0 which uses libpathrs 0.2.5 and since we use Clang as our default compiler we are now getting a build failure that appears to be in go-pathrs:

/mason/gomodcache/cyphar.com/go-pathrs@v0.2.5/internal/libpathrs/libpathrs_linux.go:256:32: cannot use (_Ciconst_PATHRS_PROC_ROOT) (untyped int constant 18446744067006164835) as int64 value in variable declaration (overflows)
/mason/gomodcache/cyphar.com/go-pathrs@v0.2.5/internal/libpathrs/libpathrs_linux.go:257:32: cannot use (_Ciconst_PATHRS_PROC_SELF) (untyped int constant 18446744065272536607) as int64 value in variable declaration (overflows)
/mason/gomodcache/cyphar.com/go-pathrs@v0.2.5/internal/libpathrs/libpathrs_linux.go:258:32: cannot use (_Ciconst_PATHRS_PROC_THREAD_SELF) (untyped int constant 18446744066171166239) as int64 value in variable declaration (overflows)

As far as we can tell the behavior of variables here is slightly different between Clang and GCC and there may need to be an explicit cast to int64 added on the go side.

This is using Golang 1.26.4 and Clang 22.1.8.

[cyphar]

Grr, the logic there was done specifically to deal with casting issues in Go. This init function exists because we cannot use the C.* values in all of the contexts we need to use them, so we use Go const values and assert at runtime they are the same as the C ones.

func assertEqual[T comparable](a, b T, msg string) {
	if a != b {
		panic(fmt.Sprintf("%s ((%T) %#v != (%T) %#v)", msg, a, a, b, b))
	}
}

// Verify that the values above match the actual C values. Unfortunately, Go
// only allows us to forcefully cast int64 to uint64 if you use a temporary
// variable, which means we cannot do it in a const context and thus need to do
// it at runtime (even though it is a check that fundamentally could be done at
// compile-time)...
func init() {
	var (
		actualProcRoot       int64 = C.PATHRS_PROC_ROOT
		actualProcSelf       int64 = C.PATHRS_PROC_SELF
		actualProcThreadSelf int64 = C.PATHRS_PROC_THREAD_SELF
	)

	assertEqual(ProcRoot, ProcBase(actualProcRoot), "PATHRS_PROC_ROOT")
	assertEqual(ProcSelf, ProcBase(actualProcSelf), "PATHRS_PROC_SELF")
	assertEqual(ProcThreadSelf, ProcBase(actualProcThreadSelf), "PATHRS_PROC_THREAD_SELF")

	var (
		actualProcBaseTypeMask uint64 = C.__PATHRS_PROC_TYPE_MASK
		actualProcBaseTypePid  uint64 = C.__PATHRS_PROC_TYPE_PID
	)

	assertEqual(ProcBaseTypeMask, ProcBase(actualProcBaseTypeMask), "__PATHRS_PROC_TYPE_MASK")
	assertEqual(ProcBaseTypePid, ProcBase(actualProcBaseTypePid), "__PATHRS_PROC_TYPE_PID")

	assertEqual(ProcDefaultRootFd, int(C.PATHRS_PROC_DEFAULT_ROOTFD), "PATHRS_PROC_DEFAULT_ROOTFD")
}

The errors are coming from the actual... int64 = C.PATHS_... lines -- using uint64 causes the following errors with GCC:

# cyphar.com/go-pathrs/internal/libpathrs
../../go-pathrs/internal/libpathrs/libpathrs_linux.go:256:33: cannot use (_Ciconst_PATHRS_PROC_ROOT) (untyped int constant -6703386781) as uint64 value in variable declaration (overflows)
../../go-pathrs/internal/libpathrs/libpathrs_linux.go:257:33: cannot use (_Ciconst_PATHRS_PROC_SELF) (untyped int constant -8437015009) as uint64 value in variable declaration (overflows)
../../go-pathrs/internal/libpathrs/libpathrs_linux.go:258:33: cannot use (_Ciconst_PATHRS_PROC_THREAD_SELF) (untyped int constant -7538385377) as uint64 value in variable declaration (overflows)

🤔

[cyphar]

Note that we are doing an explicit int64 cast, that is what's causing the issue:

var (
	actualProcRoot       int64 = C.PATHRS_PROC_ROOT
	actualProcSelf       int64 = C.PATHRS_PROC_SELF
	actualProcThreadSelf int64 = C.PATHRS_PROC_THREAD_SELF
)

(Moving int64 to the right side has the same behaviour.)