Introduce fips build tag (#14495)

Unlike fips_140_3, fips will be a (FIPS) version-agnostic build tag.
The listener support will remain in 140-3 only, but the IsFIPS() check
should apply regardless of FIPS version.

We add two FIPS-only build files which validate the constraints of FIPS
builds here: fips must be specified with either fips_140_2 or fips_140_3
build tags, and fips and cgo must also be specified together.
Additionally, using only a version-specific FIPS build tag without the
version-agnostic FIPS tag should be a failure.

Signed-off-by: Alexander Scheel <alex.scheel@hashicorp.com>

helper/constants/fips.go

@@ -1,4 +1,4 @@
-//go:build !fips_140_3
+//go:build !fips
 
 package constants
 

helper/constants/fips_build_check.go

@@ -0,0 +1,38 @@
+//go:build (!fips && (fips_140_2 || fips_140_3)) || (fips && !fips_140_2 && !fips_140_3) || (fips_140_2 && fips_140_3)
+
+package constants
+
+import "C"
+
+// This function is the equivalent of an external (CGo) function definition,
+// without implementation in any imported or built library. This results in
+// a linker err if the above build constraints are satisfied:
+//
+//   /home/cipherboy/GitHub/cipherboy/vault-enterprise/helper/constants/fips_build_check.go:10: undefined reference to `github.com/hashicorp/vault/helper/constants.VaultFIPSBuildRequiresVersionAgnosticTagAndOneVersionTag'
+//
+// This indicates that a build error has occurred due to mismatched tags.
+//
+// In particular, we use this to enforce the following restrictions on build
+// tags:
+//
+//  - If a versioned fips_140_* tag is specified, the unversioned tag must
+//    also be.
+//  - If the unversioned tag is specified, a versioned tag must be.
+//  - Both versioned flags cannot be specified at the same time.
+//
+// In the unlikely event that a FFI implementation for this function exists
+// in the future, it should be renamed to a new function which does not
+// exist.
+//
+// This approach was chosen above the other implementation in fips_cgo_check.go
+// because this version does not break static analysis tools: most tools do not
+// cross the CGo boundary and thus do not know that the below function is
+// missing an implementation. However, in the other file, the function call is
+// not marked as CGo (in part large because the lack of a cgo build tag
+// prohibits us from using the same technique) and thus it must be a Go
+// declaration, that is missing.
+func VaultFIPSBuildRequiresVersionAgnosticTagAndOneVersionTag()
+
+func init() {
+	VaultFIPSBuildRequiresVersionAgnosticTagAndOneVersionTag()
+}

helper/constants/fips_cgo_check.go

@@ -0,0 +1,18 @@
+//go:build (fips || fips_140_2 || fips_140_3) && !cgo
+
+package constants
+
+func init() {
+	// See note in fips_build_check.go.
+	//
+	// This function call is missing a declaration, causing the build to
+	// fail on improper tags (fips specified but cgo not specified). This
+	// ensures Vault fails to build if a FIPS build is requested but CGo
+	// support is not enabled.
+	//
+	// Note that this could confuse static analysis tools as this function
+	// should not ever be defined. If this function is defined in the future,
+	// the below reference should be renamed to a new name that is not
+	// defined to ensure we get a build failure.
+	VaultFIPSBuildTagMustEnableCGo()
+}