mkuimage: advise GOAMD64=v1 for GOARCH=amd64

If the user builds an initrd for amd64, and their golang toolchain is
configured for an amd64 ABI version greater than 1, then issue a warning.

Binaries built for GOAMD64=v2+ may crash with an #UD exception on old
CPUs, or exit immediately (gracefully) with an error message like "This
program can only be run on AMD64 processors with v2 microarchitecture
support" [1]. Such a dysfunctional "init" (gobusybox) binary renders a
u-root initrd effectively unbootable. qemu-system-x86_64 with no
particular "-cpu" option is affected by this, for example.

GOAMD64 defaults to v1 [2], but it may be overridden in at least three
places (in increasing order of importance):

- the system-wide "go.env" file, such as "/usr/lib/golang/go.env",

- the user's "go.env" file, such as "$HOME/.config/go/env",

- the env var GOAMD64.

For example, RHEL9 and its derivative distros set GOAMD64 to v2 in
"/usr/lib/golang/go.env".

It seems like the canonical way for deducing GOAMD64 programmatically is
to invoke "go env GOAMD64" [3].

[1] golang/go@8c8baad927b2
[2] https://go.dev/wiki/MinimumRequirements#amd64
[3] golang/go#72791

Signed-off-by: Laszlo Ersek <[email protected]>

Author: lersek

README.md

@@ -260,6 +260,33 @@ $ mkuimage -files "$HOME/hello.ko:etc/hello.ko" -files "$HOME/hello2.ko:etc/hell
 $ qemu-system-x86_64 -kernel /boot/vmlinuz-$(uname -r) -initrd /tmp/initramfs.linux_amd64.cpio
 ```
 
+## AMD64 Architecture Level
+
+Before building for AMD64, verify that the command
+
+```shell
+go env GOAMD64
+```
+
+prints `v1`. A [`GOAMD64` setting](https://go.dev/wiki/MinimumRequirements#amd64)
+of any higher version may produce such binaries that don't execute on old AMD64
+processors (including the default CPU model of QEMU).
+
+`GOAMD64` can be reset to `v1` with one of the following methods:
+
+*   through the `GOAMD64` environment variable:
+
+    ```shell
+    export GOAMD64=v1
+    ```
+
+*   through `go env` (only takes effect if the `GOAMD64` environment variable
+    is not set):
+
+    ```shell
+    go env -w GOAMD64=v1
+    ```
+
 ## Cross Compilation (targeting different architectures and OSes)
 
 Just like standard Go tooling, cross compilation is easy and supported.

uimage/mkuimage/cmd.go

@@ -5,12 +5,15 @@
 package mkuimage
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
 	"os"
+	"os/exec"
 	"runtime"
 	"strings"
 
+	"github.com/u-root/gobusybox/src/pkg/golang"
 	"github.com/u-root/mkuimage/uimage"
 	"github.com/u-root/mkuimage/uimage/builder"
 	"github.com/u-root/mkuimage/uimage/templates"
@@ -56,6 +59,32 @@ func uimageOpts(l *llog.Logger, m []uimage.Modifier, tpl *templates.Templates, f
 	return uimage.OptionsFor(append(m, more...)...)
 }
 
+func checkAmd64Level(l *llog.Logger, env *golang.Environ) {
+	if env.GOARCH != "amd64" {
+		return
+	}
+
+	// Looking for "amd64.v2" in "env.ToolTags" is unreliable; see
+	// <https://github.com/golang/go/issues/72791>. Invoke "go env" instead.
+	var bad string
+	abiLevel, err := exec.Command("go", "env", "GOAMD64").Output()
+	if err == nil {
+		if bytes.Equal(abiLevel, []byte("v1\n")) {
+			return
+		}
+		bad = "is not"
+	} else {
+		if exerr, isExErr := err.(*exec.ExitError); isExErr {
+			l.Warnf("\"go env\" failed: %s", exerr.Stderr)
+		} else {
+			l.Warnf("couldn't execute \"go env\": %s", err)
+		}
+		bad = "may not be"
+	}
+	l.Warnf("GOAMD64 %s set to v1; on older CPUs, binaries built into " +
+		"the initrd may crash or refuse to run.", bad)
+}
+
 // CreateUimage creates a uimage with the given base modifiers and flags, using args as the list of commands.
 func CreateUimage(l *llog.Logger, base []uimage.Modifier, tf *TemplateFlags, f *Flags, args []string) error {
 	tpl, err := tf.Get()
@@ -95,6 +124,8 @@ func CreateUimage(l *llog.Logger, base []uimage.Modifier, tf *TemplateFlags, f *
 		l.Warnf("GOOS is not linux. Did you mean to set GOOS=linux?")
 	}
 
+	checkAmd64Level(l, env);
+
 	v, err := env.Version()
 	if err != nil {
 		l.Infof("Could not get environment's Go version, using runtime's version: %v", err)