cmd/go/internal/work: use pie link mode when using MSAN on arm64

Currently, when running the "CC=clang go run -msan misc/cgo/
testsanitizers/testdata/msan.go" command on arm64, it will
report an error and the error is reported by llvm/compiler-rt/
lib/msan and it is "Make sure to compile with -fPIE and to link
with -pie".

This CL fixes this issue, using PIE link mode when using MSAN
on arm64.

This CL also updates the related document and go build help message.

Fixes #33712

Change-Id: I0cc9d95f3fa264d6c042c27a40ccbb82826922fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/190482
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/alldocs.go

@@ -110,11 +110,13 @@
 // 		The default is the number of CPUs available.
 // 	-race
 // 		enable data race detection.
-// 		Supported only on linux/amd64, freebsd/amd64, darwin/amd64 and windows/amd64.
+// 		Supported only on linux/amd64, freebsd/amd64, darwin/amd64, windows/amd64,
+// 		linux/ppc64le and linux/arm64 (only for 48-bit VMA).
 // 	-msan
 // 		enable interoperation with memory sanitizer.
 // 		Supported only on linux/amd64, linux/arm64
 // 		and only with Clang/LLVM as the host C compiler.
+// 		On linux/arm64, pie build mode will be used.
 // 	-v
 // 		print the names of packages as they are compiled.
 // 	-work

src/cmd/go/internal/work/build.go

@@ -62,11 +62,13 @@ and test commands:
 		The default is the number of CPUs available.
 	-race
 		enable data race detection.
-		Supported only on linux/amd64, freebsd/amd64, darwin/amd64 and windows/amd64.
+		Supported only on linux/amd64, freebsd/amd64, darwin/amd64, windows/amd64,
+		linux/ppc64le and linux/arm64 (only for 48-bit VMA).
 	-msan
 		enable interoperation with memory sanitizer.
 		Supported only on linux/amd64, linux/arm64
 		and only with Clang/LLVM as the host C compiler.
+		On linux/arm64, pie build mode will be used.
 	-v
 		print the names of packages as they are compiled.
 	-work

src/cmd/go/internal/work/init.go

@@ -60,6 +60,11 @@ func instrumentInit() {
 	mode := "race"
 	if cfg.BuildMSan {
 		mode = "msan"
+		// MSAN does not support non-PIE binaries on ARM64.
+		// See issue #33712 for details.
+		if cfg.Goos == "linux" && cfg.Goarch == "arm64" && cfg.BuildBuildmode == "default" {
+			cfg.BuildBuildmode = "pie"
+		}
 	}
 	modeFlag := "-" + mode