[release-branch.go1.15] cmd/go: don't lookup the path for CC when invoking cgo

Previously, if CC was a path without separators (like gcc or clang),
we'd look it up in PATH in cmd/go using internal/execabs.LookPath,
then pass the resolved path to cgo in CC.

This caused a regression: if the directory in PATH containing CC has a
space, cgo splits it and interprets it as multiple arguments.

With this change, cmd/go no longer resolves CC before invoking
cgo. cgo does the path lookup on each invocation. This reverts the
security fix CL 284780, but that was redundant with the addition of
internal/execabs (CL 955304), which still protects us.

NOTE: This CL includes a related test fix from CL 286292.

Fixes #43860

Change-Id: I65d91a1e303856df8653881eb6e2e75a3bf95c49
Reviewed-on: https://go-review.googlesource.com/c/go/+/285873
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
(cherry picked from commit a2cef9b544708ecae983ed8836ee2425a28aab68)
Reviewed-on: https://go-review.googlesource.com/c/go/+/285954
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>

4 files changed, 72 insertions, 26 deletions

diff --git a/src/cmd/go/internal/work/action.go b/src/cmd/go/internal/work/action.go
index 03ca301bdd8..6b5f9e48079 100644
--- a/src/cmd/go/internal/work/action.go
+++ b/src/cmd/go/internal/work/action.go
@@ -56,9 +56,6 @@ type Builder struct {
 	id           sync.Mutex
 	toolIDCache  map[string]string // tool name -> tool ID
 	buildIDCache map[string]string // file name -> build ID
-
-	cgoEnvOnce  sync.Once
-	cgoEnvCache []string
 }
 
 // NOTE: Much of Action would not need to be exported if not for test.
diff --git a/src/cmd/go/internal/work/exec.go b/src/cmd/go/internal/work/exec.go
index f5f9951afa3..eb1efd9f824 100644
--- a/src/cmd/go/internal/work/exec.go
+++ b/src/cmd/go/internal/work/exec.go
@@ -1081,7 +1081,10 @@ func (b *Builder) vet(a *Action) error {
 	}
 
 	// TODO(rsc): Why do we pass $GCCGO to go vet?
-	env := b.cgoEnv()
+	env := b.cCompilerEnv()
+	if cfg.BuildToolchainName == "gccgo" {
+		env = append(env, "GCCGO="+BuildToolchain.compiler())
+	}
 
 	p := a.Package
 	tool := VetTool
@@ -2015,24 +2018,6 @@ func (b *Builder) cCompilerEnv() []string {
 	return []string{"TERM=dumb"}
 }
 
-// cgoEnv returns environment variables to set when running cgo.
-// Some of these pass through to cgo running the C compiler,
-// so it includes cCompilerEnv.
-func (b *Builder) cgoEnv() []string {
-	b.cgoEnvOnce.Do(func() {
-		cc, err := exec.LookPath(b.ccExe()[0])
-		if err != nil || filepath.Base(cc) == cc { // reject relative path
-			cc = "/missing-cc"
-		}
-		gccgo := GccgoBin
-		if filepath.Base(gccgo) == gccgo { // reject relative path
-			gccgo = "/missing-gccgo"
-		}
-		b.cgoEnvCache = append(b.cCompilerEnv(), "CC="+cc, "GCCGO="+gccgo)
-	})
-	return b.cgoEnvCache
-}
-
 // mkdir makes the named directory.
 func (b *Builder) Mkdir(dir string) error {
 	// Make Mkdir(a.Objdir) a no-op instead of an error when a.Objdir == "".
@@ -2622,7 +2607,7 @@ func (b *Builder) cgo(a *Action, cgoExe, objdir string, pcCFLAGS, pcLDFLAGS, cgo
 	// along to the host linker. At this point in the code, cgoLDFLAGS
 	// consists of the original $CGO_LDFLAGS (unchecked) and all the
 	// flags put together from source code (checked).
-	cgoenv := b.cgoEnv()
+	cgoenv := b.cCompilerEnv()
 	if len(cgoLDFLAGS) > 0 {
 		flags := make([]string, len(cgoLDFLAGS))
 		for i, f := range cgoLDFLAGS {
@@ -2843,7 +2828,7 @@ func (b *Builder) dynimport(a *Action, p *load.Package, objdir, importGo, cgoExe
 	if p.Standard && p.ImportPath == "runtime/cgo" {
 		cgoflags = []string{"-dynlinker"} // record path to dynamic linker
 	}
-	return b.run(a, p.Dir, p.ImportPath, b.cgoEnv(), cfg.BuildToolexec, cgoExe, "-dynpackage", p.Name, "-dynimport", dynobj, "-dynout", importGo, cgoflags)
+	return b.run(a, p.Dir, p.ImportPath, b.cCompilerEnv(), cfg.BuildToolexec, cgoExe, "-dynpackage", p.Name, "-dynimport", dynobj, "-dynout", importGo, cgoflags)
 }
 
 // Run SWIG on all SWIG input files.
diff --git a/src/cmd/go/testdata/script/cgo_path.txt b/src/cmd/go/testdata/script/cgo_path.txt
index 0d159984262..3eba71bc158 100644
--- a/src/cmd/go/testdata/script/cgo_path.txt
+++ b/src/cmd/go/testdata/script/cgo_path.txt
@@ -1,12 +1,20 @@
 [!cgo] skip
 
+# Set CC explicitly to something that requires a PATH lookup.
+# Normally, the default is gcc or clang, but if CC was set during make.bash,
+# that becomes the default.
+[exec:clang] env CC=clang
+[exec:gcc] env CC=gcc
+[!exec:clang] [!exec:gcc] skip 'Unknown C compiler'
+
 env GOCACHE=$WORK/gocache  # Looking for compile flags, so need a clean cache.
 [!windows] env PATH=.:$PATH
-[!windows] chmod 0777 p/gcc p/clang
+[!windows] chmod 0755 p/gcc p/clang
 [!windows] exists -exec p/gcc p/clang
 [windows] exists -exec p/gcc.bat p/clang.bat
 ! exists p/bug.txt
-go build -x
+! go build -x
+stderr '^cgo: exec (clang|gcc): (clang|gcc) resolves to executable in current directory \(.[/\\](clang|gcc)(.bat)?\)$'
 ! exists p/bug.txt
 
 -- go.mod --
diff --git a/src/cmd/go/testdata/script/cgo_path_space.txt b/src/cmd/go/testdata/script/cgo_path_space.txt
new file mode 100644
index 00000000000..654295dc692
--- /dev/null
+++ b/src/cmd/go/testdata/script/cgo_path_space.txt
@@ -0,0 +1,56 @@
+# Check that if the PATH directory containing the C compiler has a space,
+# we can still use that compiler with cgo.
+# Verifies #43808.
+[!cgo] skip
+
+# Set CC explicitly to something that requires a PATH lookup.
+# Normally, the default is gcc or clang, but if CC was set during make.bash,
+# that becomes the default.
+[exec:clang] env CC=clang
+[exec:gcc] env CC=gcc
+[!exec:clang] [!exec:gcc] skip 'Unknown C compiler'
+
+[!windows] chmod 0755 $WORK/'program files'/clang
+[!windows] chmod 0755 $WORK/'program files'/gcc
+[!windows] exists -exec $WORK/'program files'/clang
+[!windows] exists -exec $WORK/'program files'/gcc
+[!windows] env PATH=$WORK/'program files':$PATH
+[windows] exists -exec $WORK/'program files'/gcc.bat
+[windows] exists -exec $WORK/'program files'/clang.bat
+[windows] env PATH=$WORK\'program files';%PATH%
+
+! exists $WORK/log.txt
+? go build -x
+exists $WORK/log.txt
+rm $WORK/log.txt
+
+# TODO(#41400, #43078): when CC is set explicitly, it should be allowed to
+# contain spaces separating arguments, and it should be possible to quote
+# arguments with spaces (including the path), as in CGO_CFLAGS and other
+# variables. For now, this doesn't work.
+[!windows] env CC=$WORK/'program files'/gcc
+[windows] env CC=$WORK\'program files'\gcc.bat
+! go build -x
+! exists $WORK/log.txt
+
+-- go.mod --
+module m
+
+-- m.go --
+package m
+
+// #define X 1
+import "C"
+
+-- $WORK/program files/gcc --
+#!/bin/sh
+
+echo ok >$WORK/log.txt
+-- $WORK/program files/clang --
+#!/bin/sh
+
+echo ok >$WORK/log.txt
+-- $WORK/program files/gcc.bat --
+echo ok >%WORK%\log.txt
+-- $WORK/program files/clang.bat --
+echo ok >%WORK%\log.txt