cmd/link: avoid deadcode of global map vars for programs using plugins

If a program imports the plugin package, the mechanisms in place for
detecting and deleting unused global map variables are no longer safe,
since it's possibly for a given global map var to be unreferenced in
the main program but referenced by a plugin. This patch changes the
linker to test for plugin use and to avoid removing any unused global
map variables if the main program could possibly load up a plugin.

Fixes #62430.

Change-Id: Ie00b18b681cb0d259e3c859ac947ade5778cd6c8
Reviewed-on: https://go-review.googlesource.com/c/go/+/526115
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

1 files changed, 20 insertions, 4 deletions

diff --git a/src/cmd/link/internal/ld/deadcode.go b/src/cmd/link/internal/ld/deadcode.go
index b365a3b39e..a051e43401 100644
--- a/src/cmd/link/internal/ld/deadcode.go
+++ b/src/cmd/link/internal/ld/deadcode.go
@@ -143,10 +143,26 @@ func (d *deadcodePass) flood() {
 		methods = methods[:0]
 		for i := 0; i < relocs.Count(); i++ {
 			r := relocs.At(i)
-			// When build with "-linkshared", we can't tell if the interface
-			// method in itab will be used or not. Ignore the weak attribute.
-			if r.Weak() && !(d.ctxt.linkShared && d.ldr.IsItab(symIdx)) {
-				continue
+			if r.Weak() {
+				convertWeakToStrong := false
+				// When build with "-linkshared", we can't tell if the
+				// interface method in itab will be used or not.
+				// Ignore the weak attribute.
+				if d.ctxt.linkShared && d.ldr.IsItab(symIdx) {
+					convertWeakToStrong = true
+				}
+				// If the program uses plugins, we can no longer treat
+				// relocs from pkg init functions to outlined map init
+				// fragments as weak, since doing so can cause package
+				// init clashes between the main program and the
+				// plugin. See #62430 for more details.
+				if d.ctxt.canUsePlugins && r.Type().IsDirectCall() {
+					convertWeakToStrong = true
+				}
+				if !convertWeakToStrong {
+					// skip this reloc
+					continue
+				}
 			}
 			t := r.Type()
 			switch t {