cmd/internal/ld: reserve space for package list note when -buildmode=shared

This makes the intermediate object file a little bigger but it doesn't waste
any space in the final shared library.

Fixes #10691

Change-Id: Ic51a571d60291f1ac2dad1b50dba4679643168ae
Reviewed-on: https://go-review.googlesource.com/9710Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/6l/obj.go

@@ -169,6 +169,14 @@ func archinit() {
 		ld.Elfinit()
 
 		ld.HEADR = ld.ELFRESERVE
+		if ld.Buildmode == ld.BuildmodeShared {
+			// When building a shared library we write a package list
+			// note that can get quite large. The external linker will
+			// re-layout all the sections anyway, so making this larger
+			// just wastes a little space in the intermediate object
+			// file, not the final shared library.
+			ld.HEADR *= 3
+		}
 		if ld.INITTEXT == -1 {
 			ld.INITTEXT = (1 << 22) + int64(ld.HEADR)
 		}

src/cmd/internal/ld/elf.go

@@ -1909,8 +1909,9 @@ func Asmbelf(symo int64) {
 		eh.machine = EM_PPC64
 	}
 
+	elfreserve := int64(ELFRESERVE)
 	startva := INITTEXT - int64(HEADR)
-	resoff := int64(ELFRESERVE)
+	resoff := elfreserve
 
 	var pph *ElfPhdr
 	var pnote *ElfPhdr
@@ -1921,6 +1922,12 @@ func Asmbelf(symo int64) {
 		eh.phentsize = 0
 
 		if Buildmode == BuildmodeShared {
+			// The package list note we make space for here can get quite
+			// large. The external linker will re-layout all the sections
+			// anyway, so making this larger just wastes a little space
+			// in the intermediate object file, not the final shared
+			// library.
+			elfreserve *= 3
 			sh := elfshname(".note.go.pkg-list")
 			resoff -= int64(elfgopkgnote(sh, uint64(startva), uint64(resoff)))
 		}
@@ -2336,8 +2343,8 @@ elfobj:
 		a += int64(elfwritegopkgnote())
 	}
 
-	if a > ELFRESERVE {
-		Diag("ELFRESERVE too small: %d > %d", a, ELFRESERVE)
+	if a > elfreserve {
+		Diag("ELFRESERVE too small: %d > %d", a, elfreserve)
 	}
 }