cmd/link: check mmap error

We already check mmap errors on some code paths, but we missed
one. Add error check there.

Change-Id: Ic0e9cb0eb03c805de40802cfc5d5500e3e065d99
Reviewed-on: https://go-review.googlesource.com/c/go/+/319290
Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/link/internal/ld/asmb.go

@@ -167,7 +167,10 @@ func sizeExtRelocs(ctxt *Link, relsize uint32) {
 		}
 	}
 	filesz := ctxt.Out.Offset() + sz
-	ctxt.Out.Mmap(uint64(filesz))
+	err := ctxt.Out.Mmap(uint64(filesz))
+	if err != nil {
+		Exitf("mapping output file failed: %v", err)
+	}
 }
 
 // relocSectFn wraps the function writing relocations of a section

src/cmd/link/internal/ld/main.go

@@ -334,7 +334,7 @@ func Main(arch *sys.Arch, theArch Arch) {
 		// Don't mmap if we're building for Wasm. Wasm file
 		// layout is very different so filesize is meaningless.
 		if err := ctxt.Out.Mmap(filesize); err != nil {
-			panic(err)
+			Exitf("mapping output file failed: %v", err)
 		}
 	}
 	// asmb will redirect symbols to the output file mmap, and relocations

src/cmd/link/internal/ld/outbuf.go

@@ -160,7 +160,7 @@ func (out *OutBuf) copyHeap() bool {
 	total := uint64(bufLen + heapLen)
 	if heapLen != 0 {
 		if err := out.Mmap(total); err != nil { // Mmap will copy out.heap over to out.buf
-			panic(err)
+			Exitf("mapping output file failed: %v", err)
 		}
 	}
 	return true