consensus/ethash: avoid runtime errors due to OOD on mmap writes (#23799)

When we map a file for generating the DAG, we do a simple truncate to e.g. 1Gb. This is fine, even if we have nowhere near 1Gb disk available, as the actual file doesn't take up the full 1Gb, merely a few bytes. When we start generating into it, however, it eventually crashes with a unexpected fault address .

This change fixes it (on linux systems) by using the Fallocate syscall, which preallocates suffcient space on disk to avoid that situation. 


Co-authored-by: Felix Lange <fjl@twurst.com>

consensus/ethash/ethash.go

@@ -136,13 +136,16 @@ func memoryMapAndGenerate(path string, size uint64, lock bool, generator func(bu
 	if err != nil {
 		return nil, nil, nil, err
 	}
-	if err = dump.Truncate(int64(len(dumpMagic))*4 + int64(size)); err != nil {
+	if err = ensureSize(dump, int64(len(dumpMagic))*4+int64(size)); err != nil {
+		dump.Close()
+		os.Remove(temp)
 		return nil, nil, nil, err
 	}
 	// Memory map the file for writing and fill it with the generator
 	mem, buffer, err := memoryMapFile(dump, true)
 	if err != nil {
 		dump.Close()
+		os.Remove(temp)
 		return nil, nil, nil, err
 	}
 	copy(buffer, dumpMagic)
@@ -358,7 +361,7 @@ func (d *dataset) generate(dir string, limit int, lock bool, test bool) {
 		if err != nil {
 			logger.Error("Failed to generate mapped ethash dataset", "err", err)
 
-			d.dataset = make([]uint32, dsize/2)
+			d.dataset = make([]uint32, dsize/4)
 			generateDataset(d.dataset, d.epoch, cache)
 		}
 		// Iterate over all previous instances and delete old ones

consensus/ethash/mmap_help_linux.go

@@ -0,0 +1,35 @@
+// Copyright 2021 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+//go:build linux
+// +build linux
+
+package ethash
+
+import (
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+// ensureSize expands the file to the given size. This is to prevent runtime
+// errors later on, if the underlying file expands beyond the disk capacity,
+// even though it ostensibly is already expanded, but due to being sparse
+// does not actually occupy the full declared size on disk.
+func ensureSize(f *os.File, size int64) error {
+	// Docs: https://www.man7.org/linux/man-pages/man2/fallocate.2.html
+	return unix.Fallocate(int(f.Fd()), 0, 0, size)
+}

consensus/ethash/mmap_help_other.go

@@ -0,0 +1,36 @@
+// Copyright 2021 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+//go:build !linux
+// +build !linux
+
+package ethash
+
+import (
+	"os"
+)
+
+// ensureSize expands the file to the given size. This is to prevent runtime
+// errors later on, if the underlying file expands beyond the disk capacity,
+// even though it ostensibly is already expanded, but due to being sparse
+// does not actually occupy the full declared size on disk.
+func ensureSize(f *os.File, size int64) error {
+	// On systems which do not support fallocate, we merely truncate it.
+	// More robust alternatives  would be to
+	// - Use posix_fallocate, or
+	// - explicitly fill the file with zeroes.
+	return f.Truncate(size)
+}