4 files changed, 65 insertions, 6 deletions
diff --git a/src/debug/macho/fat.go b/src/debug/macho/fat.go
index 6bd730dc0b..775beaf12c 100644
--- a/src/debug/macho/fat.go
+++ b/src/debug/macho/fat.go
@@ -7,6 +7,7 @@ package macho
 import (
 	"encoding/binary"
 	"fmt"
+	"internal/saferio"
 	"io"
 	"os"
 )
@@ -85,9 +86,13 @@ func NewFatFile(r io.ReaderAt) (*FatFile, error) {
 
 	// Following the fat_header comes narch fat_arch structs that index
 	// Mach-O images further in the file.
-	ff.Arches = make([]FatArch, narch)
+	c := saferio.SliceCap(FatArch{}, uint64(narch))
+	if c < 0 {
+		return nil, &FormatError{offset, "too many images", nil}
+	}
+	ff.Arches = make([]FatArch, 0, c)
 	for i := uint32(0); i < narch; i++ {
-		fa := &ff.Arches[i]
+		var fa FatArch
 		err = binary.Read(sr, binary.BigEndian, &fa.FatArchHeader)
 		if err != nil {
 			return nil, &FormatError{offset, "invalid fat_arch header", nil}
@@ -115,6 +120,8 @@ func NewFatFile(r io.ReaderAt) (*FatFile, error) {
 				return nil, &FormatError{offset, fmt.Sprintf("Mach-O type for architecture #%d (type=%#x) does not match first (type=%#x)", i, fa.Type, machoType), nil}
 			}
 		}
+
+		ff.Arches = append(ff.Arches, fa)
 	}
 
 	return &ff, nil
diff --git a/src/go/build/deps_test.go b/src/go/build/deps_test.go
index e911f2f341..061345f64b 100644
--- a/src/go/build/deps_test.go
+++ b/src/go/build/deps_test.go
@@ -123,9 +123,6 @@ var depsRules = `
 
 	unicode !< strconv;
 
-	io
-	< internal/saferio;
-
 	# STR is basic string and buffer manipulation.
 	RUNTIME, io, unicode/utf8, unicode/utf16, unicode
 	< bytes, strings
@@ -242,6 +239,9 @@ var depsRules = `
 	encoding/binary, regexp
 	< index/suffixarray;
 
+	io, reflect
+	< internal/saferio;
+
 	# executable parsing
 	FMT, encoding/binary, compress/zlib, internal/saferio
 	< runtime/debug
diff --git a/src/internal/saferio/io.go b/src/internal/saferio/io.go
index 019216f352..0361011e95 100644
--- a/src/internal/saferio/io.go
+++ b/src/internal/saferio/io.go
@@ -9,7 +9,10 @@
 // untrustworthy attacker.
 package saferio
 
-import "io"
+import (
+	"io"
+	"reflect"
+)
 
 // chunk is an arbitrary limit on how much memory we are willing
 // to allocate without concern.
@@ -91,3 +94,27 @@ func ReadDataAt(r io.ReaderAt, n uint64, off int64) ([]byte, error) {
 	}
 	return buf, nil
 }
+
+// SliceCap returns the capacity to use when allocating a slice.
+// After the slice is allocated with the capacity, it should be
+// built using append. This will avoid allocating too much memory
+// if the capacity is large and incorrect.
+//
+// A negative result means that the value is always too big.
+//
+// The element type is described by passing a value of that type.
+// This would ideally use generics, but this code is built with
+// the bootstrap compiler which need not support generics.
+func SliceCap(v any, c uint64) int {
+	if int64(c) < 0 || c != uint64(int(c)) {
+		return -1
+	}
+	size := reflect.TypeOf(v).Size()
+	if uintptr(c)*size > chunk {
+		c = uint64(chunk / size)
+		if c == 0 {
+			c = 1
+		}
+	}
+	return int(c)
+}
diff --git a/src/internal/saferio/io_test.go b/src/internal/saferio/io_test.go
index 301b798834..9214e735c2 100644
--- a/src/internal/saferio/io_test.go
+++ b/src/internal/saferio/io_test.go
@@ -81,3 +81,28 @@ func TestReadDataAt(t *testing.T) {
 		}
 	})
 }
+
+func TestSliceCap(t *testing.T) {
+	t.Run("small", func(t *testing.T) {
+		c := SliceCap(0, 10)
+		if c != 10 {
+			t.Errorf("got capacity %d, want %d", c, 10)
+		}
+	})
+
+	t.Run("large", func(t *testing.T) {
+		c := SliceCap(byte(0), 1<<30)
+		if c < 0 {
+			t.Error("SliceCap failed unexpectedly")
+		} else if c == 1<<30 {
+			t.Errorf("got capacity %d which is too high", c)
+		}
+	})
+
+	t.Run("maxint", func(t *testing.T) {
+		c := SliceCap(byte(0), 1<<63)
+		if c >= 0 {
+			t.Errorf("SliceCap returned %d, expected failure", c)
+		}
+	})
+}