syscall: validate ParseDirent inputs

Don't panic, crash, or return references to uninitialized memory when
ParseDirent is passed invalid input.

Move common dirent parsing to syscall.go with minimal platform-specific
functions in syscall_$GOOS.go.

Fixes #15653

Change-Id: I5602475e02321fe381064488401c14b33bec6886
Reviewed-on: https://go-review.googlesource.com/23780
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

1 files changed, 10 insertions, 26 deletions

diff --git a/src/syscall/syscall_openbsd.go b/src/syscall/syscall_openbsd.go
index 68218cf93b..bd25fbf87a 100644
--- a/src/syscall/syscall_openbsd.go
+++ b/src/syscall/syscall_openbsd.go
@@ -50,32 +50,16 @@ func nametomib(name string) (mib []_C_int, err error) {
 	return nil, EINVAL
 }
 
-// ParseDirent parses up to max directory entries in buf,
-// appending the names to names. It returns the number
-// bytes consumed from buf, the number of entries added
-// to names, and the new names slice.
-func ParseDirent(buf []byte, max int, names []string) (consumed int, count int, newnames []string) {
-	origlen := len(buf)
-	for max != 0 && len(buf) > 0 {
-		dirent := (*Dirent)(unsafe.Pointer(&buf[0]))
-		if dirent.Reclen == 0 {
-			buf = nil
-			break
-		}
-		buf = buf[dirent.Reclen:]
-		if dirent.Fileno == 0 { // File absent in directory.
-			continue
-		}
-		bytes := (*[10000]byte)(unsafe.Pointer(&dirent.Name[0]))
-		var name = string(bytes[0:dirent.Namlen])
-		if name == "." || name == ".." { // Useless names
-			continue
-		}
-		max--
-		count++
-		names = append(names, name)
-	}
-	return origlen - len(buf), count, names
+func direntIno(buf []byte) (uint64, bool) {
+	return readInt(buf, unsafe.Offsetof(Dirent{}.Fileno), unsafe.Sizeof(Dirent{}.Fileno))
+}
+
+func direntReclen(buf []byte) (uint64, bool) {
+	return readInt(buf, unsafe.Offsetof(Dirent{}.Reclen), unsafe.Sizeof(Dirent{}.Reclen))
+}
+
+func direntNamlen(buf []byte) (uint64, bool) {
+	return readInt(buf, unsafe.Offsetof(Dirent{}.Namlen), unsafe.Sizeof(Dirent{}.Namlen))
 }
 
 //sysnb pipe(p *[2]_C_int) (err error)