internal/syscall/unix: allow calling getrandom(..., 0, ...)

Calling getrandom() with a zero length is actually valid and useful:

- Calling getrandom(..., 0, 0) will block until the RNG is initialized.
- Calling getrandom(..., 0, GRND_NONBLOCK) will query whether the RNG
  is initialized.

So instead of short circuiting execution for these circumstances, pass
this through to the syscall.

Change-Id: I15178f087908a2d8be6c020a1ef800cc0a074742
Reviewed-on: https://go-review.googlesource.com/c/go/+/615315
Run-TryBot: Jason Donenfeld <Jason@zx2c4.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Auto-Submit: Jason Donenfeld <Jason@zx2c4.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

diff --git a/src/internal/syscall/unix/getrandom.go b/src/internal/syscall/unix/getrandom.go
index e83f0cd6f95733783e9a2e1fc79e278966311176..4270898b7e5c96675ac20004f8b66db7b87aac53 100644 (file)
--- a/src/internal/syscall/unix/getrandom.go
+++ b/src/internal/syscall/unix/getrandom.go
@@ -19,14 +19,11 @@ type GetRandomFlag uintptr
 
 // GetRandom calls the getrandom system call.
 func GetRandom(p []byte, flags GetRandomFlag) (n int, err error) {
-       if len(p) == 0 {
-               return 0, nil
-       }
        if getrandomUnsupported.Load() {
                return 0, syscall.ENOSYS
        }
        r1, _, errno := syscall.Syscall(getrandomTrap,
-               uintptr(unsafe.Pointer(&p[0])),
+               uintptr(unsafe.Pointer(unsafe.SliceData(p))),
                uintptr(len(p)),
                uintptr(flags))
        if errno != 0 {