runtime: support riscv64 SV57 mode

riscv64 has SV57 mode when user-space VA is 56 bits.
Linux kernel recently got support for this mode and Go binaries started crashing as:

runtime: lfstack.push invalid packing: node=0xffffff5908a940 cnt=0x1
packed=0xffff5908a9400001 -> node=0xffff5908a940

Adjust lfstack code to use only 8 top bits of pointers on riscv64.

For context see:
https://groups.google.com/g/syzkaller-bugs/c/lU0GQTZoNQQ/m/O_c3vmE3AAAJ

Update #54104

Change-Id: Ib5d3d6a79c0c6eddf11618d73fcc8bc1832a9c25
Reviewed-on: https://go-review.googlesource.com/c/go/+/409055


Reviewed-by: Joel Sing <joel@sing.id.au>
Reviewed-by: Meng Zhuo <mzh@golangcn.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/runtime/lfstack_64bit.go

@@ -36,12 +36,21 @@ const (
 	// We use one bit to distinguish between the two ranges.
 	aixAddrBits = 57
 	aixCntBits  = 64 - aixAddrBits + 3
+
+	// riscv64 SV57 mode gives 56 bits of userspace VA.
+	// lfstack code supports it, but broader support for SV57 mode is incomplete,
+	// and there may be other issues (see #54104).
+	riscv64AddrBits = 56
+	riscv64CntBits  = 64 - riscv64AddrBits + 3
 )
 
 func lfstackPack(node *lfnode, cnt uintptr) uint64 {
 	if GOARCH == "ppc64" && GOOS == "aix" {
 		return uint64(uintptr(unsafe.Pointer(node)))<<(64-aixAddrBits) | uint64(cnt&(1<<aixCntBits-1))
 	}
+	if GOARCH == "riscv64" {
+		return uint64(uintptr(unsafe.Pointer(node)))<<(64-riscv64AddrBits) | uint64(cnt&(1<<riscv64CntBits-1))
+	}
 	return uint64(uintptr(unsafe.Pointer(node)))<<(64-addrBits) | uint64(cnt&(1<<cntBits-1))
 }
 
@@ -54,5 +63,8 @@ func lfstackUnpack(val uint64) *lfnode {
 	if GOARCH == "ppc64" && GOOS == "aix" {
 		return (*lfnode)(unsafe.Pointer(uintptr((val >> aixCntBits << 3) | 0xa<<56)))
 	}
+	if GOARCH == "riscv64" {
+		return (*lfnode)(unsafe.Pointer(uintptr(val >> riscv64CntBits << 3)))
+	}
 	return (*lfnode)(unsafe.Pointer(uintptr(val >> cntBits << 3)))
 }