RE: [PATCH 14/17] MIPS: bpf: Prevent kernel fall over for >=32bit shifts

From: Markos Chandras

Remove BUG_ON() if the shift immediate is >=32 to avoid
kernel crashes due to malicious user input. Since the micro-assembler
will not allow an immediate greater or equal to 32, we will use the
maximum value which is 31. This will do the correct thing on either 32-
or 64-bit cores since no 64-bit instructions are being used in JIT.

I'm not sure that bounding the shift to 31 bits 'is the correct thing'.
I'd have thought that emulating the large shift or masking the shift
to 5 bits are equally 'correct'.

...

 {
 	/* sa is 5-bits long */
-	BUG_ON(sa >= BIT(5));
+	if (sa >= BIT(5))
+		sa = BIT(5) - 1;
 	emit_instr(ctx, sll, dst, src, sa);

...

	David