futex: futex_wake_op, do not fail on invalid op

In commit 30d6e0a4 ("futex: Remove duplicated code and fix undefined
behaviour"), I let FUTEX_WAKE_OP to fail on invalid op.  Namely when op
should be considered as shift and the shift is out of range (< 0 or > 31).

But strace's test suite does this madness:

  futex(0x7fabd78bcffc, 0x5, 0xfacefeed, 0xb, 0x7fabd78bcffc, 0xa0caffee);
  futex(0x7fabd78bcffc, 0x5, 0xfacefeed, 0xb, 0x7fabd78bcffc, 0xbadfaced);
  futex(0x7fabd78bcffc, 0x5, 0xfacefeed, 0xb, 0x7fabd78bcffc, 0xffffffff);

When I pick the first 0xa0caffee, it decodes as:

  0x80000000 & 0xa0caffee: oparg is shift
  0x70000000 & 0xa0caffee: op is FUTEX_OP_OR
  0x0f000000 & 0xa0caffee: cmp is FUTEX_OP_CMP_EQ
  0x00fff000 & 0xa0caffee: oparg is sign-extended 0xcaf = -849
  0x00000fff & 0xa0caffee: cmparg is sign-extended 0xfee = -18

That means the op tries to do this:

  (futex |= (1 << (-849))) == -18

which is completely bogus. The new check of op in the code is:

        if (encoded_op & (FUTEX_OP_OPARG_SHIFT << 28)) {
                if (oparg < 0 || oparg > 31)
                        return -EINVAL;
                oparg = 1 << oparg;
        }

which results obviously in the "Invalid argument" errno:

  FAIL: futex
  ===========

  futex(0x7fabd78bcffc, 0x5, 0xfacefeed, 0xb, 0x7fabd78bcffc, 0xa0caffee) = -1: Invalid argument
  futex.test: failed test: ../futex failed with code 1

So let us soften the failure to print only a (ratelimited) message, crop
the value and continue as if it were right.  When userspace keeps up, we
can switch this to return -EINVAL again.

[v2] Do not return 0 immediatelly, proceed with the cropped value.

Fixes: 30d6e0a4 ("futex: Remove duplicated code and fix undefined behaviour")
Signed-off-by: NJiri Slaby <jslaby@suse.cz>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Darren Hart <dvhart@infradead.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/futex.c

@@ -1570,8 +1570,16 @@ static int futex_atomic_op_inuser(unsigned int encoded_op, u32 __user *uaddr)
 	int oldval, ret;
 
 	if (encoded_op & (FUTEX_OP_OPARG_SHIFT << 28)) {
-		if (oparg < 0 || oparg > 31)
-			return -EINVAL;
+		if (oparg < 0 || oparg > 31) {
+			char comm[sizeof(current->comm)];
+			/*
+			 * kill this print and return -EINVAL when userspace
+			 * is sane again
+			 */
+			pr_info_ratelimited("futex_wake_op: %s tries to shift op by %d; fix this program\n",
+					get_task_comm(comm, current), oparg);
+			oparg &= 31;
+		}
 		oparg = 1 << oparg;
 	}