crypto: fix bogus error benchmarking pbkdf on fast machines

We're seeing periodic reports of errors like:

$ qemu-img create -f luks --object secret,data=123456,id=sec0 \
                  -o key-secret=sec0 luks-info.img 1M
  Formatting 'luks-info.img', fmt=luks size=1048576 key-secret=sec0
  qemu-img: luks-info.img: Unable to get accurate CPU usage

This error message comes from a recent attempt to workaround a
kernel bug with measuring rusage in long running processes:

  commit c72cab5ad9f849bbcfcf4be7952b8b8946cc626e
  Author: Tiago Pasqualini <tiago.pasqualini@canonical.com>
  Date:   Wed Sep 4 20:52:30 2024 -0300

    crypto: run qcrypto_pbkdf2_count_iters in a new thread

Unfortunately this has a subtle bug on machines which are very fast.

On the first time around the loop, the 'iterations' value is quite
small (1 << 15), and so will run quite fast. Testing has shown that
some machines can complete this benchmarking task in as little as
7 milliseconds.

Unfortunately the 'getrusage' data is not updated at the time of
the 'getrusage' call, it is done asynchronously by the scheduler.
The 7 millisecond completion time for the benchmark is short
enough that 'getrusage' sometimes reports 0 accumulated execution
time.

As a result the 'delay_ms == 0' sanity check in the above commit
is triggering non-deterministically on such machines.

The benchmarking loop intended to run multiple times, increasing
the 'iterations' value until the benchmark ran for > 500 ms, but
the sanity check doesn't allow this to happen.

To fix it, we keep a loop counter and only run the sanity check
after we've been around the loop more than 5 times. At that point
the 'iterations' value is high enough that even with infrequent
updates of 'getrusage' accounting data on fast machines, we should
see a non-zero value.

Fixes: https://lore.kernel.org/qemu-devel/ffe542bb-310c-4616-b0ca-13182f849fd1@redhat.com/
Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=2336437
Reported-by: Thomas Huth <thuth@redhat.com>
Reported-by: Richard W.M. Jones <rjones@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-ID: <20250109093746.1216300-1-berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

crypto/pbkdf.c

@@ -107,7 +107,7 @@ static void *threaded_qcrypto_pbkdf2_count_iters(void *data)
     size_t nsalt = iters_data->nsalt;
     size_t nout = iters_data->nout;
     Error **errp = iters_data->errp;
-
+    size_t scaled = 0;
     uint64_t ret = -1;
     g_autofree uint8_t *out = g_new(uint8_t, nout);
     uint64_t iterations = (1 << 15);
@@ -131,7 +131,17 @@ static void *threaded_qcrypto_pbkdf2_count_iters(void *data)
 
         delta_ms = end_ms - start_ms;
 
-        if (delta_ms == 0) { /* sanity check */
+        /*
+         * For very small 'iterations' values, CPU (or crypto
+         * accelerator) might be fast enough that the scheduler
+         * hasn't incremented getrusage() data, or incremented
+         * it by a very small amount, resulting in delta_ms == 0.
+         * Once we've scaled 'iterations' x10, 5 times, we really
+         * should be seeing delta_ms != 0, so sanity check at
+         * that point.
+         */
+        if (scaled > 5 &&
+            delta_ms == 0) { /* sanity check */
             error_setg(errp, "Unable to get accurate CPU usage");
             goto cleanup;
         } else if (delta_ms > 500) {
@@ -141,6 +151,7 @@ static void *threaded_qcrypto_pbkdf2_count_iters(void *data)
         } else {
             iterations = (iterations * 1000 / delta_ms);
         }
+        scaled++;
     }
 
     iterations = iterations * 1000 / delta_ms;