lkml.org 
[lkml]   [2023]   [Jul]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [PATCH 2/2] selftests/bpf: Add test for bpf_rdtsc
From

On 04/07/2023 01:00, John Fastabend wrote:
> Tero Kristo wrote:
>> Add selftest for bpf_rdtsc() which reads the TSC (Time Stamp Counter) on
>> x86_64 architectures. The test reads the TSC from both userspace and the
>> BPF program, and verifies the TSC values are in incremental order as
>> expected. The test is automatically skipped on architectures that do not
>> support the feature.
>>
>> Signed-off-by: Tero Kristo <tero.kristo@linux.intel.com>
>> ---
>> .../selftests/bpf/prog_tests/test_rdtsc.c | 67 +++++++++++++++++++
>> .../testing/selftests/bpf/progs/test_rdtsc.c | 21 ++++++
>> 2 files changed, 88 insertions(+)
>> create mode 100644 tools/testing/selftests/bpf/prog_tests/test_rdtsc.c
>> create mode 100644 tools/testing/selftests/bpf/progs/test_rdtsc.c
>>
>> diff --git a/tools/testing/selftests/bpf/prog_tests/test_rdtsc.c b/tools/testing/selftests/bpf/prog_tests/test_rdtsc.c
>> new file mode 100644
>> index 000000000000..2b26deb5b35a
>> --- /dev/null
>> +++ b/tools/testing/selftests/bpf/prog_tests/test_rdtsc.c
>> @@ -0,0 +1,67 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +/* Copyright(c) 2023 Intel Corporation */
>> +
>> +#include "test_progs.h"
>> +#include "test_rdtsc.skel.h"
>> +
>> +#ifdef __x86_64__
>> +
>> +static inline u64 _rdtsc(void)
>> +{
>> + u32 low, high;
>> +
>> + __asm__ __volatile__("rdtscp" : "=a" (low), "=d" (high));
> I think its ok but note this could fail if user doesn't have
> access to rdtscp and iirc that can be restricted?

It is possible to restrict RDTSC access from userspace by enabling the
TSD bit in CR4 register, and it will cause the userspace process to trap
with general protection fault.

However, the usage of RDTSC appears to be built-in to C standard
libraries (probably some timer routines) and enabling the CR4 TSD makes
the system near unusable. Things like sshd + systemd also start
generating the same general protection faults if RDTSC is blocked. Also,
attempting to run anything at all with the BPF selftest suite causes the
same general protection fault; not only the rdtsc test.

I tried this with couple of setups, one system running a minimalistic
buildroot and another one running a fedora37 installation and the
results were similar.

-Tero

>
>> + return ((u64)high << 32) | low;
>> +}

\
 
 \ /
  Last update: 2023-07-04 10:55    [W:0.046 / U:2.568 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site