[Mesa-dev] [PATCH] RFC: Workaround for pthread_setaffinity_np() seccomp filtering

[Marc-André Lureau]

Hi

On Thu, Feb 28, 2019 at 1:17 AM Marek Olšák <maraeo at gmail.com> wrote:
>
> I'd rather have something more robust than an env var, like catching SIGSYS.

With current qemu in most distros, it defaults to SIGSYS (we switched
away from SCMP_ACT_KILL, which had other problems). With more recent
qemu/libseccomp, it will default to SCMP_ACT_KILL_PROCESS. In those
KILL action cases, mesa will not be able to catch the failing
syscalls.

>
> Marek
>
> On Wed, Feb 27, 2019 at 6:13 PM <marcandre.lureau at redhat.com> wrote:
>>
>> From: Marc-André Lureau <marcandre.lureau at redhat.com>
>>
>> Since commit d877451b48a59ab0f9a4210fc736f51da5851c9a ("util/u_queue:
>> add UTIL_QUEUE_INIT_SET_FULL_THREAD_AFFINITY"), mesa calls
>> sched_setaffinity syscall. Unfortunately, qemu crashes with SIGSYS
>> when sandboxing is enabled (by default with libvirt), as this syscall
>> is filtered.
>>
>> There doesn't seem to be a way to check for the seccomp rule other
>> than doing a call, which may result in various behaviour depending on
>> seccomp actions. There is a PTRACE_SECCOMP_GET_FILTER, but it is
>> low-level and a priviledged operation (but there might be a way to use
>> it?). A safe way would be to try the call in a subprocess,
>> unfortunately, qemu also prohibits fork(). Also this could be subject
>> to TOCTOU.
>>
>> There seems to be few solutions, but the issue can be considered a
>> regression for various libvirt/Boxes users.
>>
>> Introduce MESA_NO_THREAD_AFFINITY environment variable to prevent the
>> offending call. Wrap pthread_setaffinity_np() in a utility function
>> u_pthread_setaffinity_np(), returning a EACCESS error if the variable
>> is set.
>>
>> Note: one call is left with a FIXME, as I didn't investigate how to
>> build and test it, help welcome!
>>
>> See also:
>> https://bugs.freedesktop.org/show_bug.cgi?id=109695
>>
>> Signed-off-by: Marc-André Lureau <marcandre.lureau at redhat.com>
>> ---
>>  .../drivers/swr/rasterizer/core/threads.cpp       |  1 +
>>  src/util/u_queue.c                                |  2 +-
>>  src/util/u_thread.h                               | 15 ++++++++++++++-
>>  3 files changed, 16 insertions(+), 2 deletions(-)
>>
>> diff --git a/src/gallium/drivers/swr/rasterizer/core/threads.cpp b/src/gallium/drivers/swr/rasterizer/core/threads.cpp
>> index e30c1170568..d10c79512a1 100644
>> --- a/src/gallium/drivers/swr/rasterizer/core/threads.cpp
>> +++ b/src/gallium/drivers/swr/rasterizer/core/threads.cpp
>> @@ -364,6 +364,7 @@ void bindThread(SWR_CONTEXT* pContext,
>>      CPU_ZERO(&cpuset);
>>      CPU_SET(threadId, &cpuset);
>>
>> +    /* FIXME: use u_pthread_setaffinity_np() if possible */
>>      int err = pthread_setaffinity_np(thread, sizeof(cpu_set_t), &cpuset);
>>      if (err != 0)
>>      {
>> diff --git a/src/util/u_queue.c b/src/util/u_queue.c
>> index 3812c824b6d..dea8d2bb4ae 100644
>> --- a/src/util/u_queue.c
>> +++ b/src/util/u_queue.c
>> @@ -249,7 +249,7 @@ util_queue_thread_func(void *input)
>>        for (unsigned i = 0; i < CPU_SETSIZE; i++)
>>           CPU_SET(i, &cpuset);
>>
>> -      pthread_setaffinity_np(pthread_self(), sizeof(cpuset), &cpuset);
>> +      u_pthread_setaffinity_np(pthread_self(), sizeof(cpuset), &cpuset);
>>     }
>>  #endif
>>
>> diff --git a/src/util/u_thread.h b/src/util/u_thread.h
>> index a46c18d3db2..a4e6dbae5d7 100644
>> --- a/src/util/u_thread.h
>> +++ b/src/util/u_thread.h
>> @@ -70,6 +70,19 @@ static inline void u_thread_setname( const char *name )
>>     (void)name;
>>  }
>>
>> +#if defined(HAVE_PTHREAD_SETAFFINITY)
>> +static inline int u_pthread_setaffinity_np(pthread_t thread, size_t cpusetsize,
>> +                                           const cpu_set_t *cpuset)
>> +{
>> +   if (getenv("MESA_NO_THREAD_AFFINITY")) {
>> +      errno = EACCES;
>> +      return -1;
>> +   }
>> +
>> +   return pthread_setaffinity_np(thread, cpusetsize, cpuset);
>> +}
>> +#endif
>> +
>>  /**
>>   * An AMD Zen CPU consists of multiple modules where each module has its own L3
>>   * cache. Inter-thread communication such as locks and atomics between modules
>> @@ -89,7 +102,7 @@ util_pin_thread_to_L3(thrd_t thread, unsigned L3_index, unsigned cores_per_L3)
>>     CPU_ZERO(&cpuset);
>>     for (unsigned i = 0; i < cores_per_L3; i++)
>>        CPU_SET(L3_index * cores_per_L3 + i, &cpuset);
>> -   pthread_setaffinity_np(thread, sizeof(cpuset), &cpuset);
>> +   u_pthread_setaffinity_np(thread, sizeof(cpuset), &cpuset);
>>  #endif
>>  }
>>
>> --
>> 2.21.0
>>
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>> mesa-dev at lists.freedesktop.org
>> https://lists.freedesktop.org/mailman/listinfo/mesa-dev
>
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-- 
Marc-André Lureau