linux-kernel - Re: [PATCH v6 1/9] kernel: Support TIF_SYSCALL

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Date:   Tue, 08 Sep 2020 00:59:49 -0400
From:   Gabriel Krisman Bertazi <krisman@...labora.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     luto@...nel.org, tglx@...utronix.de, keescook@...omium.org,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, willy@...radead.org,
        linux-kselftest@...r.kernel.org, shuah@...nel.org,
        kernel@...labora.com
Subject: Re: [PATCH v6 1/9] kernel: Support TIF_SYSCALL_INTERCEPT flag

Christian Brauner <christian.brauner@...ntu.com> writes:

> On Fri, Sep 04, 2020 at 04:31:39PM -0400, Gabriel Krisman Bertazi wrote:
>> index afe01e232935..3511c98a7849 100644
>> --- a/include/linux/sched.h
>> +++ b/include/linux/sched.h
>> @@ -959,7 +959,11 @@ struct task_struct {
>>  	kuid_t				loginuid;
>>  	unsigned int			sessionid;
>>  #endif
>> -	struct seccomp			seccomp;
>> +
>> +	struct {
>> +		unsigned int			syscall_intercept;
>> +		struct seccomp			seccomp;
>> +	};
>
> If there's no specific reason to do this I'd not wrap this in an
> anonymous struct. It doesn't really buy anything and there doesn't seem
> to be  precedent in struct task_struct right now. Also, if this somehow
> adds padding it seems you might end up increasing the size of struct
> task_struct more than necessary by accident? (I might be wrong
> though.)

Hi Christian,

Thanks for your review on this and on the other patches of this series.

I wrapped these to prevent struct layout randomization from separating
the flags field from seccomp, as they are going to be used together and
I was trying to reduce overhead to seccomp entry due to two cache misses
when reading this structure.  Measuring it seccomp_benchmark didn't show
any difference with the unwrapped version, so perhaps it was a bit of
premature optimization?

>> diff --git a/include/linux/syscall_intercept.h b/include/linux/syscall_intercept.h
>> new file mode 100644
>> index 000000000000..725d157699da
>> --- /dev/null
>> +++ b/include/linux/syscall_intercept.h
>> @@ -0,0 +1,70 @@
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +/*
>> + * Copyright (C) 2020 Collabora Ltd.
>> + */
>> +#ifndef _SYSCALL_INTERCEPT_H
>> +#define _SYSCALL_INTERCEPT_H
>> +
>> +#include <linux/sched.h>
>> +#include <linux/sched/signal.h>
>> +#include <linux/thread_info.h>
>> +
>> +#define SYSINT_SECCOMP		0x1
>
> <bikeshed>
>
> Can we maybe use a better name for this? I noone minds the extra
> characters I'd suggest:
> SYSCALL_INTERCEPT_SECCOMP
> or
> SYS_INTERCEPT_SECCOMP
>
> </bikeshed>
>

will do.

Thanks,

-- 
Gabriel Krisman Bertazi