linux-kernel - Re: [PATCH] x86/vdso: Add prctl to set per-process VDSO load

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Date:	Tue, 16 Sep 2014 17:27:42 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Richard Larocque <rlarocque@...gle.com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, filbranden@...gle.com,
	md@...gle.com, gthelen@...gle.com, X86 ML <x86@...nel.org>,
	Linux API <linux-api@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/vdso: Add prctl to set per-process VDSO load

On Tue, Sep 16, 2014 at 5:05 PM, Richard Larocque <rlarocque@...gle.com> wrote:
> Adds new prctl calls to enable or disable VDSO loading for a process
> and its children.
>
> The PR_SET_DISABLE_VDSO call takes one argument, which is interpreted as
> a boolean value.  If true, it disables the loading of the VDSO on exec()
> for this process and any children created after this call.  A false
> value unsets the flag.
>
> The PR_GET_DISABLE_VDSO option returns a non-negative true value if VDSO
> loading has been disabled for this process, zero if it has not been
> disabled, and a negative value in case of error.
>
> These prctl calls are hidden behind a new Kconfig,
> CONFIG_VDSO_DISABLE_PRCTL.  This feature is available only on x86.
>
> The command line option vdso=0 overrides the behavior of
> PR_SET_DISABLE_VDSO, however, PR_GET_DISABLE_VDSO will coninue to return
> whetever setting was last set with PR_SET_DISABLE_VDSO.
>
> Signed-off-by: Richard Larocque <rlarocque@...gle.com>
> ---
> This patch is part of some work to better handle times and CRIU migration.
> I suspect that there are other use cases out there, so I'm offering this
> patch separately.
>
> When considering CRIU migration and times, we put some thought into how
> to handle the rdtsc instruction.  If we migrate between machines or across
> reboots, the migrated process will see values that could break its assumptions
> about how rdtsc is supposed to work.

I don't get it.

If __vdso_clock_gettime returns the wrong value in any scenario, we
should fix that.  Simiarly, CRIU *already works*, unless there's
something I don't know of.

That being said, I would like an option to gate off RDTSC for a
process and its children in order to make PR_TSC_SIGSEGV more useful.
All the prerequisites are there now.

What problem are you trying to solve exactly?

--Andy
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