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

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

[Richard Larocque]

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@google.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.  To deal with this, we could:
 * let the application handle it
 * ban the instruction (ie. PR_TSC_SIGSEGV), to make sure the application
   doesn't use it by accident
 * trap the instruction then mark the process as "tainted" for migration
   purposes
 * trap the instruction then apply an adjustment to keep values consistent
   across machines
 * do something really crazy involving the VMCS

There's no great option here.  Which one we choose probably depends on
what kind of process is being migrated.

Many of these options involve setting a trap for the rdtsc instruction in the
process, which creates problems for the vDSO.  The vDSO implementations of
clock_gettime() make use of that instruction from userspace.  We can use
workarounds in the kernel to turn the trap into a no-op when it comes from vDSO
code, but that somewhat defeats the purpose of having a vDSO in the first
place.  It was supposed to avoid unnecessary calls to kernel space.  Trapping
its instructions goes against that goal.

So we think it would be nice to disable the vDSO for some processes on a
machine.  This would allow us to implement some of the rdtsc handling options
without having to worry about the vDSO.

We have some additional plans for clock_gettime() and friends that may or
may not depend on disabling the vDSO, but it might be best if we defer that
part of the discussion to the next patch set.  I think this patch and its
use case can stand on their own.

 arch/x86/Kconfig           |  8 ++++++++
 arch/x86/vdso/vma.c        | 18 ++++++++++++++++++
 include/linux/sched.h      |  5 +++++
 include/uapi/linux/prctl.h |  9 +++++++++
 kernel/fork.c              |  4 ++++
 kernel/sys.c               | 16 ++++++++++++++++
 6 files changed, 60 insertions(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 3632743..ff54ead 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1875,6 +1875,14 @@ config COMPAT_VDSO
 	  If unsure, say N: if you are compiling your own kernel, you
 	  are unlikely to be using a buggy version of glibc.
 
+config VDSO_DISABLE_PRCTL
+	depends on X86
+	bool "prctl to disable VDSO loading"
+	---help---
+	  Enabling this option adds support for prctl calls that
+	  set and retrieve a per-process flag to disable VDSO loading on
+	  exec() for this process and all of its children.
+
 config CMDLINE_BOOL
 	bool "Built-in kernel command line"
 	---help---
diff --git a/arch/x86/vdso/vma.c b/arch/x86/vdso/vma.c
index 970463b..496c48b 100644
--- a/arch/x86/vdso/vma.c
+++ b/arch/x86/vdso/vma.c
@@ -23,6 +23,15 @@ unsigned int __read_mostly vdso64_enabled = 1;
 extern unsigned short vdso_sync_cpuid;
 #endif
 
+static int vdso_enabled_for_current_process(void)
+{
+#if defined(CONFIG_VDSO_DISABLE_PRCTL)
+	return !current->signal->disable_vdso;
+#else
+	return 1;
+#endif
+}
+
 void __init init_vdso_image(const struct vdso_image *image)
 {
 	int i;
@@ -185,6 +194,9 @@ static int load_vdso32(void)
 	if (vdso32_enabled != 1)  /* Other values all mean "disabled" */
 		return 0;
 
+	if (!vdso_enabled_for_current_process())
+		return 0;
+
 	ret = map_vdso(selected_vdso32, false);
 	if (ret)
 		return ret;
@@ -204,6 +216,9 @@ int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
 	if (!vdso64_enabled)
 		return 0;
 
+	if (!vdso_enabled_for_current_process())
+		return 0;
+
 	return map_vdso(&vdso_image_64, true);
 }
 
@@ -216,6 +231,9 @@ int compat_arch_setup_additional_pages(struct linux_binprm *bprm,
 		if (!vdso64_enabled)
 			return 0;
 
+		if (!vdso_enabled_for_current_process())
+			return 0;
+
 		return map_vdso(&vdso_image_x32, true);
 	}
 #endif
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 5c2c885..37f6a7a 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -708,6 +708,11 @@ struct signal_struct {
 	struct mutex cred_guard_mutex;	/* guard against foreign influences on
 					 * credential calculations
 					 * (notably. ptrace) */
+
+#ifdef CONFIG_VDSO_DISABLE_PRCTL
+	unsigned int disable_vdso;     /* If true, prevents loading of VDSO on
+					  next exec() */
+#endif
 };
 
 /*
diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 58afc04..3dbbeda 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -152,4 +152,13 @@
 #define PR_SET_THP_DISABLE	41
 #define PR_GET_THP_DISABLE	42
 
+/*
+ * These can be used to flag a process so that neither it nor its children will
+ * receive VDSO mappings on their next exec() call.
+ */
+#define PR_SET_VDSO		43
+#define PR_GET_VDSO		44
+# define PR_VDSO_DISABLE	0	/* prevent loading of VDSO on exec() */
+# define PR_VDSO_ENABLE		1	/* allow loading of VDSO on exec() */
+
 #endif /* _LINUX_PRCTL_H */
diff --git a/kernel/fork.c b/kernel/fork.c
index 0cf9cdb..11ede19 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1091,6 +1091,10 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
 	sig->has_child_subreaper = current->signal->has_child_subreaper ||
 				   current->signal->is_child_subreaper;
 
+#ifdef CONFIG_VDSO_DISABLE_PRCTL
+	sig->disable_vdso = current->signal->disable_vdso;
+#endif
+
 	mutex_init(&sig->cred_guard_mutex);
 
 	return 0;
diff --git a/kernel/sys.c b/kernel/sys.c
index ce81291..eb94a96 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2011,6 +2011,22 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 			me->mm->def_flags &= ~VM_NOHUGEPAGE;
 		up_write(&me->mm->mmap_sem);
 		break;
+#ifdef CONFIG_VDSO_DISABLE_PRCTL
+	case PR_SET_VDSO:
+		if (arg2 == PR_VDSO_ENABLE)
+			me->signal->disable_vdso = 0;
+		else if (arg2 == PR_VDSO_DISABLE)
+			me->signal->disable_vdso = 1;
+		else
+			return -EINVAL;
+		break;
+	case PR_GET_VDSO:
+		if (!me->signal->disable_vdso)
+			error = put_user(PR_VDSO_ENABLE, (int __user *)arg2);
+		else
+			error = put_user(PR_VDSO_DISABLE, (int __user *)arg2);
+		break;
+#endif
 	default:
 		error = -EINVAL;
 		break;
-- 
2.1.0.rc2.206.gedb03e5

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

[Andi Kleen]

Richard Larocque <rlarocque@google.com> writes:

Perhaps I'm missing something, but how do you modify the AUX vector
for the children?
  
> +config VDSO_DISABLE_PRCTL
> +	depends on X86
> +	bool "prctl to disable VDSO loading"
> +	---help---
> +	  Enabling this option adds support for prctl calls that
> +	  set and retrieve a per-process flag to disable VDSO loading on
> +	  exec() for this process and all of its children.

I don't think it makes any sense to have a config for a simple
feature like this. Just do it unconditionally.

-Andi

-- 
ak@linux.intel.com -- Speaking for myself only

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

[Richard Larocque]

On Tue, Sep 16, 2014 at 5:13 PM, Andi Kleen <andi-Vw/NltI1exuRpAAqCnN02g@public.gmane.org> wrote:
> Richard Larocque <rlarocque-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> writes:
>
> Perhaps I'm missing something, but how do you modify the AUX vector
> for the children?
>
>> +config VDSO_DISABLE_PRCTL
>> +     depends on X86
>> +     bool "prctl to disable VDSO loading"
>> +     ---help---
>> +       Enabling this option adds support for prctl calls that
>> +       set and retrieve a per-process flag to disable VDSO loading on
>> +       exec() for this process and all of its children.
>
> I don't think it makes any sense to have a config for a simple
> feature like this. Just do it unconditionally.
>
> -Andi
>
> --
> ak-VuQAYsv1563Yd54FQh9/CA@public.gmane.org -- Speaking for myself only
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/

The AUX vector issue is a mistake on my part.

Speaking of mistakes, the IDs in the commit message do not match those
in the patch itself.  That needs to be fixed, too.

Point taken about the config.  I'll remove it in the next version of the patch.

Thanks for the feedback.

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

[Andy Lutomirski]

On Tue, Sep 16, 2014 at 5:05 PM, Richard Larocque <rlarocque-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> 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-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
> ---
> 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

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

[Richard Larocque]

On Tue, Sep 16, 2014 at 5:27 PM, Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org> wrote:
> On Tue, Sep 16, 2014 at 5:05 PM, Richard Larocque <rlarocque-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> 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-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
>> ---
>> 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.

Right.  As far as I know, there's nothing wrong with the use of RDTSC
in the vDSO following a migration.  The problem is that some
applications might use RDTSC outside of the vDSO.  If they save the
returned values, then compare pre- and post- migration values, bad
things could happen (in theory).

Anything we do to try to trap and handle the use of RDTSC in wider
userspace will affect its use in the vDSO, too.  In some situations,
it might be nice to run applications with no vDSO and PR_TSC_SIGSEGV,
just to make sure they don't have any heavy reliance on the TSC.  It
would be nice if those applications didn't crash when they called
clock_gettime().

Another alternative is to trap and adjust the RDTSC.  That might be a
viable option for applications that care about reliable RDTSC behavior
and migration, but don't care about performance.  I think it makes
sense to disable the vDSO in that case, rather than trap on every call
that it makes.

> 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.

Agreed.  That's what this patch is attempting to do, and that's the
main reason why I figured it was worth submitting independent of any
other time-related work.

> What problem are you trying to solve exactly?

Eventually, we'd like to make it so that neither RDTSC nor
CLOCK_MONOTONIC can go backwards following a migration.

The fix for RDTSC starts here.  Building on this patch as a base, we
can either ban it from being used entirely, or write some code to
adjust its value as necessary.

The CLOCK_MONOTONIC fix will be a different patch stack.  We're
currently hoping to do that without disable the vDSO, but that's
another discussion.

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

[Andy Lutomirski]

On Tue, Sep 16, 2014 at 6:18 PM, Richard Larocque <rlarocque-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> wrote:
> On Tue, Sep 16, 2014 at 5:27 PM, Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org> wrote:
>> On Tue, Sep 16, 2014 at 5:05 PM, Richard Larocque <rlarocque-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> 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-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
>>> ---
>>> 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.
>
> Right.  As far as I know, there's nothing wrong with the use of RDTSC
> in the vDSO following a migration.  The problem is that some
> applications might use RDTSC outside of the vDSO.  If they save the
> returned values, then compare pre- and post- migration values, bad
> things could happen (in theory).

These applications are broken, full stop.  They will misbehave on VMs,
or older machines, and even on the rather new piece of sh*t MSI
motherboard under my desk.  I think that CRIU is just icing on the
cake.  Also, they'll probably just crash if you turn off RDTSC.

>
> Anything we do to try to trap and handle the use of RDTSC in wider
> userspace will affect its use in the vDSO, too.  In some situations,
> it might be nice to run applications with no vDSO and PR_TSC_SIGSEGV,
> just to make sure they don't have any heavy reliance on the TSC.  It
> would be nice if those applications didn't crash when they called
> clock_gettime().

Agreed.  But let's do it without turning off the vdso.  Also, turning
off the 32-bit vdso could break a lot of things.

>
> Another alternative is to trap and adjust the RDTSC.  That might be a
> viable option for applications that care about reliable RDTSC behavior
> and migration, but don't care about performance.  I think it makes
> sense to disable the vDSO in that case, rather than trap on every call
> that it makes.

Here I disagree.  Let's just tweak the vdso not to use rdtsc in this case.

>
>> 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.
>
> Agreed.  That's what this patch is attempting to do, and that's the
> main reason why I figured it was worth submitting independent of any
> other time-related work.
>
>> What problem are you trying to solve exactly?
>
> Eventually, we'd like to make it so that neither RDTSC nor
> CLOCK_MONOTONIC can go backwards following a migration.
>
> The fix for RDTSC starts here.  Building on this patch as a base, we
> can either ban it from being used entirely, or write some code to
> adjust its value as necessary.
>
> The CLOCK_MONOTONIC fix will be a different patch stack.  We're
> currently hoping to do that without disable the vDSO, but that's
> another discussion.

I think that the patch should instead tweak the vvar mapping to tell
the vdso not to use rdtsc.  It should be based on this:

https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/log/?h=x86/vsyscall

and I'll talk to hpa tomorrow about about getting that, or something
like it, into the tip tree.  In particular, you'll need this:

https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/commit/?h=x86/vsyscall&id=0cc410a05cb95e073ebfe099c9e03cef48d2be0f

Also, this kind of inheritable restriction may end up requiring
no_new_privs or CAP_SYS_ADMIN to be secure.

--Andy

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

[Andy Lutomirski]

On Tue, Sep 16, 2014 at 10:00 PM, Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org> wrote:
> On Tue, Sep 16, 2014 at 6:18 PM, Richard Larocque <rlarocque-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> wrote:
>> On Tue, Sep 16, 2014 at 5:27 PM, Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org> wrote:
> I think that the patch should instead tweak the vvar mapping to tell
> the vdso not to use rdtsc.  It should be based on this:
>
> https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/log/?h=x86/vsyscall
>
> and I'll talk to hpa tomorrow about about getting that, or something
> like it, into the tip tree.  In particular, you'll need this:
>
> https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/commit/?h=x86/vsyscall&id=0cc410a05cb95e073ebfe099c9e03cef48d2be0f

Crud.  Now that I've said that, I realize that this won't work right
if rdtscp is off.  I'll drop that patch.

I don't think that this changes the conclusion.  It should be possible
to swap out the vvar page to keep the vdso working even without rdtsc
available.

--Andy

>
> Also, this kind of inheritable restriction may end up requiring
> no_new_privs or CAP_SYS_ADMIN to be secure.
>
> --Andy



-- 
Andy Lutomirski
AMA Capital Management, LLC

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

[Filipe Brandenburger]

Hi Andy,

On Tue, Sep 16, 2014 at 10:00 PM, Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org> wrote:
> I think that the patch should instead tweak the vvar mapping to tell
> the vdso not to use rdtsc.  It should be based on this:

I've been working on this approach which extends the vvar from 2 to 3
pages. The third page would initially be mapped to a zero page but
then through a prctl a task could replace it with a real page that
could then be inherited through fork and exec.

That would make it possible to have per-task vvar contents.

We could use some of those values as flags to indicate whether vdso
routines may use RDTSC or not.

In the future, we're planning to also use that to store clock offsets
so that we can ensure CLOCK_MONOTONIC works after CRIU migration
without having to turn off the VDSO or have to always fallback to full
syscalls on every case.

Do you think that would be a reasonable way to accomplish that?

Thanks,
Filipe

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

[H. Peter Anvin]

On 09/16/2014 11:21 PM, Filipe Brandenburger wrote:
> Hi Andy,
> 
> On Tue, Sep 16, 2014 at 10:00 PM, Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org> wrote:
>> I think that the patch should instead tweak the vvar mapping to tell
>> the vdso not to use rdtsc.  It should be based on this:
> 
> I've been working on this approach which extends the vvar from 2 to 3
> pages. The third page would initially be mapped to a zero page but
> then through a prctl a task could replace it with a real page that
> could then be inherited through fork and exec.
> 
> That would make it possible to have per-task vvar contents.
> 
> We could use some of those values as flags to indicate whether vdso
> routines may use RDTSC or not.
> 
> In the future, we're planning to also use that to store clock offsets
> so that we can ensure CLOCK_MONOTONIC works after CRIU migration
> without having to turn off the VDSO or have to always fallback to full
> syscalls on every case.
> 
> Do you think that would be a reasonable way to accomplish that?
> 

Why would we need/want per process vvar contents?  It seems better to
have the code swapped out.

	-hpa

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

[Kevin Easton]

On Tue, Sep 16, 2014 at 05:05:51PM -0700, Richard Larocque wrote:
> +	case PR_SET_VDSO:
> +		if (arg2 == PR_VDSO_ENABLE)
> +			me->signal->disable_vdso = 0;
> +		else if (arg2 == PR_VDSO_DISABLE)
> +			me->signal->disable_vdso = 1;
> +		else
> +			return -EINVAL;
> +		break;
> +	case PR_GET_VDSO:
> +		if (!me->signal->disable_vdso)
> +			error = put_user(PR_VDSO_ENABLE, (int __user *)arg2);
> +		else
> +			error = put_user(PR_VDSO_DISABLE, (int __user *)arg2);
> +		break;

Perhaps both of those should do

if (arg3 || arg4 || arg5)
	return -EINVAL;

    - Kevin

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

[Filipe Brandenburger]

Hi,

On Wed, Sep 17, 2014 at 1:46 AM, H. Peter Anvin <hpa@zytor.com> wrote:
> Why would we need/want per process vvar contents?  It seems better to
> have the code swapped out.

We are looking at the migration use case (CRIU).

In specific, we want to make CLOCK_MONOTONIC keep the "monotonic"
promise after migration. In order to accomplish that, we may need to
add an offset to CLOCK_MONOTONIC calls after migration.

By keeping a per process vvar page we should be able to implement that
without having to fallback to a real syscall for calls to
clock_gettime(CLOCK_MONOTONIC).

One other thought was that we could use this process vvar page to
expose a migration counter that would get incremented after migration.
That way we could allow (some) userspace tasks (and possibly some VDSO
routines) to use RDTSC but check the contents of that counter
before/after taking measurements to check whether the elapsed time is
valid or not. That means we could still provide tasks with a fast time
counter if they really need one.

Cheers,
Filipe

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

[Andy Lutomirski]

On Sep 17, 2014 1:46 AM, "H. Peter Anvin" <hpa@zytor.com> wrote:
>
> On 09/16/2014 11:21 PM, Filipe Brandenburger wrote:
> > Hi Andy,
> >
> > On Tue, Sep 16, 2014 at 10:00 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> >> I think that the patch should instead tweak the vvar mapping to tell
> >> the vdso not to use rdtsc.  It should be based on this:
> >
> > I've been working on this approach which extends the vvar from 2 to 3
> > pages. The third page would initially be mapped to a zero page but
> > then through a prctl a task could replace it with a real page that
> > could then be inherited through fork and exec.
> >
> > That would make it possible to have per-task vvar contents.
> >
> > We could use some of those values as flags to indicate whether vdso
> > routines may use RDTSC or not.
> >
> > In the future, we're planning to also use that to store clock offsets
> > so that we can ensure CLOCK_MONOTONIC works after CRIU migration
> > without having to turn off the VDSO or have to always fallback to full
> > syscalls on every case.
> >
> > Do you think that would be a reasonable way to accomplish that?
> >
>
> Why would we need/want per process vvar contents?  It seems better to
> have the code swapped out.

That seems messier from a build perspective.  Also, if we ever want to
switch this dynamically, swapping out data is much easier than
swapping out code.  I think we should be able to replace the vvar page
with the zero page, though.

One tricky bit: currently we can only easily do this on exec, but we
should be able to do it immediately if we start tracking mremap of the
vdso.  Should we make that a prerequisite?  I don't really want this
to end up being permanently weird.

As for an actual post-migration offset, I'd rather add support for
per-mm forced syscall fallback and then get something into the code
timing code before even thinking about an x86 vdso fast path.  I don't
think that a feature like per-mm timing offsets should happen as an
arch-specific thing first.

--Andy

>
>         -hpa
>
>

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

[Andy Lutomirski]

On Wed, Sep 17, 2014 at 7:28 AM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Sep 17, 2014 1:46 AM, "H. Peter Anvin" <hpa@zytor.com> wrote:
>>
>> On 09/16/2014 11:21 PM, Filipe Brandenburger wrote:
>> > Hi Andy,
>> >
>> > On Tue, Sep 16, 2014 at 10:00 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>> >> I think that the patch should instead tweak the vvar mapping to tell
>> >> the vdso not to use rdtsc.  It should be based on this:
>> >
>> > I've been working on this approach which extends the vvar from 2 to 3
>> > pages. The third page would initially be mapped to a zero page but
>> > then through a prctl a task could replace it with a real page that
>> > could then be inherited through fork and exec.
>> >
>> > That would make it possible to have per-task vvar contents.
>> >
>> > We could use some of those values as flags to indicate whether vdso
>> > routines may use RDTSC or not.
>> >
>> > In the future, we're planning to also use that to store clock offsets
>> > so that we can ensure CLOCK_MONOTONIC works after CRIU migration
>> > without having to turn off the VDSO or have to always fallback to full
>> > syscalls on every case.
>> >
>> > Do you think that would be a reasonable way to accomplish that?
>> >
>>
>> Why would we need/want per process vvar contents?  It seems better to
>> have the code swapped out.
>
> That seems messier from a build perspective.  Also, if we ever want to
> switch this dynamically, swapping out data is much easier than
> swapping out code.  I think we should be able to replace the vvar page
> with the zero page, though.
>
> One tricky bit: currently we can only easily do this on exec, but we
> should be able to do it immediately if we start tracking mremap of the
> vdso.  Should we make that a prerequisite?  I don't really want this
> to end up being permanently weird.

I have this (special mapping tracking) 3/4 implemented.  I'm planning
on making it fully functional for 64-bit programs and almost correct
for 32-bit.  (You'll still crash if you have multiple threads, you use
sysenter, and you remap the vdso, but I think that this is essentially
unavoidable until someone lets mremap work on multiple vmas at once.)

>
> As for an actual post-migration offset, I'd rather add support for
> per-mm forced syscall fallback and then get something into the code
> timing code before even thinking about an x86 vdso fast path.  I don't
> think that a feature like per-mm timing offsets should happen as an
> arch-specific thing first.
>
> --Andy
>
>>
>>         -hpa
>>
>>



-- 
Andy Lutomirski
AMA Capital Management, LLC

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

[Richard Larocque]

On Fri, Sep 19, 2014 at 12:27 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Wed, Sep 17, 2014 at 7:28 AM, Andy Lutomirski <luto@amacapital.net> wrote:
>> On Sep 17, 2014 1:46 AM, "H. Peter Anvin" <hpa@zytor.com> wrote:
>>>
>>> On 09/16/2014 11:21 PM, Filipe Brandenburger wrote:
>>> > Hi Andy,
>>> >
>>> > On Tue, Sep 16, 2014 at 10:00 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>>> >> I think that the patch should instead tweak the vvar mapping to tell
>>> >> the vdso not to use rdtsc.  It should be based on this:
>>> >
>>> > I've been working on this approach which extends the vvar from 2 to 3
>>> > pages. The third page would initially be mapped to a zero page but
>>> > then through a prctl a task could replace it with a real page that
>>> > could then be inherited through fork and exec.
>>> >
>>> > That would make it possible to have per-task vvar contents.
>>> >
>>> > We could use some of those values as flags to indicate whether vdso
>>> > routines may use RDTSC or not.
>>> >
>>> > In the future, we're planning to also use that to store clock offsets
>>> > so that we can ensure CLOCK_MONOTONIC works after CRIU migration
>>> > without having to turn off the VDSO or have to always fallback to full
>>> > syscalls on every case.
>>> >
>>> > Do you think that would be a reasonable way to accomplish that?
>>> >
>>>
>>> Why would we need/want per process vvar contents?  It seems better to
>>> have the code swapped out.
>>
>> That seems messier from a build perspective.  Also, if we ever want to
>> switch this dynamically, swapping out data is much easier than
>> swapping out code.  I think we should be able to replace the vvar page
>> with the zero page, though.
>>
>> One tricky bit: currently we can only easily do this on exec, but we
>> should be able to do it immediately if we start tracking mremap of the
>> vdso.  Should we make that a prerequisite?  I don't really want this
>> to end up being permanently weird.
>
> I have this (special mapping tracking) 3/4 implemented.  I'm planning
> on making it fully functional for 64-bit programs and almost correct
> for 32-bit.  (You'll still crash if you have multiple threads, you use
> sysenter, and you remap the vdso, but I think that this is essentially
> unavoidable until someone lets mremap work on multiple vmas at once.)
>

Thanks!  I look forward to seeing the result.

I have some per-process clock offset patches that currently work only
when the vDSO is disabled.  It sounds like your patches will provide a
clean solution to deal with that issue.  I'll try to rebase my work on
top of your changes when they're ready.

We've also got some patches to apply an offset to the TSC that could
benefit from your changes, but I guess there's not much appetite for
merging them.  That's fine with me.  I don't see any need for that
feature until we have a few examples of applications that could be
broken by TSC changes during migration.

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

[Filipe Brandenburger]

Hi Andy,

On Fri, Sep 19, 2014 at 12:27 PM, Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org> wrote:
> I have this (special mapping tracking) 3/4 implemented.  I'm planning
> on making it fully functional for 64-bit programs and almost correct
> for 32-bit.  (You'll still crash if you have multiple threads, you use
> sysenter, and you remap the vdso, but I think that this is essentially
> unavoidable until someone lets mremap work on multiple vmas at once.)

In case that's useful, I was looking at swapping the vvar page by
changing the vm_special_mapping to change the pages array between the
actual vvar page and the zero page and using zap_page_range to force
the next access to go through a page fault that would remap it.

I didn't have all the details figured out (I was closer to 1/4 of it
implemented) but I didn't see any issues on 32-bit programs.

Let me know if you'd like to see some of my patches or if you think I
should keep working on them.

Cheers,
Filipe

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

[Andy Lutomirski]

On Fri, Sep 19, 2014 at 3:02 PM, Filipe Brandenburger
<filbranden@google.com> wrote:
> Hi Andy,
>
> On Fri, Sep 19, 2014 at 12:27 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>> I have this (special mapping tracking) 3/4 implemented.  I'm planning
>> on making it fully functional for 64-bit programs and almost correct
>> for 32-bit.  (You'll still crash if you have multiple threads, you use
>> sysenter, and you remap the vdso, but I think that this is essentially
>> unavoidable until someone lets mremap work on multiple vmas at once.)
>
> In case that's useful, I was looking at swapping the vvar page by
> changing the vm_special_mapping to change the pages array between the
> actual vvar page and the zero page and using zap_page_range to force
> the next access to go through a page fault that would remap it.

That will do it globally, since the vm_special_mapping is global.  I
was just thinking of using remap_pfn_range.

>
> I didn't have all the details figured out (I was closer to 1/4 of it
> implemented) but I didn't see any issues on 32-bit programs.

The 32-bit issue comes from mremap.

>
> Let me know if you'd like to see some of my patches or if you think I
> should keep working on them.

Give me another day or two to straighten out the vma stuff, although
it shouldn't impact your patches too much.  The main effect will be
that you'll be able to rely on mm->context.vdso (renamed to
vvar_vma_start, most likey) being correct.  Currently, you should
*not* rely on it, especially if CRIU is involved.

--Andy

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

[Filipe Brandenburger]

Hi,

On Fri, Sep 19, 2014 at 3:09 PM, Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org> wrote:
> On Fri, Sep 19, 2014 at 3:02 PM, Filipe Brandenburger <filbranden-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> wrote:
>> In case that's useful, I was looking at swapping the vvar page by
>> changing the vm_special_mapping to change the pages array between the
>> actual vvar page and the zero page and using zap_page_range to force
>> the next access to go through a page fault that would remap it.
>
> That will do it globally, since the vm_special_mapping is global.  I
> was just thinking of using remap_pfn_range.

Sorry if I wasn't clear... That's the exact point of my patch, to make
vm_special_mapping local to the task.

I started with an approach of keeping a struct vm_special_mapping + a
struct page * array per mm_struct.

I was also looking at keeping two static vm_special_mapping structs,
one with the actual vvar page and the other with the zero page and
then swapping vma->vm_private_data to point to the appropriate one.

> Give me another day or two to straighten out the vma stuff, although
> it shouldn't impact your patches too much.  The main effect will be
> that you'll be able to rely on mm->context.vdso (renamed to
> vvar_vma_start, most likey) being correct.  Currently, you should
> *not* rely on it, especially if CRIU is involved.

Cool! As I said, let me know if you'd like to see some of my
incomplete patches or if you'd like me to keep working on them to show
you something more complete.

Cheers,
Filipe

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

[Andy Lutomirski]

On Fri, Sep 19, 2014 at 3:19 PM, Filipe Brandenburger
<filbranden-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> wrote:
> Hi,
>
> On Fri, Sep 19, 2014 at 3:09 PM, Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org> wrote:
>> On Fri, Sep 19, 2014 at 3:02 PM, Filipe Brandenburger <filbranden-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> wrote:
>>> In case that's useful, I was looking at swapping the vvar page by
>>> changing the vm_special_mapping to change the pages array between the
>>> actual vvar page and the zero page and using zap_page_range to force
>>> the next access to go through a page fault that would remap it.
>>
>> That will do it globally, since the vm_special_mapping is global.  I
>> was just thinking of using remap_pfn_range.
>
> Sorry if I wasn't clear... That's the exact point of my patch, to make
> vm_special_mapping local to the task.
>
> I started with an approach of keeping a struct vm_special_mapping + a
> struct page * array per mm_struct.
>
> I was also looking at keeping two static vm_special_mapping structs,
> one with the actual vvar page and the other with the zero page and
> then swapping vma->vm_private_data to point to the appropriate one.

This sounds like it may be more complicated than necessary.  Is there
any reason that just doing remap_pfn_range on the vvar page isn't
enough?

Changing out the text is a whole can of worms involving self-modifying
code, although it may be completely safe if done through the page
tables.  But I don't think you can't use remap_pfn_range for that.

>
>> Give me another day or two to straighten out the vma stuff, although
>> it shouldn't impact your patches too much.  The main effect will be
>> that you'll be able to rely on mm->context.vdso (renamed to
>> vvar_vma_start, most likey) being correct.  Currently, you should
>> *not* rely on it, especially if CRIU is involved.
>
> Cool! As I said, let me know if you'd like to see some of my
> incomplete patches or if you'd like me to keep working on them to show
> you something more complete.

Maybe better: what exactly are you trying to do?

--Andy

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

[Filipe Brandenburger]

Hi Andy,

On Fri, Sep 19, 2014 at 3:31 PM, Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org> wrote:
>> Sorry if I wasn't clear... That's the exact point of my patch, to make
>> vm_special_mapping local to the task.
>>
>> I started with an approach of keeping a struct vm_special_mapping + a
>> struct page * array per mm_struct.
>>
>> I was also looking at keeping two static vm_special_mapping structs,
>> one with the actual vvar page and the other with the zero page and
>> then swapping vma->vm_private_data to point to the appropriate one.
>
> This sounds like it may be more complicated than necessary.  Is there
> any reason that just doing remap_pfn_range on the vvar page isn't
> enough?

I thought of doing that from the prctl but AFAICT remap_pfn_range
requires that it's unmapped before the call (remap_pte_range has
BUG_ON(!pte_none(*pte));) and doing a zap_page_range followed by
remap_pfn_range might incur a race condition if another thread of the
same process is accessing the vvar page at that time... Am I wrong
about that race?

> Changing out the text is a whole can of worms involving self-modifying
> code, although it may be completely safe if done through the page
> tables.  But I don't think you can't use remap_pfn_range for that.

No, I'm not planning to change the text, just replacing the vvar page
swapping the one where the vsyscall_gtod_data lives with a zero page
(and back depending on the parameter of the prctl.)

> Maybe better: what exactly are you trying to do?

Just replace the page mapped to the vvar address.

But nevermind my ramblings here, looks like you're almost there so go
ahead and when you have a working patch I'll take a look at it.

My offer to help still stands, so let me know if you'd like to see
some of my code (unfinished as it is now or if you'd like me to show
you something that works in a few days.)

Cheers,
Filipe

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

[Andy Lutomirski]

On Fri, Sep 19, 2014 at 8:10 PM, Filipe Brandenburger
<filbranden-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> wrote:
> Hi Andy,
>
> On Fri, Sep 19, 2014 at 3:31 PM, Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org> wrote:
>>> Sorry if I wasn't clear... That's the exact point of my patch, to make
>>> vm_special_mapping local to the task.
>>>
>>> I started with an approach of keeping a struct vm_special_mapping + a
>>> struct page * array per mm_struct.
>>>
>>> I was also looking at keeping two static vm_special_mapping structs,
>>> one with the actual vvar page and the other with the zero page and
>>> then swapping vma->vm_private_data to point to the appropriate one.
>>
>> This sounds like it may be more complicated than necessary.  Is there
>> any reason that just doing remap_pfn_range on the vvar page isn't
>> enough?
>
> I thought of doing that from the prctl but AFAICT remap_pfn_range
> requires that it's unmapped before the call (remap_pte_range has
> BUG_ON(!pte_none(*pte));) and doing a zap_page_range followed by
> remap_pfn_range might incur a race condition if another thread of the
> same process is accessing the vvar page at that time... Am I wrong
> about that race?

No, you're right.  Ugh.

It might pay to add an explicit .fault callback to vm_special_mapping,
but your approach will work, too.  The main downside is more memory
overhead per mm.

>
>> Changing out the text is a whole can of worms involving self-modifying
>> code, although it may be completely safe if done through the page
>> tables.  But I don't think you can't use remap_pfn_range for that.
>
> No, I'm not planning to change the text, just replacing the vvar page
> swapping the one where the vsyscall_gtod_data lives with a zero page
> (and back depending on the parameter of the prctl.)

Hmm.  This will break the _COARSE timing functions as well as
__vdso_time.  That'll need fixing, either by swapping out the code
(yuck!) or by adding a branch to all of those code paths.

Maybe there's a non-branchy way, though.  Let me think.

>
>> Maybe better: what exactly are you trying to do?
>
> Just replace the page mapped to the vvar address.
>
> But nevermind my ramblings here, looks like you're almost there so go
> ahead and when you have a working patch I'll take a look at it.

See:

https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/log/?h=vdso/vma_tracking

>
> My offer to help still stands, so let me know if you'd like to see
> some of my code (unfinished as it is now or if you'd like me to show
> you something that works in a few days.)
>
> Cheers,
> Filipe



-- 
Andy Lutomirski
AMA Capital Management, LLC

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

[Filipe Brandenburger]

On Fri, Sep 19, 2014 at 8:27 PM, Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org> wrote:
>> I thought of doing that from the prctl but AFAICT remap_pfn_range
>> requires that it's unmapped before the call (remap_pte_range has
>> BUG_ON(!pte_none(*pte));) and doing a zap_page_range followed by
>> remap_pfn_range might incur a race condition if another thread of the
>> same process is accessing the vvar page at that time... Am I wrong
>> about that race?
>
> No, you're right.  Ugh.
>
> It might pay to add an explicit .fault callback to vm_special_mapping,
> but your approach will work, too.  The main downside is more memory
> overhead per mm.

One way to limit that overhead was to keep only two static
vm_special_mapping structs (one for the real vvar page and one for the
zero page) and then change the vma->vm_private_data to point to one or
the other when the prctl is called. I just thought of that recently
and haven't tried it yet so not sure if it will cause other
problems...

>>> Changing out the text is a whole can of worms involving self-modifying
>>> code, although it may be completely safe if done through the page
>>> tables.  But I don't think you can't use remap_pfn_range for that.
>>
>> No, I'm not planning to change the text, just replacing the vvar page
>> swapping the one where the vsyscall_gtod_data lives with a zero page
>> (and back depending on the parameter of the prctl.)
>
> Hmm.  This will break the _COARSE timing functions as well as
> __vdso_time.  That'll need fixing, either by swapping out the code
> (yuck!) or by adding a branch to all of those code paths.
>
> Maybe there's a non-branchy way, though.  Let me think.

So this is what I was thinking of extending the vvar region to include
one extra page (right now it's two pages, the one with the
vsyscall_gtod_data and the one with the hrtimer I/O mapping, so this
would be a third page.) This third page would be initially mapped to
the zero page, so it wouldn't cost an extra page of memory per process
or even globally.

The prctl would swap that page with another page allocated as the
prctl is called. That one page that's allocated with the prctl would
be inherited over forks and execs, so if you have a bunch of processes
that need the same offset then they'd all share that one single page.

The vdso clock routines would get the base clock information from
vsyscall_gtod_data, hpet, tsc, etc. and would simply add the offsets
from the task_vvar page (not too branch-y, just a few extra adds.) In
the regular case, that would be a zero page so it would be simply
adding zero. If we need to apply offsets, then we replace that page
with one that has a struct with the right offsets for each clock type.

When running in kernel mode, the clock code (and timer code, etc.)
would find that page through the mm_struct and find the clock offsets
that way.

I was working on this approach before you suggested swapping the
actual vvar page (and not sure if you were actually looking at
hot-swapping the vdso itself.) I can go back to working on it and send
you a few commits since I still have some of that code around.

Cheers,
Filipe

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

[Andy Lutomirski]

On Sep 19, 2014 8:46 PM, "Filipe Brandenburger" <filbranden-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> wrote:
>
> On Fri, Sep 19, 2014 at 8:27 PM, Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org> wrote:
> >> I thought of doing that from the prctl but AFAICT remap_pfn_range
> >> requires that it's unmapped before the call (remap_pte_range has
> >> BUG_ON(!pte_none(*pte));) and doing a zap_page_range followed by
> >> remap_pfn_range might incur a race condition if another thread of the
> >> same process is accessing the vvar page at that time... Am I wrong
> >> about that race?
> >
> > No, you're right.  Ugh.
> >
> > It might pay to add an explicit .fault callback to vm_special_mapping,
> > but your approach will work, too.  The main downside is more memory
> > overhead per mm.
>
> One way to limit that overhead was to keep only two static
> vm_special_mapping structs (one for the real vvar page and one for the
> zero page) and then change the vma->vm_private_data to point to one or
> the other when the prctl is called. I just thought of that recently
> and haven't tried it yet so not sure if it will cause other
> problems...

Sneaky :)

This might have interesting effects if someone races with you.
mmap_sem could be enough.

On the other hand, holding mmap_sem for write could be enough to let
you zap the pte and remap it.

>
> >>> Changing out the text is a whole can of worms involving self-modifying
> >>> code, although it may be completely safe if done through the page
> >>> tables.  But I don't think you can't use remap_pfn_range for that.
> >>
> >> No, I'm not planning to change the text, just replacing the vvar page
> >> swapping the one where the vsyscall_gtod_data lives with a zero page
> >> (and back depending on the parameter of the prctl.)
> >
> > Hmm.  This will break the _COARSE timing functions as well as
> > __vdso_time.  That'll need fixing, either by swapping out the code
> > (yuck!) or by adding a branch to all of those code paths.
> >
> > Maybe there's a non-branchy way, though.  Let me think.
>
> So this is what I was thinking of extending the vvar region to include
> one extra page (right now it's two pages, the one with the
> vsyscall_gtod_data and the one with the hrtimer I/O mapping, so this
> would be a third page.) This third page would be initially mapped to
> the zero page, so it wouldn't cost an extra page of memory per process
> or even globally.
>
> The prctl would swap that page with another page allocated as the
> prctl is called. That one page that's allocated with the prctl would
> be inherited over forks and execs, so if you have a bunch of processes
> that need the same offset then they'd all share that one single page.
>
> The vdso clock routines would get the base clock information from
> vsyscall_gtod_data, hpet, tsc, etc. and would simply add the offsets
> from the task_vvar page (not too branch-y, just a few extra adds.) In
> the regular case, that would be a zero page so it would be simply
> adding zero. If we need to apply offsets, then we replace that page
> with one that has a struct with the right offsets for each clock type.
>
> When running in kernel mode, the clock code (and timer code, etc.)
> would find that page through the mm_struct and find the clock offsets
> that way.

This seems like a lot of complexity and non-negligible overhead (extra
adds, cacheline misses, and even TLB misses) in the common case just
to have fast timing under very unusual circumstances.

I'd much rather do something to the vvar page that causes all the
timing calls to turn into normal syscalls and then handle the offset
in the timing syscalls.  Are you really doing something that's
performance-sensitive enough to justify all of the complexity of
making this work without syscalls?

To do that, the only real tricky part is handling the coarse timing
calls.  The could be done with extra branches, text patches, or
trickery involving setting up an alternate vvar page that always fails
the seqcount retry check.

>
> I was working on this approach before you suggested swapping the
> actual vvar page (and not sure if you were actually looking at
> hot-swapping the vdso itself.) I can go back to working on it and send
> you a few commits since I still have some of that code around.
>
> Cheers,
> Filipe