[uml-devel] (implementation idea) Re: uml and nonstandard memory splits?

[uml-devel] (implementation idea) Re: uml and nonstandard memory splits?

[Blaisorblade]

On Friday 24 March 2006 01:40, Jason Lunz wrote:
> On Thu, Mar 23, 2006 at 06:50:44PM -0500, Jeff Dike wrote:
> > You can make some progress by disabling CONFIG_MODE_TT - that will
> > cause UML to load at a normal address rather than the 0xa0000000 that
> > CONFIG_MODE_TT will force.
> >
> > That should get you to the point of trying to run init.
> >
> > Since you have skas3, my guess is that UML will be fine at that point.
> >
> > On a non-skas3 host, it will run in skas0 mode, and will expect to be
> > able to map a couple of pages at the top of the process address space
> > (hardcoded to 0xbfffe000 and 0xbffff000).  These offsets would need to
> > be calculated based on the host's split in order for skas0 to work.
>
> So I guess getting a 2.4 uml to work on any split other than 1G/3G is
> pretty unlikely. Oh well, I already knew 2.4 was on its way out. So this
> might be the thing that really forces me to use 2.6.

No, that's wrong - what David Lang described:
> I don't know about other non-standard memory configs, but there is an 
> explicit config option when compiling a uml kernel to support the 2G/2G 
> memory split.
was also supported by UML 2.4. I think unusual split and the UML trivial 
support for that were born and used when 4G/4G didn't exist yet (or maybe not 
even highmem?).

> 2.4 support aside, my aim is to have a single uml binary that will work
> on arbitrary host kernels. How difficult will it be to implement support
> for dynamic offsets for these pages in skas0?

I don't think it's easy - however, it can be done. Below my idea - Jeff, 
please comment on this

In assembly.
It goes like this:

call label /*this is coded like a relative jump, i.e. call +1*/
label: pop $eax /*we got the saved EIP*/

/* Now we can round EIP and subtract an offset to calculate the data page 
location.
*/

The only problem is that it must be done in assembly and at times this must be 
redone (we don't have enough registers to save the value across a 6-param 
syscall), and touching this code is a bit dangerous.

> Jason

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[uml-devel] Re: (implementation idea) Re: uml and nonstandard memory splits?

[Jeff Dike]

On Fri, Mar 24, 2006 at 01:56:32AM +0100, Blaisorblade wrote:
> I don't think it's easy - however, it can be done. Below my idea - Jeff, 
> please comment on this
> 
> In assembly.
> It goes like this:
> 
> call label /*this is coded like a relative jump, i.e. call +1*/
> label: pop $eax /*we got the saved EIP*/
> 
> /* Now we can round EIP and subtract an offset to calculate the data page 
> location.
> */

How does this give us the location of the data page?

You really want to know the end of the process address space, which
you can do by taking the address of a convenient local and rounding up
to the nearest 1G or .5G or so.

				Jeff


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[uml-devel] Re: (implementation idea) Re: uml and nonstandard memory splits?

[Blaisorblade]

On Friday 24 March 2006 03:30, Jeff Dike wrote:
> On Fri, Mar 24, 2006 at 01:56:32AM +0100, Blaisorblade wrote:
> > I don't think it's easy - however, it can be done. Below my idea - Jeff,
> > please comment on this

> > In assembly.
> > It goes like this:

> > call label /*this is coded like a relative jump, i.e. call +1*/
> > label: pop $eax /*we got the saved EIP*/
> >
> > /* Now we can round EIP and subtract an offset to calculate the data page
> > location.
> > */

> How does this give us the location of the data page?

The EIP is inside the stub code page, and the two pages are one near the 
other.

> You really want to know the end of the process address space, which
> you can do by taking the address of a convenient local and rounding up
> to the nearest 1G or .5G or so.

You can't do that in the stub; however, what you suggest or something similar 
is probably needed outside the stub, from the kernel code, when you need to 
map it somewhere inside the process memory.

However, currently, the rounding can't be 0.5G, it must be 0.25G with the 
current code, and it's still fragile if the host code is changed.

(arch/i386/Kconfig)
config PAGE_OFFSET
        hex
        default 0xB0000000 if VMSPLIT_3G_OPT
        default 0x78000000 if VMSPLIT_2G
        default 0x40000000 if VMSPLIT_1G
        default 0xC0000000

-- 
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[uml-devel] Re: (implementation idea) Re: uml and nonstandard memory splits?

[Jason Lunz]

blaisorblade@yahoo.it said:
> However, currently, the rounding can't be 0.5G, it must be 0.25G with the 
> current code, and it's still fragile if the host code is changed.
>
> (arch/i386/Kconfig)
> config PAGE_OFFSET
>         hex
>         default 0xB0000000 if VMSPLIT_3G_OPT
>         default 0x78000000 if VMSPLIT_2G
>         default 0x40000000 if VMSPLIT_1G
>         default 0xC0000000

Yes, I don't think other choices are too unlikely in the future.
0xB0000000 was only chosen because it prevents a typical 32-bit machine
with 1G of ram from having 1/8 of it in a little 128M highmem zone. In
the future, "typical" could likely be something else.

Jason



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[uml-devel] Re: (implementation idea) Re: uml and nonstandard memory splits?

[Jeff Dike]

On Fri, Mar 24, 2006 at 03:34:01PM +0100, Blaisorblade wrote:
> The EIP is inside the stub code page, and the two pages are one near the 
> other.

I think you're going to have to talk more slowly and use shorter words.

The problem is that we need to figure out at runtime where to put the
stub pages, correct?  And we can't use the code stub page to figure out 
where it is, because we haven't mapped it yet, because we don't know where
to put it, correct?

> However, currently, the rounding can't be 0.5G, it must be 0.25G with the 
> current code, and it's still fragile if the host code is changed.
> 
> (arch/i386/Kconfig)
> config PAGE_OFFSET
>         hex
>         default 0xB0000000 if VMSPLIT_3G_OPT
>         default 0x78000000 if VMSPLIT_2G
>         default 0x40000000 if VMSPLIT_1G
>         default 0xC0000000

How much can stack randomization vary?  If it can vary more than the
split varies, which I would expect, then rounding the stack pointer
isn't going to work.

Another approach is to start with the current top of stack and try
mapping pages at increments equal to the smallest split granularity
that we think we are going to see, until one fails.

				Jeff


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[uml-devel] Re: (implementation idea) Re: uml and nonstandard memory splits?

[Blaisorblade]

On Friday 24 March 2006 18:26, Jeff Dike wrote:
> On Fri, Mar 24, 2006 at 03:34:01PM +0100, Blaisorblade wrote:
> > The EIP is inside the stub code page, and the two pages are one near the
> > other.

> I think you're going to have to talk more slowly and use shorter words.

Yes, I've been a bit too fast :-)

> The problem is that we need to figure out at runtime where to put the
> stub pages, correct?  And we can't use the code stub page to figure out
> where it is, because we haven't mapped it yet, because we don't know where
> to put it, correct?

That is problem #1, and exists; but when answering I thought to problem #2, 
i.e. that the stub code currently hardcodes the location of the stub data 
page, and that this must be fixed; I didn't notice that we must first put the 
stubs somewhere.

So I remembered that trick to get EIP which I read time ago (the purpose was 
to make some code Position-Independent exactly injecting some code into 
another process address space).

Btw, PIC library are another story because they're told their location (IIRC, 
I forget the details).

Actually, we need to figure were to place the pages, so we could _maybe_ as 
well pass the address to the stub in a register. I'm quite unsure of this 
however as at least clone.c for i386 uses _all_ available registers for the 
6-param syscall call.

> How much can stack randomization vary?  If it can vary more than the
> split varies, which I would expect, then rounding the stack pointer
> isn't going to work.

Dunno, but even if it currently works, we'd better find a solution without the 
assumption that stack randomization doesn't create problem.

> Another approach is to start with the current top of stack

How do you get that?

> and try 
> mapping pages at increments equal to the smallest split granularity
> that we think we are going to see, until one fails.

I don't like the idea of parsing /proc/XXX/maps, but I must mention it too; 
however, the format changed slightly in the various kernels, so it's 
particularly a bad idea.

bfff9000-c0000000 rwxp bfff9000 00:00 0 [stack]

-- 
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[uml-devel] Re: (implementation idea) Re: uml and nonstandard memory splits?

[Jeff Dike]

On Mon, Mar 27, 2006 at 11:30:05PM +0100, Blaisorblade wrote:
> That is problem #1, and exists; but when answering I thought to problem #2, 
> i.e. that the stub code currently hardcodes the location of the stub data 
> page, and that this must be fixed; I didn't notice that we must first put the
> stubs somewhere.

I see - you're one problem ahead of me.

I'm wondering if we can play some linker tricks for this.  Stick a
word of data on the code page and put the data page address in it.
Then the stub just reads that in order to get the data.

> 
> So I remembered that trick to get EIP which I read time ago (the purpose was 
> to make some code Position-Independent exactly injecting some code into 
> another process address space).

That's the standard way of getting your EIP on x86 since there's no
move from EIP instruction.  That will work, too, but (as you point
out) requires a register.

> > Another approach is to start with the current top of stack

Hmmmm :-)  Maybe easier said than done.  I was thinking about figuring
that out as basically the first thing in main, but I can think of
reasons that wouldn't be robust.

Other schemes, which involve a binary search for the boundary by using
test mmaps, involve the risk of unmapping part of the stack.

				Jeff


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[uml-devel] Re: [uml-user] Re: (implementation idea) Re: uml and nonstandard memory splits?

[Blaisorblade]

On Tuesday 28 March 2006 22:02, Jeff Dike wrote:
> On Mon, Mar 27, 2006 at 11:30:05PM +0100, Blaisorblade wrote:
> > That is problem #1, and exists; but when answering I thought to problem
> > #2, i.e. that the stub code currently hardcodes the location of the stub
> > data page, and that this must be fixed; I didn't notice that we must
> > first put the stubs somewhere.

> I see - you're one problem ahead of me.

> I'm wondering if we can play some linker tricks for this.  Stick a
> word of data on the code page and put the data page address in it.
> Then the stub just reads that in order to get the data.

After you place the stubs somewhere, you can pass them the address in a 
register. Indeed, we already do that since the stack in located inside the 
data page, so you simply need to round that (hardcoding at compile-time the 
data size page, but we already do that).

Passing that in a memory word doesn't work unless you use Instruction Pointer 
relative addressing, only available (to my knowledge) in x86_64 as 
RIP-relative addressing.

> That will work, too, but (as you point
> out) requires a register.

Not a big problem - you can recalculate it at will so that's not a real 
problem as long as we can recalculate if without problems (which could 
probably require switching to full assembler).

> > > Another approach is to start with the current top of stack

> Hmmmm :-)  Maybe easier said than done.  I was thinking about figuring
> that out as basically the first thing in main, but I can think of
> reasons that wouldn't be robust.

Can we remove the assumption the pages are at the absolute top of the address 
space?

The guest process doesn't risk unmapping them because we stop it from that, 
with gate_vma's for instance, even if I know that's a work in progress from 
Bodo's comments (not yet begun).

We could block mmapping over the used address, and we must do that anyway - 
SKAS3 indeed would fail on 2G/2G and such hosts exactly because we don't do 
that and our mmap() returns addresses beyond host TASK_SIZE.

However, this doesn't happen because we already implement the "top of the 
stack" detection technique in arch/um/kernel/skas/mem.c. So we've well-tested 
code doing this, even if maybe at times suboptimal.

But how much does stack randomization changes? That's the only remaining 
doubt.
-- 
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[uml-devel] Re: [uml-user] Re: (implementation idea) Re: uml and nonstandard memory splits?

[Blaisorblade]

On Wednesday 29 March 2006 00:05, Blaisorblade wrote:
> However, this doesn't happen because we already implement the "top of the
> stack" detection technique in arch/um/kernel/skas/mem.c. So we've
> well-tested code doing this, even if maybe at times suboptimal.

About that code, 

$ find arch/um/ include/asm-um |xargs grep HOST_TASK_SIZE

finds no mention of CONFIG_HOST_TASK_SIZE anywhere else, so that should be 
removed it seems (unless it's for S390, but that port is stalling - there's 
no point in still waiting to merge it, we must either merge or drop it 
depending on the fact that we have at least 1 active user).
-- 
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