Re: [RFC PATCH 2/2] tracing/user_events: Fixup enable faults asyncly

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2022-10-28 18:35, Beau Belgrave wrote:
> On Fri, Oct 28, 2022 at 06:07:34PM -0400, Mathieu Desnoyers wrote:
>> On 2022-10-27 18:40, Beau Belgrave wrote:

[...]

> 
>> I'm still slightly unsure about using "uint32_t" for enable check, or going
>> for "unsigned long". The core question there is whether a 32-bit word test
>> would cause partial register stalls on 64-bit architectures. Going for
>> unsigned long would require that user events receives information about the
>> bitness of the word as input from userspace. (bit=63 rather than 31).
>> Perhaps this is something the user events ABI should accommodate by
>> reserving more than 5 bits to express the target bit ?
>>
> 
> Yeah, I thought about this. From the user side you can actually use any
> arbitrary bits you want by passing a 32-bit aligned value. So you could
> take the lower or top half of a 64-bit value. The reason I limit to 0-31
> bits is to ensure no page straddling occurs. Futex also does this, check
> out get_futex_key() in kernel/futex/core.c.

We can ensure no page straddling by checking the address alignment 
compared to the size of the integer (4 or 8 bytes) also received as input.

> 
> I would recommend trying out uint64 but give the upper half to
> user_events ABI and checking if that works for you if you want say 63
> bits to user space. You'd tell the ABI bit 31, but in user space it
> would be bit 63.

That's tricky because Linux supports both big and little endian, which 
will make the ABI tricky to use if we try to be too clever about this.

Also, just stating a "pointer" and "bits offset" is not enough if we 
want to support 32-bit and 64-bit integer types, because the bit offset 
does not consider endianness.

I would recommend to get the following information through the user 
events registration:

- pointer address,
- size of integer type (4 or 8 bytes),
- bit offset (currently 31 or 63).

This way we have all the information we need to set the right bit in 
both little and big endian. We also have all the information we need to 
validate natural alignment, e.g.:

/* Check alignment */
if (addr & (type_size - 1))
	return -EINVAL;
/* Check bit offset range */
if (bit_offset > (type_size * CHAR_BIT) - 1)
	return -EINVAL;
switch (type_size) {
case 4:
	/* Update 32-bit integer */
	break;
#if BITS_PER_LONG >= 64
case 8:
	/* Update 64-bit integer */
	break;
#endif
default:
	return -EINVAL;
}


> 
>>>
>>> The enable address is disabled while the async fault-in occurs. This
>>> ensures that we don't attempt fault-in more than is necessary. Once the
>>> page has been faulted in, the address write is attempted again. If the
>>> page couldn't fault-in, then we wait until the next time the event is
>>> enabled to prevent any potential infinite loops.
>>
>> So if the page is removed from the page cache between the point where it is
>> faulted in and the moment the write is re-attempted, that will not trigger
>> another attempt at paging in the page, am I correct ?
>>
> 
> If we fault and the fixup user fault fails still with retries, then we
> give up until the next enablement of that site.
> 
> However, if we fault and the fixup user fault with retries works, and
> then we fault again trying to write, that will retry.
> 
>> I would think this is unexpected. I would expect that failing to fault in
>> the page would stop any further attempts, but simply failing to pin the page
>> after faulting it in should simply try again.
>>
> 
> The issue I repro is mmap() register the enable at that location, then
> unmap(). All the faulting errors just tell you -EFAULT in this case,
> even though it was maliciously removed. In this case you could get the
> kernel to infinite loop trying to page it in.
> 
>> Thoughts ?
>>
> 
> We need to have some sanity toward giving up faulting in data that never
> will make it. The code currently assigns that line to if
> fixup_user_fault with retries fails, we give up. pin_user_pages_remote
> will not stop it from being attempted again.

What are the legitimate cases that can make fixup_user_fault fail ? If 
there are none, and the only case that can make this fail is userspace 
unmapping memory, then we should very well kill the offending process.

An example here is futex fault_in_user_writeable(). When it fails, it 
appears that the futex_wake_op simply returns -EFAULT to the caller.

Thanks,

Mathieu

> 
>> Thanks,
>>
>> Mathieu
>>
> 
> Thanks,
> -Beau

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com