RFR(xxs): 8167650: NMT should check for invalid MEMFLAGS.

Wed Nov 23 02:08:25 UTC 2016

Hi Thomas,

> David did not like that, he thought it strange to check an enum for
> correct enum values. I can see his point. He would have preferred
> instead an assert inside NMTUtil::flag_to_index() which before
> returning checks the to-be-returned index for
> 0>=i>=mt_number_of_types. I did not like that because strictly
> speaking inside of NMTUtil::flag_to_index() it is not known that the
> returned integer will be used by the caller as an index into an array
> of mt_number_of_types length.

Sorry but how is NMTUtil::flag_to_index anything but a mapping from 0 to 
mt_number_of_types ??? MEMFLAGS aka MemoryType is an enum that spans 
zero to mt_number_of_types! We're validating the post condition for the 
method.

As I said on Oct 19, and which Chris indicated he agreed with, it makes 
no sense to me to put the same assert in all the callers of 
flag_to_index instead of doing the validation inside flag_to_index. 
Whether you choose to validate the flag or the index I don't care. If we 
were in a decent language the bug would have been impossible to begin with.

David
-----

On 22/11/2016 1:21 AM, Thomas Stüfe wrote:
> Hi all,
>
> this small issue got fc extension, so lets wrap this up. Thanks to
> all for reviewing!
>
> New webrev:
>
> http://cr.openjdk.java.net/~stuefe/webrevs/8167650-NMT-should-check_MEMFLAGS/webrev.02/webrev/
>
>  No changes to the last one but the updated copyrights Chris did ask
> for. Solution still follows Max' suggestion of checking the index
> right before consumption.
>
> Thanks, and Kind Regards, Thomas
>
>
> On Mon, Nov 21, 2016 at 4:17 PM, Thomas Stüfe
> <thomas.stuefe at gmail.com <mailto:thomas.stuefe at gmail.com>> wrote:
>
> Hi Chris,
>
> On Thu, Nov 17, 2016 at 8:54 PM, Chris Plummer
> <chris.plummer at oracle.com <mailto:chris.plummer at oracle.com>> wrote:
>
> On 10/18/16 11:49 PM, David Holmes wrote:
>
>
>
> On 19/10/2016 3:17 PM, Thomas Stüfe wrote:
>
>
>
> On Wed, Oct 19, 2016 at 3:21 AM, David Holmes
> <david.holmes at oracle.com <mailto:david.holmes at oracle.com>
> <mailto:david.holmes at oracle.com <mailto:david.holmes at oracle.com>>>
> wrote:
>
> On 18/10/2016 3:39 PM, Thomas Stüfe wrote:
>
> Hi David, Max,
>
> I changed the asserts according to Max' suggestion. Instead of
> checking inside flag_to_index, now I check before callers of this
> function use this value to access memory.
>
>
> I don't see where Max suggested that??
>
>
> Max wrote: " I think the decision on whether to access a slot should
> occur as close to memory accessing code as possible." and proceeded
> to suggest fixing VirtualMemorySnapshot::by_type() as well.
>
>
> I did not interpret that comment that way, and was puzzled by the
> reference to by_type.
>
>
> It doesn't make sense to me to have all the callers of flag_to_index
> check what it returned instead of doing it inside flag_to_index.
>
>
> I disagree. Imho it makes sense to either check the Memflags
> enumeration input argument in flag_to_index() or the returned index
> before consumption. In both cases one knows the valid value range.
> Strictly speaking checking the index in flag_to_index() cannot be
> done because it is a faceless int type whose valid values are not yet
> known.
>
>
> The index has to fall in the range 0 <= index <= mt_number_of_types,
> and I was suggesting that it makes more sense to verify this once in
> flag_to_index() than in all the callers of flag_to_index.
>
> Hi Thomas,
>
> Just catching up on this thread. This is the same conclusion I came
> to. I don't understand what you mean by "it is a faceless int type
> whose valid values are not yet known".
>
>
> Thank you for looking into this!
>
> "it is a faceless int type whose valid values are not yet known" was
> maybe expressed sloppily:
>
> My first patch checked the input enum "flag" argument inside
> NMTUtil::flag_to_index() for correct enum values :
> http://cr.openjdk.java.net/~stuefe/webrevs/8167650-NMT-should-check_MEMFLAGS/webrev.00/webrev/
>
>
<http://cr.openjdk.java.net/~stuefe/webrevs/8167650-NMT-should-check_MEMFLAGS/webrev.00/webrev/>
>
> David did not like that, he thought it strange to check an enum for
> correct enum values. I can see his point. He would have preferred
> instead an assert inside NMTUtil::flag_to_index() which before
> returning checks the to-be-returned index for
> 0>=i>=mt_number_of_types. I did not like that because strictly
> speaking inside of NMTUtil::flag_to_index() it is not known that the
> returned integer will be used by the caller as an index into an array
> of mt_number_of_types length.
>
> It is all extreme nitpicking and in the end amounts to the same :)
>
> The current patch does neither, but follows Max' suggestion of
> checking the index right before it is consumed to access an array. I
> think this is a good solution and very clear.
>
>
>
> BTW, I'll sponsor this fix for you once it is finalized. Please
> update the copyright dates first.
>
>
> Thank you. I'll update the webrev and repost.
>
>
>
> thanks,
>
> Chris
>
>
> Kind Regards, Thomas
>
>
>
> David
>
> It is all academical and mostly a matter of taste.
>
> http://cr.openjdk.java.net/~stuefe/webrevs/8167650-NMT-should-check_MEMFLAGS/webrev.01/webrev/index.html
>
>
<http://cr.openjdk.java.net/~stuefe/webrevs/8167650-NMT-should-check_MEMFLAGS/webrev.01/webrev/index.html>
> <http://cr.openjdk.java.net/~stuefe/webrevs/8167650-NMT-should-check_MEMFLAGS/webrev.01/webrev/index.html
>
>
<http://cr.openjdk.java.net/~stuefe/webrevs/8167650-NMT-should-check_MEMFLAGS/webrev.01/webrev/index.html>>
>
> As David correctly writes, this is technically not a bug, so I guess
> this will have to wait until java 10.
>
>
> Yes, afraid so.
>
>
> The fix is trivial and I will try to get fc extension for this (now
> that Goetz explained to me how to do this :). It seems this is done
> for many other non-bug issues as well.
>
> ..Thomas
>
>
> Thanks, David
>
> Kind Regards, Thomas
>
>
>
> On Fri, Oct 14, 2016 at 12:57 AM, David Holmes
> <david.holmes at oracle.com <mailto:david.holmes at oracle.com>
> <mailto:david.holmes at oracle.com <mailto:david.holmes at oracle.com>>
> <mailto:david.holmes at oracle.com <mailto:david.holmes at oracle.com>
> <mailto:david.holmes at oracle.com <mailto:david.holmes at oracle.com>>>>
> wrote:
>
> On 13/10/2016 10:53 PM, Thomas Stüfe wrote:
>
> Hi David,
>
> On Thu, Oct 13, 2016 at 12:08 PM, David Holmes
> <david.holmes at oracle.com <mailto:david.holmes at oracle.com>
> <mailto:david.holmes at oracle.com <mailto:david.holmes at oracle.com>>
> <mailto:david.holmes at oracle.com <mailto:david.holmes at oracle.com>
> <mailto:david.holmes at oracle.com <mailto:david.holmes at oracle.com>>>
> <mailto:david.holmes at oracle.com <mailto:david.holmes at oracle.com>
> <mailto:david.holmes at oracle.com <mailto:david.holmes at oracle.com>>
>
> <mailto:david.holmes at oracle.com <mailto:david.holmes at oracle.com>
> <mailto:david.holmes at oracle.com <mailto:david.holmes at oracle.com>>>>>
> wrote:
>
> Hi Thomas,
>
> On 13/10/2016 3:49 PM, Thomas Stüfe wrote:
>
> Hi all,
>
> may I have plase a review for this tiny change? It just adds some
> assert to NMT.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8167650
> <https://bugs.openjdk.java.net/browse/JDK-8167650>
>
> <https://bugs.openjdk.java.net/browse/JDK-8167650
> <https://bugs.openjdk.java.net/browse/JDK-8167650>>
> <https://bugs.openjdk.java.net/browse/JDK-8167650
> <https://bugs.openjdk.java.net/browse/JDK-8167650>
> <https://bugs.openjdk.java.net/browse/JDK-8167650
> <https://bugs.openjdk.java.net/browse/JDK-8167650>>>
>
>
> <https://bugs.openjdk.java.net/browse/JDK-8167650
> <https://bugs.openjdk.java.net/browse/JDK-8167650>
>
> <https://bugs.openjdk.java.net/browse/JDK-8167650
> <https://bugs.openjdk.java.net/browse/JDK-8167650>>
> <https://bugs.openjdk.java.net/browse/JDK-8167650
> <https://bugs.openjdk.java.net/browse/JDK-8167650>
> <https://bugs.openjdk.java.net/browse/JDK-8167650
> <https://bugs.openjdk.java.net/browse/JDK-8167650>>>> webrev:
>
>
> http://cr.openjdk.java.net/~stuefe/webrevs/8167650-NMT-should-check_
> <http://cr.openjdk.java.net/~stuefe/webrevs/8167650-NMT-should-check_>
>
>
<http://cr.openjdk.java.net/~stuefe/webrevs/8167650-NMT-should-check_
> <http://cr.openjdk.java.net/~stuefe/webrevs/8167650-NMT-should-check_>>
>
>
> <http://cr.openjdk.java.net/~stuefe/webrevs/8167650-NMT-should-check_
>
>
<http://cr.openjdk.java.net/~stuefe/webrevs/8167650-NMT-should-check_>
> <http://cr.openjdk.java.net/~stuefe/webrevs/8167650-NMT-should-check_
>
>
<http://cr.openjdk.java.net/~stuefe/webrevs/8167650-NMT-should-check_>>>
>
>
> <http://cr.openjdk.java.net/~stuefe/webrevs/8167650-NMT-should-check_
>
>
<http://cr.openjdk.java.net/~stuefe/webrevs/8167650-NMT-should-check_>
> <http://cr.openjdk.java.net/~stuefe/webrevs/8167650-NMT-should-check_
>
>
<http://cr.openjdk.java.net/~stuefe/webrevs/8167650-NMT-should-check_>>
>
> <http://cr.openjdk.java.net/~stuefe/webrevs/8167650-NMT-should-check_
>
>
<http://cr.openjdk.java.net/~stuefe/webrevs/8167650-NMT-should-check_>
> <http://cr.openjdk.java.net/~stuefe/webrevs/8167650-NMT-should-check_
>
>
<http://cr.openjdk.java.net/~stuefe/webrevs/8167650-NMT-should-check_>>>>
> MEMFLAGS/webrev.00/webrev/
>
> We had an ugly memory overwrite caused by this - ultimately our
> fault, because we fed an invalid memory flag to NMT - but it was
> difficult to find. An assert would have saved some time.
>
>
> I'm a little perplexed with asserting that something of MEMFLAGS type
> must be an actual MEMFLAGS value - it implies the caller is coercing
> plain int to MEMFLAGS, and I don't have much sympathy if they mess
> that up. Can't help wondering if there is some clever C++ trick to
> flag bad conversions at compile-time?
>
>
> The error was caused by an uninitialized variable of type MEMFLAGS.
> This was our fault, we have heavily modified allocation.hpp and
> introduced an error then merging changes from upstream. Due to a
> merging error this lead to a case where Arena::_flags was not
> initialized and contained a very large value.
>
>
> Ah I see. Lack of default initialization can be annoying :)
>
> I admit it looks funny. If it bothers you, I could instead check the
> returned index to be in the range for the size of the _malloc array
> in MallocMemorySnapshot::by_type(). Technically, it would mean the
> same.
>
>
> So I just realized that here:
>
> 62   // Map memory type to human readable name 63   static const
> char* flag_to_name(MEMFLAGS flag) { 64     assert(flag >= 0 && flag
> < mt_number_of_types, "Invalid flag value %d.", (int)flag); 65
> return _memory_type_names[flag_to_index(flag)]; 66   }
>
> we call flag_to_index, so the assert is redundant as it is already in
> flag_to_index. Then presumably we change flag_to_index to something
> like this:
>
> static inline int flag_to_index(MEMFLAGS flag) { int index = (flag &
> 0xff); assert(index >= 0 && index < mt_number_of_types, "Invalid flag
> value %d.", (int)flag); return index; }
>
> so we're validating the index rather than the flag.
>
> Cheers, David
>
>
>
>
> The function that takes the index should validate the index, so that
> is fine.
>
> Which one were you actually passing the bad value to? :)
>
> This isn't a strong objection just musing if we can do better. And as
> the hs repos are still closed, and likely to remain so till early
> next week, we have some slack time :)
>
>
> :) Sure.
>
> Kind Regards, Thomas
>
>
> Cheers, David
>
> Thank you!
>
> Thomas
>
>
>
>
>
>
>
>