[llvm-dev] [cfe-dev] Identifying objects within BumpPtrAllocator.

[David Blaikie via llvm-dev]

Mostly what Richard said.

One thing I'd be a bit careful of - these numbers may still not be stable
in some small number of cases (eg: if objects are created based on
iteration order of a pointer-based hashing container - which may still be
valid if that ordering doesn't leak into the output of the program). So
this might provide a slightly false sense of security & make those minority
cases more painful - but perhaps they're rare enough that it's worth the
tradeoff.

(& as Richard said - debuggers will tend to disable ASLR anyway, making it
relatively easy to work with)

On Tue, Aug 28, 2018 at 6:16 PM Richard Smith via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> On Tue, 28 Aug 2018, 17:14 Artem Dergachev via cfe-dev, <
> cfe-dev at lists.llvm.org> wrote:
>
>> In various debug dumps (eg., Clang's -ast-dump), various objects (eg.,
>> Stmts and Decls in that -ast-dump) are identified by pointers. It's very
>> reliable in the sense that no two objects would ever have the same
>> pointer at the same time, but it's unpleasant that pointers change
>> across runs. Having deterministic identifiers instead of pointers would
>> aid debugging: imagine a conditional break by object identifier that has
>> not yet been constructed, or simply trying to align two debug dumps of
>> different kind from different runs together. Additionally, pointers are
>> hard to read and memorize; it's hard to notice the difference between
>> 0x7f80a28325e0 and 0x7f80a28325a0, especially when they're a few screens
>> apart.
>>
>> Hence the idea: why don't we print the offset into the allocator's
>> memory slab instead of a pointer?
>
>
> Make this "as well as" rather than "instead of" and it sounds great to me.
> When debugging, it's useful to be able to dump a large complex object, find
> the piece you want, grab its address and start accessing it directly.
>
> (For the pointer stability problem, at least on Linux you can turn off
> ASLR. When running under gdb, that's typically done for you, and you can do
> it manually with setarch. But it would be nice to have an easier way to
> identify objects than a long, essentially meaningless address.)
>
> We use BumpPtrAllocator all over the
>> place, which boils down to a set of slabs on which all objects are
>> placed in the order in which they are allocated. It is easy for the
>> allocator to identify if a pointer belongs to that allocator, and if so,
>> deteremine which slab it belongs to and at what offset the object is in
>> that slab. Therefore it is possible to identify the object by its (slab
>> index, offset) pair. Eg., "TypedefDecl 0:528" (you already memorized it)
>> instead of "TypedefDecl 0x7f80a28325e0". This could be applied to all
>> sorts of objects that live in BumpPtrAllocators.
>>
>> In order to compute such identifier, we only need access to the object
>> and to the allocator. No additional memory is used to store such
>> identifier. Such identifier would also be persistent across runs as long
>> as the same objects are allocated in the same order, which is, i
>> suspect, often the case.
>>
>> One of the downsides of this identifier is that it's not going to be the
>> same on different machines, because the same data structure may require
>> different amounts of memory on different hosts. So it wouldn't
>> necessarily help understanding a dump that the user sent you. But it
>> still seems to be better than pointers.
>>
>> Should we go ahead and try to implement it?
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