Regression in object archive writer

[Dylan McKay via llvm-commits]

Here’s one of the places we create a Archive::Child with all nulls

Archive.cpp:771
<https://github.com/llvm-mirror/llvm/blob/master/lib/Object/Archive.cpp#L771>

Archive::child_iterator Archive::child_end() const {
return child_iterator(Child(this, nullptr, nullptr), nullptr);
}

If you look at the constructor in question

Archive.cpp:308
<https://github.com/llvm-mirror/llvm/blob/master/lib/Object/Archive.cpp#L308>
.

Archive::Child::Child(const Archive *Parent, const char *Start, Error *Err)
: Parent(Parent), Header(Parent, Start, Parent->getData().size() -
(Start - Parent->getData().data()), Err) {

/// …
}

We are always dereferencing the given parent, even when it is null.
​

On Wed, Oct 5, 2016 at 8:36 AM, Lang Hames <lhames at gmail.com> wrote:

> Hi Dylan,
>
> Where exactly are you seeing the segfault? Are you passing a non-null Data
> field? Archive::Child values can be sentinels (all null fields) or concrete
> children (both data and a parent), but you can't create an Archive::Child
> with a data pointer but no parent.
>
> - Lang.
>
>
> On Tue, Oct 4, 2016 at 11:01 AM, Kevin Enderby via llvm-commits <
> llvm-commits at lists.llvm.org> wrote:
>
>> Hi Dylan,
>>
>> I worked with Lang Hames on this back in July when this change was made.
>> I don’t recall the exact details off the top of my head, but one key points
>> of the thinking was you could not use this without handling the Error.
>> I’ll seem to recall the only case one of the default constructors was
>> allowed was for a sentinel value.
>>
>> I’ll try to grab Lang today and try to look at this and get back to you.
>> Sorry the details are not fresh on my mind as we did make these changes
>> over 2 months ago.
>>
>> Kev
>>
>> On Oct 3, 2016, at 6:48 PM, Dylan McKay <dylanmckay34 at gmail.com> wrote:
>>
>> r276686 introduced a regression into the object archive writer.
>>
>> r276686 <http://276686/> on Phabricator
>>
>> It causes the constructor Archive::Child(Parent, Data, StartOfFile) to
>> segfault if it is given a parent equalling NULL.
>>
>> LLVM does this in a few places. The most prominent example is the
>> Archive::child_iterator default constructor. This will never execute
>> without causing a segfault.
>>
>> I’m in the process of updating Rust to LLVM 4.0. We are currently using
>> Archive::child_iterator, and it causes us to segfault when writing
>> object files.
>> ​
>>
>>
>>
>> _______________________________________________
>> llvm-commits mailing list
>> llvm-commits at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>>
>>
>
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