Re: Address the -Wuse-after-free warning in ATExecAttachPartition()

On Tue, Jul 9, 2024 at 6:18 PM Amit Langote <amitlangote09(at)gmail(dot)com> wrote:
>
> Hi Junwang,
>
> On Mon, Jul 8, 2024 at 7:08 PM Junwang Zhao <zhjwpku(at)gmail(dot)com> wrote:
> > On Mon, Jul 8, 2024 at 3:22 PM Nitin Jadhav
> > <nitinjadhavpostgres(at)gmail(dot)com> wrote:
> > >
> > > In [1], Andres reported a -Wuse-after-free bug in the
> > > ATExecAttachPartition() function. I've created a patch to address it
> > > with pointers from Amit offlist.
> > >
> > > The issue was that the partBoundConstraint variable was utilized after
> > > the list_concat() function. This could potentially lead to accessing
> > > the partBoundConstraint variable after its memory has been freed.
> > >
> > > The issue was resolved by using the return value of the list_concat()
> > > function, instead of using the list1 argument of list_concat(). I
> > > copied the partBoundConstraint variable to a new variable named
> > > partConstraint and used it for the previous references before invoking
> > > get_proposed_default_constraint(). I confirmed that the
> > > eval_const_expressions(), make_ands_explicit(),
> > > map_partition_varattnos(), QueuePartitionConstraintValidation()
> > > functions do not modify the memory location pointed to by the
> > > partBoundConstraint variable. Therefore, it is safe to use it for the
> > > next reference in get_proposed_default_constraint()
> > >
> > > Attaching the patch. Please review and share the comments if any.
> > > Thanks to Andres for spotting the bug and some off-list advice on how
> > > to reproduce it.
> >
> > The patch LGTM.
> >
> > Curious how to reproduce the bug ;)
>
> Download and apply (`git am`) Andres' patch to "add allocator
> attributes" here (note it's not merged into the tree yet!):
> https://github.com/anarazel/postgres/commit/99067d5c944e7bd29a8702689f470f623723f4e7.patch
>
> Then configure using meson with -Dc_args="-Wuse-after-free=3"
> --buildtype=release
>
> Compiling with gcc-12 or gcc-13 should give you the warning that looks
> as follows:
>
> [713/2349] Compiling C object
> src/backend/postgres_lib.a.p/commands_tablecmds.c.o
> ../src/backend/commands/tablecmds.c: In function ‘ATExecAttachPartition’:
> ../src/backend/commands/tablecmds.c:18593:25: warning: pointer
> ‘partBoundConstraint’ may be used after ‘list_concat’
> [-Wuse-after-free]
> 18593 |
> get_proposed_default_constraint(partBoundConstraint);
> |
> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ../src/backend/commands/tablecmds.c:18546:26: note: call to ‘list_concat’ here
> 18546 | partConstraint = list_concat(partBoundConstraint,
> | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 18547 |
> RelationGetPartitionQual(rel));
> |
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> --
> Thanks, Amit Langote

Thanks Amit,

Good to know.

When Nitin says:

```Thanks to Andres for spotting the bug and some off-list advice on how
to reproduce it.```

I thought maybe there is some test case that can really trigger the
use-after-free bug, I might get it wrong though ;)

--
Regards
Junwang Zhao