Re: [musl] mallocng switchover - opportunity to test

[Szabolcs Nagy <nsz@port70.net>]

* Rich Felker <dalias@libc.org> [2020-06-09 20:58:26 -0400]:

> On Tue, Jun 09, 2020 at 04:08:00PM -0400, Rich Felker wrote:
> > On Tue, Jun 09, 2020 at 01:09:14PM +0200, Szabolcs Nagy wrote:
> > > * Rich Felker <dalias@libc.org> [2020-06-08 23:50:10 -0400]:
> > > > This produces a near-fully-integrated malloc, including support for
> > > > reclaim_gaps donation from ldso. The only functionality missing, which
> > > > I expect to flesh out before actual import, is handling of the case of
> > > > incomplete malloc replacement by interposition (__malloc_replaced!=0).
> > > 
> > > i would actually prefer if we didn't check for __malloc_replaced
> > > in aligned alloc, because i think it does not provide significant
> > > safety, but it prevents the simple RTLD_NEXT wrappers which are
> > > commonly used for simple malloc debugging/tracing/etc (and while
> > > unsafe in general depending on what libc api calls they make,
> > > they likely work in practice).
> > > 
> > > (the check does not provide safety because existing interposers
> > > written for glibc likely work with musl too without issues:
> > > the only problem is if musl uses aligned alloc somewhere where
> > > glibc does not so an interposer may work on glibc without
> > > interposing aligned alloc but not on musl. for newly written
> > > interposers we just need to document the api contract.)
> > 
> > I'm not sure about this, and how it interacts with our definition of
> > posix_memalign and memalign in terms of aligned_alloc.
> 
> What do you think of this proposal:
> 
> Have ldso track both whether malloc was replaced and whether
> aligned_alloc was replaced. If malloc was replaced but aligned_alloc
> wasn't, aligned_alloc fails with ENOMEM. If both were replaced and our
> internal aligned_alloc still gets called, assume some sort of wrapping
> is going on and allow it to proceed.
> 
> With mallocng, this is "safe" against misuse in the sense that it will
> trap rather than corrupting memory if the contract is violated.

sounds good to me.