Re: [PATCH 0/2] mremap: Fix newaddr hint with MREMAP_DONTUNMAP

[Brian Geffon <bgeffon@google.com>]

On Fri, Dec 6, 2024 at 10:52 AM Lorenzo Stoakes
<lorenzo.stoakes@oracle.com> wrote:
>
> On Fri, Dec 06, 2024 at 07:42:51PM +0100, Jann Horn wrote:
> > +mmap maintainers (maybe mm/mremap.c should be added to the file
> > pattern for "MEMORY MAPPING" in "MAINTAINERS"? I'm not sure)
>
> Yeah I think it's actually right to group together _all_ VMA-related operations
> under the VMA entry, because we have interaction between them all mprotect,
> mlock, etc. etc. etc.
>
> I will send a patch in a second for this, because we do keep getting bitten by
> this.
>
> >
> > On Fri, Dec 6, 2024 at 4:20 PM Brian Geffon <bgeffon@google.com> wrote:
> > > mmap(2) allows for a destination address to be specified without
> > > MAP_FIXED and in this situation it's a hint to get_unmapped_area().
> > > This address need not be page aligned because get_unmapped_area() will
> > > align the hint.
> > >
> > > In the case of mremap(2) with MREMAP_DONTUNMAP it shares a code path
> > > with MREMAP_FIXED in mremap_to(), which means this function can be
> > > called in 3 different scenarios: MREMAP_FIXED only, MREMAP_DONTUNMAP
> > > only, or MREMAP_FIXED | MREMAP_DONTUNMAP. In the second case when only
> > > MREMAP_DONTUNMAP is specified we don't need to do alignment or size
> > > checks on newaddr because they will be passed to get_unmapped_area() and
> > > dealt with appropriately.
> > >
> > > This patch corrects that behavior to match what non-MREMAP_DONTUNMAP
> > > mremap(2) and mmap(2) do. This odd behavioral difference was reported by
> > > Marco Vanotti. Additionally, I've included a self test to validate this
> > > behavior.
>
> Yeah if this is user-facing - I don't think we can change this. Can we do any v2
> as an RFC for now until we can get a chance to look at this? And please cc- the
> VMA/mmap maintainers on future revisions (sorry this wasn't at all clear, we
> need to update MAINTAINERS here).

Sure, I'll mail the next series as an RFC in the next few days. This
behavior was not introduced intentionally.

>
> Thanks!
>
> >
> > Marco pointed me to this; I had no idea mremap() had this undocumented
> > behavior where it takes a hint address. The mremap() manpage is
> > currently wrong about this, it sort of implies that the new_address
> > argument is only used if MREMAP_FIXED is set.
> >
> > Marco also noticed that upstream glibc now assumes this behavior:
> > https://sourceware.org/git/?p=glibc.git;a=commit;h=6c40cb0e9f893d49dc7caee580a055de53562206
> >
> > Debian also has a test that explicitly checks for this behavior:
> > https://sources.debian.org/src/glibc/2.40-4/debian/patches/git-updates.diff/?hl=22820#L22818
> >
> > I guess it's too late to remove that behavior at this point, and the
> > right thing to do is to update the manpage?
>
> Yeah, if user-facing we can't fundamentally change behaviour even if it's
> strange I'd say.

Definitely, no matter what happens we'll need a man page update. I
think to make things consistent we'll probably want to consider
allowing all variants of mremap(2) (without MREMAP_FIXED) to use
newaddr as a hint, like mmap(2). But I'll mail the RFC with much more
detail in the cover letter about the history and impact.

Brian