RE: [RFC PATCH 0/5] madvise MADV_DOEXEC

David Laight
2021-08-16T08:07:52Z

From: Longpeng
> Sent: 16 August 2021 01:26
> Hi David,
>
> 在 2021/8/15 4:07, David Laight 写道:
> > ...
> >>>>> Let me describe my use case more clearly (just ignore if you're not
> >>>>> interested in it):
> >>>>>
> >>>>> 1. Prog A mmap() 4GB memory (anon or file-mapping), suppose the
> >>>>> allocated VA range is [0x40000000,0x140000000)
> >>>>>
> >>>>> 2. Prog A specifies [0x48000000,0x50000000) and
> >>>>> [0x80000000,0x100000000) will be shared by its child.
> >>>>>
> >>>>> 3. Prog A fork() Prog B and then Prog B exec() a new ELF binary.
> >>>>>
> >>>>> 4. Prog B notice the shared ranges (e.g. by input parameters or
> >>>>> ...)
> >>>>> and remap them to a continuous VA range.
> >
> > Remapping to contiguous VA is going to be difficult in the
> > general case for (IIRC) VIVT caches.
> > The required cache coherence may only be attainable by
> > using uncached mappings.
> >
>
> The Prog B uses mremap() syscall to remapping the shared ranges to other places,
> this is a common case for mremap in userspace.
> The cache coherence should already be processed in mremap core logic, otherwise
> there's maybe something wrong in mremap().

Maybe it does, and probably mremap() makes it work.
But with VIVT caches if a pages gets mapped in two processes
at the same time at different offsets into the cache then
then both mappings end up being uncached.
This was always a problem with normal file mmap.
I don't know if Linux manages to pick the VA after finding the
page has another mapping - SVR4 didn't.

David

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