[Bug dynamic-link/31076] Extra struct vm_area_struct with ---p created when PAGE_SIZE < max-page-size

["fweimer at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=31076

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |fweimer at redhat dot com

--- Comment #5 from Florian Weimer <fweimer at redhat dot com> ---
I don't see how a 152 byte struct results in 30 MB of unreclaimable kernel
memory.  Wouldn't that need ~200,000 instances? That seems really large. I've
only got ~70,000 lines in /proc/*/maps on this desktop system, and not all
these mappings will exhibit this issue.

Would it help if we use MAP_FIXED with PROT_NONE to map over these unused
parts? But as far as I understand it, these tails have not been written to, so
it shouldn't matter if the underlying memory needs to be preserved by the
kernel or not.

Regarding not doing the mprotect altogether, I believe this would result in a
loss of functionality. Today, you can use the current behavior to get as few
gadgets as possible in the process image on systems with smaller page sizes,
while still maintaining run-time compatibility with larger page sizes and
avoiding on-disk padding (which would increase file size). If we stop doing the
mprotect, then the gadgets would become visible even with smaller page sizes.
At least in principle, it should be possible for a link editor to produce
objects that do not require tail adjustment because the load segments are
usable as-is (but I understand that there are link editor limitations in this
area today).

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