[Bug dynamic-link/15097] New: The loader should avoid writing over already-valid relocations

["sourceware.org at plagman dot net" <sourceware-bugzilla@sourceware.org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=15097

             Bug #: 15097
           Summary: The loader should avoid writing over already-valid
                    relocations
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: dynamic-link
        AssignedTo: unassigned@sourceware.org
        ReportedBy: sourceware.org@plagman.net
    Classification: Unclassified


When pre-relocating a shared object to a certain base address using `prelink
-r` (and successfully loading it there), the loader will still compute and
overwrite all relocations. This marks the pages of the mapping dirty, triggers
copy-on-write and ends up committing private memory to all processes mapping
that shared object that could otherwise have been cleanly shared.

If the loader did a comparison between the relocation it computed and the
existing contents of the destination before performing the write, this would be
avoided.

I suspect only having such a check for RELATIVE relocations would be sufficient
in practice, but in theory applying it to all types of relocations could
exhibit memory benefits with a small runtime cost.

See this email exchange for more details about the usecase, and the attached
example patch:

http://lists.freedesktop.org/archives/wayland-devel/2012-March/002798.html

http://lists.freedesktop.org/archives/wayland-devel/attachments/20120327/a8bedd3f/attachment.patch

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