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

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

[sourceware.org at plagman dot net]

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

[schwab@linux-m68k.org]

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

--- Comment #1 from Andreas Schwab <schwab@linux-m68k.org> 2013-02-05 11:16:31 UTC ---
Perhaps it is sufficient to check for l_addr == 0.

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

[fweimer at redhat dot com]

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

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |fweimer at redhat dot com
              Flags|                            |security-

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

[fweimer at redhat dot com]

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

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |WONTFIX
             Status|NEW                         |RESOLVED

--- Comment #2 from Florian Weimer <fweimer at redhat dot com> ---
We have removed prelink support, so I do not think this is an issue anymore.

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