222c4ee3d2
R=nodir BUG=524256 Review URL: https://codereview.chromium.org/1318503005 Cr-Commit-Position: refs/heads/master@{#345353}
59 lines
2.8 KiB
Markdown
59 lines
2.8 KiB
Markdown
# Handling repeated failures of rebaseall to allow cygwin remaps
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Sometimes DLLs over which cygwin has no control get mapped into cygwin
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processes at locations that cygwin has chosen for its libraries.
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This has been seen primarily with anti-virus DLLs. When this occurs,
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cygwin must be instructed during the rebase to avoid the area of
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memory where that DLL is mapped.
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## Background
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Some background for this is available on
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http://www.dont-panic.cc/capi/2007/10/29/git-svn-fails-with-fatal-error-unable-to-remap/
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Because of unix fork semantics (presumably), cygwin libraries must be
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mapped in the same location in both parent and child of a fork. All
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cygwin libraries have hints in them as to where they should be mapped
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in a processes address space; if those hints are followed, each
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library will be mapped in the same location in both address spaces.
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However, Windows is perfectly happy mapping a DLL anywhere in the
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address space; the hint is not considered controlling. The remapping
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error occurs when a cygwin process starts and one of its libraries
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cannot be mapped to the location specified by its hint.
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/usr/bin/rebaseall changes the DLL hints for all of the cygwin
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libraries so that there are no inter-library conflicts; it does this
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by choosing a contiguous but not overlapping library layout starting
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at a base address and working down. This process makes sure there are
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no intra-cygwin conflicts, but cannot deal with conflicts with
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external DLLs that are in cygwin process address spaces
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(e.g. anti-virus DLLs).
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To handle this case, you need to figure out what the problematic
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non-cygwin library is, where it is in the address space, and do the
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rebase all so that no cygwin hints map libraries to that location.
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## Details
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* Download the ListDLLs executable from
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[sysinternals](http://technet.microsoft.com/en-us/sysinternals/bb896656.aspx)
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* Run it as administrator while some cygwin commands are running.
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* Scan the output for the cygwin process (identifiable by the command) and for
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DLLs in that process that do not look like cygwin DLLs (like an AV). Note
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the location of those libraries (there will usually only be the one).
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* Pick an address space location lower than its starting address.
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* Quit all cygwin processes.
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* Run a windows command shell as administrator
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* cd in \cygwin\bin
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* Run `ash /usr/bin/rebaseall -b <base address>` (This command can also take a
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`-v` flag if you want to see the DLL layout.)
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That should fix the problem.
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## Failed rebaseall
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If you pick a base address that is too low, you may end up with a broken cygwin
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install. You can reinstall it by running cygwin's setup.exe again, and on the
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package selection page, clicking the "All" entry to Reinstall. You may have to
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do this twice, as you may get errors on the first reinstall pass.
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