226f69422a
This patch alters the behaviour of dll_list::topsort to preserve the order of dlopen'd units. The load order of unrelated DLLs is reversed every time fork is called, since dll_list::topsort finds the tail of the list and then unwinds to reinsert items. My change takes advantage of what should be undefined behaviour in dll_list::populate_deps (ndeps non-zero and ndeps and deps not initialised) to allow the deps field to be initialised prior to the call and appended to, rather than overwritten. All DLLs which have been dlopen'd have their deps list initialised with the list of all previously dlopen'd units. These extra dependencies mean that the unwind preserves the order of dlopen'd units. The motivation for this is the FlexDLL linker used in OCaml. The FlexDLL linker allows a dlopen'd unit to refer to symbols in previously dlopen'd units and it resolves these symbols in DllMain before anything else has initialised (including the Cygwin DLL). This means that dependencies may exist between dlopen'd units (which the OCaml runtime system understands) but which Windows is unaware of. During fork, the process-level table which FlexDLL uses to get the symbol table of each DLL is copied over but because the load order of dlopen'd DLLs is reversed, it is possible for FlexDLL to attempt to access memory in the DLL before it has been loaded and hence it fails with an access violation. Because the list is reversed on each call to fork, it means that a subsequent call to fork puts the DLLs back into the correct order, hence "even" invocations of fork work! An interesting side-effect is that this only occurs if the DLLs load at their preferred base address - if they have to be rebased, then FlexDLL works because at the time that the dependent unit is loaded out of order, there is still in memory the "dummy" DONT_RESOLVE_DLL_REFERENCES version of the dependency which, as it happens, will contain the correct symbol table in the data section. For my tests, this initially appeared to be an x86-only problem, but that was only because the two DLLs on x64 should have been rebased. Signed-off-by: David Allsopp <david.allsopp@metastack.com>
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