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1,921 bytes added ,  14:07, 19 February 2013
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Pavel pointed
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== Summary ==
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One thing to do:
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There are two moments of time when crtools runs in somewhat strange environment
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* Parasite code execution
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* Restore of page dumped contents and yield rt-sigreturn to continue execution of the original program
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== Building PIE code blobs for crtools ==
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Parasite code executes in dumpee process context thus it needs to be [http://en.wikipedia.org/wiki/Position-independent_code PIE]
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compiled and to have own stack. The same applies to restorer code, which takes place at the very end of restore procedure.
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Thus we need to reserve stack place statically somewhere in crtools and use it at dump/checkpoint stages. To achieve
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this (and still have some human way to edit source code) we do the following tricks
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* Parasite code has own bootstrap code laid in pure assembler file (parasite_head.S)
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* Restorer bootstrap code is done a bit simplier in restorer.c file.
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For both cases we generate headers files which consist of
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* Functions offsets for export
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* C array of binary data, for example
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#define parasite_blob_offset____export_parasite_args 0x000000000000002c
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#define parasite_blob_offset____export_parasite_cmd 0x0000000000000028
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#define parasite_blob_offset____export_parasite_head_start 0x0000000000000000
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#define parasite_blob_offset____export_parasite_stack 0x0000000000006034
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static char parasite_blob[] = {
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0x48, 0x8d, 0x25, 0x2d, 0x60, 0x00, 0x00, 0x48,
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0x83, 0xec, 0x10, 0x48, 0x83, 0xe4, 0xf0, 0x6a,
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0x00, 0x48, 0x89, 0xe5, 0x8b, 0x3d, 0x0e, 0x00,
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0x00, 0x00, 0x48, 0x8d, 0x35, 0x0b, 0x00, 0x00,
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...
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};
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These headers we include in crtools compiled file and then use for checkpoint/restore.
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Generation of this files is done in several steps
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* All object files needed by are linked into 'built-in.o'
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* With help of ld script we move code and data to special layout, i.e. to sections with predefined names and addresses
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* With help of objcopy we move the section(s) we need to one binary file
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* With help of hexdump we generate C-styled array of data and put it into -blob.h header
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== Example of building procedure ==
    
   LINK    pie/parasite.built-in.o
 
   LINK    pie/parasite.built-in.o
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   GEN      pie/parasite.built-in.bin
 
   GEN      pie/parasite.built-in.bin
 
   GEN      pie/parasite-blob.h
 
   GEN      pie/parasite-blob.h
 
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   LINK    pie/restorer.built-in.o
 
   LINK    pie/restorer.built-in.o
 
   GEN      pie/restorer.built-in.bin.o
 
   GEN      pie/restorer.built-in.bin.o
 
   GEN      pie/restorer.built-in.bin
 
   GEN      pie/restorer.built-in.bin
 
   GEN      pie/restorer-blob.h
 
   GEN      pie/restorer-blob.h
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need a doc (on wiki I suppose) explaining why we need 4 files
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for each -- parasite and restorer blobs.