main/Elf.hs

    {-
    -----------------------------------------------------------------------------
    --
    -- (c) The University of Glasgow 2015
    --
    -- ELF format tools
    --
    -----------------------------------------------------------------------------
    -}

    module Elf (
        readElfSectionByName,
        readElfNoteAsString,
        makeElfNote
      ) where

    import Exception
    import DynFlags
    import Platform
    import ErrUtils
    import Maybes     (MaybeT(..),runMaybeT)
    import Util       (charToC)
    import Outputable (text,hcat,SDoc)

    import Control.Monad (when)
    import Data.Binary.Get
    import Data.Word
    import Data.Char (ord)
    import Data.ByteString.Lazy (ByteString)
    import qualified Data.ByteString.Lazy as LBS
    import qualified Data.ByteString.Lazy.Char8 as B8

    {- Note [ELF specification]
       ~~~~~~~~~~~~~~~~~~~~~~~~

       ELF (Executable and Linking Format) is described in the System V Application
       Binary Interface (or ABI). The latter is composed of two parts: a generic
       part and a processor specific part. The generic ABI describes the parts of
       the interface that remain constant across all hardware implementations of
       System V.

       The latest release of the specification of the generic ABI is the version
       4.1 from March 18, 1997:

         - http://www.sco.com/developers/devspecs/gabi41.pdf

       Since 1997, snapshots of the draft for the "next" version are published:

         - http://www.sco.com/developers/gabi/

       Quoting the notice on the website: "There is more than one instance of these
       chapters to permit references to older instances to remain valid. All
       modifications to these chapters are forward-compatible, so that correct use
       of an older specification will not be invalidated by a newer instance.
       Approximately on a yearly basis, a new instance will be saved, as it reaches
       what appears to be a stable state."

       Nevertheless we will see that since 1998 it is not true for Note sections.

       Many ELF sections
       -----------------

       ELF-4.1: the normal section number fields in ELF are limited to 16 bits,
       which runs out of bits when you try to cram in more sections than that. Two
       fields are concerned: the one containing the number of the sections and the
       one containing the index of the section that contains section's names. (The
       same thing applies to the field containing the number of segments, but we
       don't care about it here).

       ELF-next: to solve this, theses fields in the ELF header have an escape
       value (different for each case), and the actual section number is stashed
       into unused fields in the first section header.

       We support this extension as it is forward-compatible with ELF-4.1.
       Moreover, GHC may generate objects with a lot of sections with the
       "function-sections" feature (one section per function).

       Note sections
       -------------

       Sections with type "note" (SHT_NOTE in the specification) are used to add
       arbitrary data into an ELF file. An entry in a note section is composed of a
       name, a type and a value.

       ELF-4.1: "The note information in sections and program header elements holds
       any number of entries, each of which is an array of 4-byte words in the
       format of the target processor." Each entry has the following format:
             | namesz |   Word32: size of the name string (including the ending \0)
             | descsz |   Word32: size of the value
             |  type  |   Word32: type of the note
             |  name  |   Name string (with \0 padding to ensure 4-byte alignment)
             |  ...   |
             |  desc  |   Value (with \0 padding to ensure 4-byte alignment)
             |  ...   |

       ELF-next: "The note information in sections and program header elements
       holds a variable amount of entries. In 64-bit objects (files with
       e_ident[EI_CLASS] equal to ELFCLASS64), each entry is an array of 8-byte
       words in the format of the target processor. In 32-bit objects (files with
       e_ident[EI_CLASS] equal to ELFCLASS32), each entry is an array of 4-byte
       words in the format of the target processor." (from 1998-2015 snapshots)

       This is not forward-compatible with ELF-4.1. In practice, for almost all
       platforms namesz, descz and type fields are 4-byte words for both 32-bit and
       64-bit objects (see elf.h and readelf source code).

       The only exception in readelf source code is for IA_64 machines with OpenVMS
       OS: "This OS has so many departures from the ELF standard that we test it at
       many places" (comment for is_ia64_vms() in readelf.c). In this case, namesz,
       descsz and type fields are 8-byte words and name and value fields are padded
       to ensure 8-byte alignment.

       We don't support this platform in the following code. Reading a note section
       could be done easily (by testing Machine and OS fields in the ELF header).
       Writing a note section, however, requires that we generate a different
       assembly code for GAS depending on the target platform and this is a little
       bit more involved.

    -}

    -- | ELF header
    --
    -- The ELF header indicates the native word size (32-bit or 64-bit) and the
    -- endianness of the target machine. We directly store getters for words of
    -- different sizes as it is more convenient to use. We also store the word size
    -- as it is useful to skip some uninteresting fields.
    --
    -- Other information such as the target machine and OS are left out as we don't
    -- use them yet. We could add them in the future if we ever need them.
    data ElfHeader = ElfHeader
       { gw16     :: Get Word16   -- ^ Get a Word16 with the correct endianness
       , gw32     :: Get Word32   -- ^ Get a Word32 with the correct endianness
       , gwN      :: Get Word64   -- ^ Get a Word with the correct word size
                                  --   and endianness
       , wordSize :: Int          -- ^ Word size in bytes
       }

    -- | Read the ELF header
    readElfHeader :: DynFlags -> ByteString -> IO (Maybe ElfHeader)
    readElfHeader dflags bs = runGetOrThrow getHeader bs `catchIO` \_ -> do
        debugTraceMsg dflags 3 $
          text ("Unable to read ELF header")
        return Nothing
      where
        getHeader = do
          magic    <- getWord32be
          ws       <- getWord8
          endian   <- getWord8
          version  <- getWord8
          skip 9  -- skip OSABI, ABI version and padding
          when (magic /= 0x7F454C46 || version /= 1) $ fail "Invalid ELF header"

          case (ws, endian) of
              -- ELF 32, little endian
              (1,1) -> return . Just $ ElfHeader
                               getWord16le
                               getWord32le
                               (fmap fromIntegral getWord32le) 4
              -- ELF 32, big endian
              (1,2) -> return . Just $ ElfHeader
                               getWord16be
                               getWord32be
                               (fmap fromIntegral getWord32be) 4
              -- ELF 64, little endian
              (2,1) -> return . Just $ ElfHeader
                               getWord16le
                               getWord32le
                               (fmap fromIntegral getWord64le) 8
              -- ELF 64, big endian
              (2,2) -> return . Just $ ElfHeader
                               getWord16be
                               getWord32be
                               (fmap fromIntegral getWord64be) 8
              _     -> fail "Invalid ELF header"

    ------------------
    -- SECTIONS
    ------------------

    -- | Description of the section table
    data SectionTable = SectionTable
      { sectionTableOffset :: Word64  -- ^ offset of the table describing sections
      , sectionEntrySize   :: Word16  -- ^ size of an entry in the section table
      , sectionEntryCount  :: Word64  -- ^ number of sections
      , sectionNameIndex   :: Word32  -- ^ index of a special section which
                                      --   contains section's names
      }

    -- | Read the ELF section table
    readElfSectionTable :: DynFlags
                        -> ElfHeader
                        -> ByteString
                        -> IO (Maybe SectionTable)

    readElfSectionTable dflags hdr bs = action `catchIO` \_ -> do
        debugTraceMsg dflags 3 $
          text ("Unable to read ELF section table")
        return Nothing
      where
        getSectionTable :: Get SectionTable
        getSectionTable = do
          skip (24 + 2*wordSize hdr) -- skip header and some other fields
          secTableOffset <- gwN hdr
          skip 10
          entrySize      <- gw16 hdr
          entryCount     <- gw16 hdr
          secNameIndex   <- gw16 hdr
          return (SectionTable secTableOffset entrySize
                               (fromIntegral entryCount)
                               (fromIntegral secNameIndex))

        action = do
          secTable <- runGetOrThrow getSectionTable bs
          -- In some cases, the number of entries and the index of the section
          -- containing section's names must be found in unused fields of the first
          -- section entry (see Note [ELF specification])
          let
            offSize0 = fromIntegral $ sectionTableOffset secTable + 8
                                      + 3 * fromIntegral (wordSize hdr)
            offLink0 = fromIntegral $ offSize0 + fromIntegral (wordSize hdr)

          entryCount'     <- if sectionEntryCount secTable /= 0
                              then return (sectionEntryCount secTable)
                              else runGetOrThrow (gwN hdr) (LBS.drop offSize0 bs)
          entryNameIndex' <- if sectionNameIndex secTable /= 0xffff
                              then return (sectionNameIndex secTable)
                              else runGetOrThrow (gw32 hdr) (LBS.drop offLink0 bs)
          return (Just $ secTable
            { sectionEntryCount = entryCount'
            , sectionNameIndex  = entryNameIndex'
            })

    -- | A section
    data Section = Section
      { entryName :: ByteString   -- ^ Name of the section
      , entryBS   :: ByteString   -- ^ Content of the section
      }

    -- | Read a ELF section
    readElfSectionByIndex :: DynFlags
                          -> ElfHeader
                          -> SectionTable
                          -> Word64
                          -> ByteString
                          -> IO (Maybe Section)

    readElfSectionByIndex dflags hdr secTable i bs = action `catchIO` \_ -> do
        debugTraceMsg dflags 3 $
          text ("Unable to read ELF section")
        return Nothing
      where
        -- read an entry from the section table
        getEntry = do
          nameIndex <- gw32 hdr
          skip (4+2*wordSize hdr)
          offset    <- fmap fromIntegral $ gwN hdr
          size      <- fmap fromIntegral $ gwN hdr
          let bs' = LBS.take size (LBS.drop offset bs)
          return (nameIndex,bs')

        -- read the entry with the given index in the section table
        getEntryByIndex x = runGetOrThrow getEntry bs'
          where
            bs' = LBS.drop off bs
            off = fromIntegral $ sectionTableOffset secTable +
                                 x * fromIntegral (sectionEntrySize secTable)

        -- Get the name of a section
        getEntryName nameIndex = do
          let idx = fromIntegral (sectionNameIndex secTable)
          (_,nameTable) <- getEntryByIndex idx
          let bs' = LBS.drop nameIndex nameTable
          runGetOrThrow getLazyByteStringNul bs'

        action = do
          (nameIndex,bs') <- getEntryByIndex (fromIntegral i)
          name            <- getEntryName (fromIntegral nameIndex)
          return (Just $ Section name bs')

    -- | Find a section from its name. Return the section contents.
    --
    -- We do not perform any check on the section type.
    findSectionFromName :: DynFlags
                        -> ElfHeader
                        -> SectionTable
                        -> String
                        -> ByteString
                        -> IO (Maybe ByteString)
    findSectionFromName dflags hdr secTable name bs =
        rec [0..sectionEntryCount secTable - 1]
      where
        -- convert the required section name into a ByteString to perform
        -- ByteString comparison instead of String comparison
        name' = B8.pack name

        -- compare recursively each section name and return the contents of
        -- the matching one, if any
        rec []     = return Nothing
        rec (x:xs) = do
          me <- readElfSectionByIndex dflags hdr secTable x bs
          case me of
            Just e | entryName e == name' -> return (Just (entryBS e))
            _                             -> rec xs

    -- | Given a section name, read its contents as a ByteString.
    --
    -- If the section isn't found or if there is any parsing error, we return
    -- Nothing
    readElfSectionByName :: DynFlags
                         -> ByteString
                         -> String
                         -> IO (Maybe LBS.ByteString)

    readElfSectionByName dflags bs name = action `catchIO` \_ -> do
        debugTraceMsg dflags 3 $
          text ("Unable to read ELF section \"" ++ name ++ "\"")
        return Nothing
      where
        action = runMaybeT $ do
          hdr      <- MaybeT $ readElfHeader dflags bs
          secTable <- MaybeT $ readElfSectionTable dflags hdr bs
          MaybeT $ findSectionFromName dflags hdr secTable name bs

    ------------------
    -- NOTE SECTIONS
    ------------------

    -- | read a Note as a ByteString
    --
    -- If you try to read a note from a section which does not support the Note
    -- format, the parsing is likely to fail and Nothing will be returned
    readElfNoteBS :: DynFlags
                  -> ByteString
                  -> String
                  -> String
                  -> IO (Maybe LBS.ByteString)

    readElfNoteBS dflags bs sectionName noteId = action `catchIO`  \_ -> do
        debugTraceMsg dflags 3 $
             text ("Unable to read ELF note \"" ++ noteId ++
                   "\" in section \"" ++ sectionName ++ "\"")
        return Nothing
      where
        -- align the getter on n bytes
        align n = do
          m <- bytesRead
          if m `mod` n == 0
            then return ()
            else skip 1 >> align n

        -- noteId as a bytestring
        noteId' = B8.pack noteId

        -- read notes recursively until the one with a valid identifier is found
        findNote hdr = do
          align 4
          namesz <- gw32 hdr
          descsz <- gw32 hdr
          _      <- gw32 hdr -- we don't use the note type
          name   <- if namesz == 0
                      then return LBS.empty
                      else getLazyByteStringNul
          align 4
          desc  <- if descsz == 0
                      then return LBS.empty
                      else getLazyByteString (fromIntegral descsz)
          if name == noteId'
            then return $ Just desc
            else findNote hdr

        action = runMaybeT $ do
          hdr  <- MaybeT $ readElfHeader dflags bs
          sec  <- MaybeT $ readElfSectionByName dflags bs sectionName
          MaybeT $ runGetOrThrow (findNote hdr) sec

    -- | read a Note as a String
    --
    -- If you try to read a note from a section which does not support the Note
    -- format, the parsing is likely to fail and Nothing will be returned
    readElfNoteAsString :: DynFlags
                        -> FilePath
                        -> String
                        -> String
                        -> IO (Maybe String)

    readElfNoteAsString dflags path sectionName noteId = action `catchIO`  \_ -> do
        debugTraceMsg dflags 3 $
             text ("Unable to read ELF note \"" ++ noteId ++
                   "\" in section \"" ++ sectionName ++ "\"")
        return Nothing
      where
        action = do
          bs   <- LBS.readFile path
          note <- readElfNoteBS dflags bs sectionName noteId
          return (fmap B8.unpack note)

    -- | Generate the GAS code to create a Note section
    --
    -- Header fields for notes are 32-bit long (see Note [ELF specification]).
    --
    -- It seems there is no easy way to force GNU AS to generate a 32-bit word in
    -- every case. Hence we use .int directive to create them: however "The byte
    -- order and bit size of the number depends on what kind of target the assembly
    -- is for." (https://sourceware.org/binutils/docs/as/Int.html#Int)
    --
    -- If we add new target platforms, we need to check that the generated words
    -- are 32-bit long, otherwise we need to use platform specific directives to
    -- force 32-bit .int in asWord32.
    makeElfNote :: DynFlags -> String -> String -> Word32 -> String -> SDoc
    makeElfNote dflags sectionName noteName typ contents = hcat [
        text "\t.section ",
        text sectionName,
        text ",\"\",",
        text elfSectionNote,
        text "\n",

        -- note name length (+ 1 for ending \0)
        asWord32 (length noteName + 1),

        -- note contents size
        asWord32 (length contents),

        -- note type
        asWord32 typ,

        -- note name (.asciz for \0 ending string) + padding
        text "\t.asciz \"",
        text noteName,
        text "\"\n",
        text "\t.align 4\n",

        -- note contents (.ascii to avoid ending \0) + padding
        text "\t.ascii \"",
        text (escape contents),
        text "\"\n",
        text "\t.align 4\n"]
      where
        escape :: String -> String
        escape = concatMap (charToC.fromIntegral.ord)

        asWord32 :: Show a => a -> SDoc
        asWord32 x = hcat [
          text "\t.int ",
          text (show x),
          text "\n"]

        elfSectionNote :: String
        elfSectionNote = case platformArch (targetPlatform dflags) of
                                 ArchARM _ _ _ -> "%note"
                                 _             -> "@note"

    ------------------
    -- Helpers
    ------------------

    -- | runGet in IO monad that throws an IOException on failure
    runGetOrThrow :: Get a -> LBS.ByteString -> IO a
    runGetOrThrow g bs = case runGetOrFail g bs of
      Left _        -> fail "Error while reading file"
      Right (_,_,a) -> return a