{- ----------------------------------------------------------------------------- -- -- (c) The University of Glasgow 2015 -- -- ELF format tools -- ----------------------------------------------------------------------------- -} module GHC.SysTools.Elf ( readElfSectionByName, readElfNoteAsString, makeElfNote ) where import GHC.Prelude import GHC.Utils.Asm import GHC.Utils.Exception import GHC.Driver.Session import GHC.Platform import GHC.Utils.Error import GHC.Data.Maybe (MaybeT(..),runMaybeT) import GHC.Utils.Misc (charToC) import GHC.Utils.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]). makeElfNote :: Platform -> String -> String -> Word32 -> String -> SDoc makeElfNote platform sectionName noteName typ contents = hcat [ text "\t.section ", text sectionName, text ",\"\",", sectionType platform "note", text "\n", text "\t.balign 4\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.balign 4\n", -- note contents (.ascii to avoid ending \0) + padding text "\t.ascii \"", text (escape contents), text "\"\n", text "\t.balign 4\n"] where escape :: String -> String escape = concatMap (charToC.fromIntegral.ord) asWord32 :: Show a => a -> SDoc asWord32 x = hcat [ text "\t.4byte ", text (show x), text "\n"] ------------------ -- 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