Commit f03a41d4 authored by Ben Gamari's avatar Ben Gamari 🐢 Committed by Marge Bot

Elf: Fix link info note generation

Previously we would use the `.int` assembler directive to generate
32-bit words in the note section. However, `.int` is note guaranteed to
produce 4-bytes; in fact, on some platforms (e.g. AArch64) it produces
8-bytes. Use the `.4bytes` directive to avoid this.

Moreover, we used the `.align` directive, which is quite platform
dependent. On AArch64 it appears to not even be idempotent (despite what
the documentation claims). `.balign` is consequentially preferred as it
offers consistent behavior across platforms.
parent 78b67ad0
Pipeline #13518 failed with stages
in 570 minutes and 13 seconds
......@@ -408,15 +408,6 @@ readElfNoteAsString dflags path sectionName noteId = action `catchIO` \_ -> do
-- | 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 :: String -> String -> Word32 -> String -> SDoc
makeElfNote sectionName noteName typ contents = hcat [
text "\t.section ",
......@@ -424,6 +415,7 @@ makeElfNote sectionName noteName typ contents = hcat [
text ",\"\",",
sectionType "note",
text "\n",
text "\t.balign 4\n",
-- note name length (+ 1 for ending \0)
asWord32 (length noteName + 1),
......@@ -438,20 +430,20 @@ makeElfNote sectionName noteName typ contents = hcat [
text "\t.asciz \"",
text noteName,
text "\"\n",
text "\t.align 4\n",
text "\t.balign 4\n",
-- note contents (.ascii to avoid ending \0) + padding
text "\t.ascii \"",
text (escape contents),
text "\"\n",
text "\t.align 4\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.int ",
text "\t.4byte ",
text (show x),
text "\n"]
......
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