Re: How to use Data.Compact.Serialize

1) I’m not sure if anyone’s actually shared example programs where repeated executions of the same binary will fail to handle loading a compact heap in. If you can cook up a small example that runs reproducible in a way that layout is an issue due to asle, please share! 2) if aslr can actually trigger an issue, we should improve compact heaps so that this does happen! At least when the same binary is used for both reading and writing processes. On Wed, Dec 19, 2018 at 5:33 AM Christopher Done wrote:

I forgot to address the fact that libc never statically links things like name server libraries.

Therefore would GHC devs recommend using a musl-based build e.g. on Alpine Linux?

Cheers On Wed, 19 Dec 2018 at 10:31, Christopher Done wrote:

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Hi,

On the docs for Data.Compact.Serialize it says:

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Our binary representation contains direct pointers to the info tables of objects in the region. This means that the info tables of the receiving process must be laid out in exactly the same way as from the original process; in practice, this means using static linking, using the exact same binary and turning off ASLR.

It seems to me that in order to use this module in any practical way (i.e. write to file from one process, and then a later run of the process reads it), you need to know a special way to build your binary which isn't fully described here. What flags, for example, should be passed to GHC to make this viable?

* Turning off ASLR on Linux is done by writing 0 to /proc/sys/kernel/randomize_va_space, which applies to all programs. That's not the most isolated way to deploy an app, but I discovered that you can set this per process here: https://askubuntu.com/a/507954 * To compile GHC programs statically, use -optl-static -optl-pthread.

So in total the example would be:

$ stack build compact compact-0.1.0.1: download compact-0.1.0.1: configure compact-0.1.0.1: build compact-0.1.0.1: copy/register

Example file from docs:

$ cat main.hs {-# LANGUAGE TypeApplications #-} import System.Environment import Data.Compact import Data.Compact.Serialize main = do arg:_ <- getArgs case arg of "write" -> do orig_c <- compact ("I want to serialize this", True) writeCompact @(String, Bool) "somefile" orig_c "read" -> do res <- unsafeReadCompact @(String, Bool) "somefile" case res of Left err -> fail err Right c -> print (getCompact c)

Compiling statically:

$ stack ghc -- -optl-static -optl-pthread main.hs [1 of 1] Compiling Main ( main.hs, main.o ) Linking main ... [...hundred more warnings like this ...]

Check that it's static:

$ ldd main not a dynamic executable

Write the file:

$ setarch `uname -m` -R ./main write

Read the file:

$ setarch `uname -m` -R ./main read ("I want to serialize this",True)

Can a GHC dev confirm that this is the proper way to do this? If so, I'll contribute this little example as documentation to the compact

http://hackage.haskell.org/package/compact-0.1.0.1/docs/Data-Compact-Seriali... package.

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Cheers

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