Time to time internetz stumble upon a ghci bug which is seen as inability to load base haskell package:

Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... ghc: /usr/lib64/ghc-7.4.2/base-4.5.1.0/HSbase-4.5.1.0.o: unknown symbol `stat'
ghc: unable to load package `base'

But the bug was most popular across rare gentoo users. An interesting correlation!

This post is about the root of this problem: the gory implementation details of ghci dynamic loader down to libC and even ELF symbols!

Not scared? Fasten your belts and Read On!

GHC (this one) is both:

  1. a compiler (ghc binary)
  2. and REPL (ghci binary)

To put simplistic ghc allows you to create final binaries out of haskell sources while ghci allows runtime loading of haskell sources.

Typical session starts like that:

$ ghci
GHCi, version 7.6.3: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Prelude> <some code to evaluate>

ghci‘s implementation allows loading arbitrary shared library:

$ ghci -lpcre
GHCi, version 7.6.3: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Loading object (dynamic) /usr/lib/gcc/x86_64-pc-linux-gnu/4.8.1/../../../../lib64/libpcre.so ... done
final link ... done
Prelude>

and even object file:

$ echo 'void foo(){}' > a.c &&
    gcc -c a.c -o a.o &&
    ghci a.o
GHCi, version 7.6.3: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Loading object (static) a.o ... done
final link ... done
Prelude>

ghci libraries are basically the same object files built of many source files.

It took me a while reproduce the bug mentioned in the very start of the post. First time I’ve heard of a bug was in December 2012 by nand but I had no idea where it comes from.

First I though it was a problem of missing headers somewhere in C code due to glibc upgrade, but no matter what combinations of binutils/gcc/glibc I tried bug did not want to show up.

6 months after after some bugs got collected I’ve noticed dreadful CFLAGS=-Os common amongst reporters which was a trigger.

Let’s explore exported symbol difference of 2 files:

  1. CFLAGS=-O2 /usr/lib64/ghc-7.6.3/base-4.6.0.1/HSbase-4.6.0.1.o
  2. CFLAGS=-Os /usr/lib64/ghc-7.6.3/base-4.6.0.1/HSbase-4.6.0.1.o
$ nm --undefined-only /usr/lib64/ghc-7.6.3/base-4.6.0.1/HSbase-4.6.0.1.o > base-O2
$ nm --undefined-only /gentoo/chroots/amd64-unstable//usr/lib64/ghc-7.6.3/base-4.6.0.1/HSbase-4.6.0.1.o > base-Os
$ diff -U0 base-O2 base-Os
-                 U __fxstat
+                 U fstat
-                 U __lxstat
+                 U lstat
-                 U memset
-                 U __xstat
+                 U stat

Do you see it?

-Os build has stat call while -O2 has __xstat. It was the right track.

Let’s try simpler example:

cat >stat-test.c <<-EOF
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
int f()
{
    struct stat s;
    return stat("/", &s);
}
EOF
$ gcc -c stat-test.c -Os -o stat-test-Os.o
$ gcc -c stat-test.c -O2 -o stat-test-O2.o
$ nm --undefined-only stat-test-O[2s].o
stat-test-O2.o:
U __xstat
stat-test-Os.o:
U stat

The symbols differ on different types of optimization. Let’s see if ghci treats them differently:

$ ghci stat-test-O2.o
...
Loading object (static) stat-test-Os.o ... done
final link ... done
$ ghci stat-test-Os.o
...
final link ... ghc: a.o: unknown symbol `stat'
linking extra libraries/objects failed

And it does! It means that __xstat comes from libc.so.6, but stat codes from somewhere else.

After some investivation I’ve found it’s definition:

$ nm --defined-only --extern-only /usr/lib/libc_nonshared.a
...
stat.oS:
00000000 T __i686.get_pc_thunk.bx
00000000 W stat
00000000 T __stat
...

We see here the file defining two symbols for us:

  1. global weak stat (the one we really need)
  2. global __stat (useless and potentially harmful as it might lead to symbol collision)

Now we can build-up working stat-test-Os.o by linking that weak stat symbol to our ghci:

$ ar x /usr/lib/libc_nonshared.a
$ mv stat.oS stat.o # ghci dislikes non-'*.o' extensions for object files
$ ghci a.o stat.o
Loading object (static) a.o ... done
Loading object (static) stat.o ... done
final link ... ghc: stat.o: unknown symbol `stat'

Almost works! Well, no. Nothing changed. But the reason is missing support for loading weak symbols to ghci. Whick is known as bug 3333.

I’ve pulled series of patches by akio and actualized it to ghc-7.6.3 (the result)

After pulling that patch into ghc I’ve got previous example to load on x86_64:

$ ar x /usr/lib/libc_nonshared.a
$ mv stat.oS stat.o # ghci dislikes non-'*.o' extensions for object files
$ ghci a.o stat.o
Loading object (static) a.o ... done
Loading object (static) stat.o ... done
final link ... done

Ideally, I should load all those and only those libc_nonshared.a symbols into ghci as a first library. I’ve decided to biggyback on ghc-prim module and stuff all those nonshared symbols there.

Perhaps, that bit of shell is the worst piece of code I have ever written. It weakens all needed symbols, localizes all the rest, and merges the result into ghc-prim. It also known to break on i386 as I have hidden GOT and module base required for PIC code.

ghci‘s loader interface used not only for interactive use but also for TemplateHaskell (where we have seen vector package failing to compile) thus.

This post is a great example why using native system’s loader is a good idea proposed in bug 4244.

I don’t know if it fixes all our cases but it will be way more clean than it is now.

If the workaround will show itself as too fragile I’ll have to roll back to force CFLAGS=-O2 when building ghc.

UPDATE: Now x86 works as well: libc_nonshared.a contained PIC code thus I’ve picked implementation from libc.a directly.

Our workaround even passes test from bug 7072!

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