Mach-O GHCi linker lacks support for a range of relocation entries
|Reported by:||chak||Owned by:|
|Keywords:||Cc:||mnislaih@…, morrow@…, hgolden, pho@…, anton.nik@…|
|Operating System:||MacOS X||Architecture:||Unknown/Multiple|
|Type of failure:||None/Unknown||Difficulty:||Unknown|
|Test Case:||Blocked By:||#3658|
The Mach-O code of the GHCi linker rts/Linker.c lacks support for a range of relocation entries. It used to silently ignore many of them. The following patch makes it barf() when it encounters an unsupported entry:
Wed Nov 11 13:07:12 EST 2009 Manuel M T Chakravarty <firstname.lastname@example.org> * Barf on unhandled Mach-O relocations in the ghci linker - It might be worthwhile to MERGE this to 6.12, BUT somebody should validate it on PPC/Mac OS X first.
Moreover, at least one entry type —i.e., GENERIC_RELOC_LOCAL_SECTDIFF— is not correctly implemented.
This is an unsatisfactory situation as the transition from Mac OS X 10.5 (Leopard) to 10.6 (Snow Leopard) showed. In that case, changes in ld suddenly created a so far unsupported entry type. This was before the above patch; so, the ignored entry led to an incorrectly relocated image, which crashed GHCi with a SIGBUS.
Instead of trying to improve the dynamic linker, IMHO, GHC should use dynamic libraries with dlopen() and leave the implementation of dynamic linking to the OS vendor. This has a number of advantages:
- The RTS gets smaller & simpler, and we eliminate a whole category of potentially tricky bugs.
- Dynamically loaded code can use dtrace probes (and other features requiring linker trickery).
- Packages that GHC links to, don't need to be in memory twice (once statically and once dynamically linked).
- Potential performance advantage due to optimisations in the OS' dynamic linker.
The main obstacle with using dynamic libraries and dlopen() appears to be the required on-the-fly conversion of GHC-generated object files into dynamic libraries. Otherwise, SimonM says that it is already possible to compile GHC itself dynamically-linked at the moment that the linker will then use dlopen() for loading packages.
More details are in the following thread on email@example.com:
Change History (23)
comment:2 in reply to: ↑ 1 ; follow-ups: ↓ 3 ↓ 4 Changed 4 years ago by simonmar
- Difficulty set to Unknown
- Milestone set to 6.12 branch
comment:6 follow-up: ↓ 8 Changed 4 years ago by morrow
- Cc morrow@… added
- Type of failure set to None/Unknown
Changed 4 years ago by PHO
comment:15 Changed 4 years ago by igloo
- Milestone changed from 6.12.3 to 6.14.1
- Priority changed from normal to low
comment:20 Changed 2 years ago by igloo
- Milestone changed from 7.4.1 to 7.6.1
- Priority changed from low to normal