id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related 5748,ghci segfault on OS X after dlsym failed lookup,gwright,igloo,"I've had repeatable segfaults with ghci 7.2.2 (OS X 10.6) and 7.0.4 (OS X 10.7 and 10.6). The immediate cause is a failed lookup of an external symbol in {{{rts/Linker.c}}}. The failure is not detected and the NULL value returned is eventually dereferenced, leading to a segfault. The underlying bug is still present in HEAD. This is what happens: {{{ gwright-macbook> ghci -v Test.hs GHCi, version 7.2.2: http://www.haskell.org/ghc/ :? for help Glasgow Haskell Compiler, Version 7.2.2, stage 2 booted by GHC version 7.0.4 Using binary package database: /usr/local/lib/ghc-7.2.2/package.conf.d/package.cache Using binary package database: /Users/gwright/.ghc/x86_64-darwin-7.2.2/package.conf.d/package.cache hiding package Cabal-1.12.0 to avoid conflict with later version Cabal-1.13.3 wired-in package ghc-prim mapped to ghc-prim-0.2.0.0-14e0c022e5d4efa3a40ab5991f2b2a1b wired-in package integer-gmp mapped to integer-gmp-0.3.0.0-2e2b0fd56be1a5f60c50913e615691d9 wired-in package base mapped to base-4.4.1.0-5ca60b2acbb66fd59e5f81685cb72740 wired-in package rts mapped to builtin_rts wired-in package template-haskell mapped to template-haskell-2.6.0.0-e7db5d1205f362bb792ab7bd5c7bbfae wired-in package dph-seq not found. wired-in package dph-par not found. Hsc static flags: -static Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. *** Chasing dependencies: Chasing modules from: Stable obj: [] Stable BCO: [] unload: retaining objs [] unload: retaining bcos [] Ready for upsweep [] Upsweep completely successful. *** Deleting temp files: Deleting: *** Chasing dependencies: Chasing modules from: *Test.hs Stable obj: [] Stable BCO: [] unload: retaining objs [] unload: retaining bcos [] Ready for upsweep [NONREC ModSummary { ms_hs_date = Sun Jan 1 18:20:14 EST 2012 ms_mod = main:Main, ms_textual_imps = [import Prelude, import Math.Symbolic.Wheeler.TensorUtilities, import Math.Symbolic.Wheeler.TensorBasics, import Math.Symbolic.Wheeler.Tensor, import Math.Symbolic.Wheeler.IO, import Math.Symbolic.Wheeler.Symbol, import Math.Symbolic.Wheeler.Numeric, import Math.Symbolic.Wheeler.Expr, import Math.Symbolic.Wheeler.Commutativity, import Math.Symbolic.Wheeler.Canonicalize, import Math.Symbolic.Wheeler.Basic, import Data.Ratio, import Data.Maybe] ms_srcimps = [] }] *** Deleting temp files: Deleting: compile: input file Test.hs Created temporary directory: /var/folders/4j/4jmo0VgVHgu2WNrlXKFTB++++TI/-Tmp-/ghc61560_1 *** Checking old interface for main:Main: [1 of 1] Compiling Main ( Test.hs, interpreted ) *** Parser: *** Renamer/typechecker: *** Desugar: Result size of Desugar = 788 *** Simplifier: Result size of Simplifier iteration=1 = 800 Result size of Simplifier = 788 *** Tidy Core: Result size of Tidy Core = 788 *** CorePrep: Result size of CorePrep = 1010 *** ByteCodeGen: Upsweep completely successful. *** Deleting temp files: Deleting: /var/folders/4j/4jmo0VgVHgu2WNrlXKFTB++++TI/-Tmp-/ghc61560_1/ghc61560_0.c /var/folders/4j/4jmo0VgVHgu2WNrlXKFTB++++TI/-Tmp-/ghc61560_1/ghc61560_0.o Warning: deleting non-existent /var/folders/4j/4jmo0VgVHgu2WNrlXKFTB++++TI/-Tmp-/ghc61560_1/ghc61560_0.c Warning: deleting non-existent /var/folders/4j/4jmo0VgVHgu2WNrlXKFTB++++TI/-Tmp-/ghc61560_1/ghc61560_0.o Ok, modules loaded: Main. *Main> x *** Parser: *** Desugar: *** Simplify: *** CorePrep: *** ByteCodeGen: Loading package bytestring-0.9.2.0 ... linking ... done. Loading package transformers-0.2.2.0 ... linking ... done. Loading package mtl-2.0.1.0 ... linking ... done. Loading package array-0.3.0.3 ... linking ... done. Loading package deepseq-1.2.0.1 ... linking ... done. Loading package text-0.11.1.12 ... linking ... done. Loading package parsec-3.1.2 ... linking ... done. Loading package uniqueid-0.1.1 ... linking ... done. Loading package Wheeler-0.1 ... linking ... done. Segmentation fault gwright-macbook> }}} The program is trying to display a record type (the variable ""x""). The record type has a Show instance associated with it, and that is apparently not being resolved correctly, leading to a NULL dereference and a segfault. The problem is isolated to OS X as far as I can tell. The code responsible for the error is in the function {{{relocateSection}}}: {{{ else if(reloc->r_extern) { struct nlist *symbol = &nlist[reloc->r_symbolnum]; char *nm = image + symLC->stroff + symbol->n_un.n_strx; IF_DEBUG(linker, debugBelch(""relocateSection: looking up external symbol %s\n"", nm)); IF_DEBUG(linker, debugBelch("" : type = %d\n"", symbol->n_type)); IF_DEBUG(linker, debugBelch("" : sect = %d\n"", symbol->n_sect)); IF_DEBUG(linker, debugBelch("" : desc = %d\n"", symbol->n_desc)); IF_DEBUG(linker, debugBelch("" : value = %p\n"", (void *)symbol->n_value)); if ((symbol->n_type & N_TYPE) == N_SECT) { value = relocateAddress(oc, nSections, sections, symbol->n_value); IF_DEBUG(linker, debugBelch(""relocateSection, defined external symbol %s, relocated address %p\n"", nm, (void *)value)); } else { value = (uint64_t) lookupSymbol(nm); IF_DEBUG(linker, debugBelch(""relocateSection: external symbol %s, address %p\n"", nm, (void *)value)); } } }}} The returned value from {{{lookupSymbol}}} is not checked for failure. A simple check for NULL and a call to {{{errorBelch}}} is all that's needed to fix this. There's another place where the return value from {{{lookupSymbol}}} is not checked and it should be fixed similarly. In a bit more detail, the program ""Test.hs"" I was loading does some simple tests on a library. I admit to having been sloppy while sorting out the module exports, but the library compiles without warnings when -Wall is set. The library has a top-level module that re-exports the most commonly used symbols, but the test program doesn't use it, importing all of the modules it needs explicitly. Am I doing something that is known to be dangerous? The failed symbol lookup is {{{ lookupSymbol: looking up _Wheelerzm0zi1_MathziSymbolicziWheelerziSimpleSymbol_zdfShowSzuzdcshowsPrec_closure }}} which is the Show instance for a {{{SimpleSymbol}}} data type. (The library implements a symbolic algebra DSL.) The symbol is undefined in the object module: {{{ gwright-macbook> nm HSWheeler-0.1.o | grep ""_Wheelerzm0zi1_MathziSymbolicziWheelerziSimpleSymbol_zdfShowSzuzdcshowsPrec_closure"" U _Wheelerzm0zi1_MathziSymbolicziWheelerziSimpleSymbol_zdfShowSzuzdcshowsPrec_closure gwright-macbook> }}} so some sort of failure is expected when I try to show something of the {{{SimpleSymbol}}} type. I'm puzzled that the first indication of failure is a segfault, or, after I patch {{{rts/Linker.c}}}, an error from deep inside the linker. It seems that there is something else going wrong which ought to generate a warning at least. I will generate a patch against HEAD to check for the failed symbol lookups; it would be good if it were included in the final 7.4.1.",bug,closed,highest,7.4.1,GHCi,7.2.1,fixed,,pho@…,MacOS X,Unknown/Multiple,GHCi crash,Unknown,,,,