diff options
Diffstat (limited to 'rts/linker/PEi386.c')
| -rw-r--r-- | rts/linker/PEi386.c | 96 |
1 files changed, 92 insertions, 4 deletions
diff --git a/rts/linker/PEi386.c b/rts/linker/PEi386.c index 8d5238f792..b5134235b0 100644 --- a/rts/linker/PEi386.c +++ b/rts/linker/PEi386.c @@ -261,6 +261,54 @@ .asciiz "libfoo_data" + Note [GHC Linking model and import libraries] + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + The above describes how import libraries work for static linking. + Fundamentally this does not apply to dynamic linking as we do in GHC. + The issue is two-folds: + + 1. In the linking model above it is expected that the .idata sections be + materialized into PLTs during linking. However in GHC we never create + PLTs, but have out own mechanism for this which is the jump island + machinery. This is required for efficiency. For one materializing the + .idata sections would result in wasting pages. We'd use one page for + every ~100 bytes. This is extremely wasteful and also fragments the + memory. Secondly the dynamic linker is lazy. We only perform the final + loading if the symbol is used, however with an import library we can + discard the actual OC immediately after reading it. This prevents us from + keeping ~1k in memory per symbol for no reason. + + 2. GHC itself does not observe symbol visibility correctly during NGC. This + in itself isn't an academic exercise. The issue stems from GHC using one + mechanism for providing two incompatible linking modes: + a) The first mode is generating Haskell shared libraries which are + intended to be used by other Haskell code. This requires us to + export the info, data and closures. For this GHC just re-exports + all symbols. But it doesn't correcly mark data/code. Symbol + visibility is overwritten by telling the linker to export all + symbols. + b) The second code is producing code that's supposed to be call-able + through a C insterface. This in reality does not require the + export of closures and info tables. But also does not require the + inclusion of the RTS inside the DLL. Hover this is done today + because we don't properly have the RTS as a dynamic library. + i.e. GHC does not only export symbols denoted by foreign export. + Also GHC should depend on an RTS library, but at the moment it + cannot because of TNTC is incompatible with dynamic linking. + + These two issues mean that for GHC we need to take a different approach + to handling import libraries. For normal C libraries we have proper + differentiation between CODE and DATA. For GHC produced import libraries + we do not. As such the SYM_TYPE_DUP_DISCARD tells the linker that if a + duplicate symbol is found, and we were going to discard it anyway, just do + so quitely. This works because the RTS symbols themselves are provided by + the currently loaded RTS as built-in symbols. + + Secondly we cannot rely on a text symbol being available. As such we + should only depend on the symbols as defined in the .idata sections, + otherwise we would not be able to correctly link against GHC produced + import libraries. + Note [Memory allocation] ~~~~~~~~~~~~~~~~~~~~~~~~ The loading of an object begins in `preloadObjectFile`, which allocates a buffer, @@ -1658,7 +1706,10 @@ ocGetNames_PEi386 ( ObjectCode* oc ) if ( secNumber != IMAGE_SYM_UNDEFINED && secNumber > 0 && section - && section->kind != SECTIONKIND_BFD_IMPORT_LIBRARY) { + /* Skip all BFD import sections. */ + && section->kind != SECTIONKIND_IMPORT + && section->kind != SECTIONKIND_BFD_IMPORT_LIBRARY + && section->kind != SECTIONKIND_BFD_IMPORT_LIBRARY_HEAD) { /* This symbol is global and defined, viz, exported */ /* for IMAGE_SYMCLASS_EXTERNAL && !IMAGE_SYM_UNDEFINED, @@ -1691,12 +1742,49 @@ ocGetNames_PEi386 ( ObjectCode* oc ) IF_DEBUG(linker_verbose, debugBelch("bss symbol @ %p %u\n", addr, symValue)); } else if (section && section->kind == SECTIONKIND_BFD_IMPORT_LIBRARY) { - setImportSymbol(oc, sname); + /* Disassembly of section .idata$5: + + 0000000000000000 <__imp_Insert>: + ... + 0: IMAGE_REL_AMD64_ADDR32NB .idata$6 + + The first two bytes contain the ordinal of the function + in the format of lowpart highpart. The two bytes combined + for the total range of 16 bits which is the function export limit + of DLLs. See note [GHC Linking model and import libraries]. */ + sname = (SymbolName*)section->start+2; + COFF_symbol* sym = &oc->info->symbols[info->numberOfSymbols-1]; + addr = get_sym_name( getSymShortName (info, sym), oc); + + IF_DEBUG(linker, + debugBelch("addImportSymbol `%s' => `%s'\n", + sname, (char*)addr)); + /* We're going to free the any data associated with the import + library without copying the sections. So we have to duplicate + the symbol name and values before the pointers become invalid. */ + sname = strdup (sname); + addr = strdup (addr); + type = has_code_section ? SYM_TYPE_CODE : SYM_TYPE_DATA; + type |= SYM_TYPE_DUP_DISCARD; + if (!ghciInsertSymbolTable(oc->fileName, symhash, sname, + addr, false, type, oc)) { + releaseOcInfo (oc); + stgFree (oc->image); + oc->image = NULL; + return false; + } + setImportSymbol (oc, sname); + + /* Don't process this oc any further. Just exit. */ + oc->n_symbols = 0; + oc->symbols = NULL; + stgFree (oc->image); + oc->image = NULL; + releaseOcInfo (oc); // There is nothing that we need to resolve in this object since we // will never call the import stubs in its text section oc->status = OBJECT_DONT_RESOLVE; - - IF_DEBUG(linker_verbose, debugBelch("import symbol %s\n", sname)); + return true; } else if (secNumber > 0 && section |