From 6ab89c3c5df8b679e6ee240a13356309c048fc71 Mon Sep 17 00:00:00 2001 From: Fangrui Song Date: Fri, 17 Jan 2020 11:50:00 -0800 Subject: [PATCH] [ELF] Allow R_PLT_PC (R_PC) to a hidden undefined weak symbol This essentially reverts b841e119d77ed0502e3a2e710f26a899bef28b3c. Such code construct can be used in the following way: // glibc/stdlib/exit.c // clang -fuse-ld=lld => succeeded // clang -fuse-ld=lld -fpie -pie => relocation R_PLT_PC cannot refer to absolute symbol __attribute__((weak, visibility("hidden"))) extern void __call_tls_dtors(); void __run_exit_handlers() { if (__call_tls_dtors) __call_tls_dtors(); } Since we allow R_PLT_PC in -no-pie mode, it makes sense to allow it in -pie mode as well. Reviewed By: pcc Differential Revision: https://reviews.llvm.org/D72943 --- lld/ELF/Relocations.cpp | 8 ++++++++ lld/test/ELF/weak-undef-hidden.s | 5 +++++ 2 files changed, 13 insertions(+) diff --git a/lld/ELF/Relocations.cpp b/lld/ELF/Relocations.cpp index 510d515ad9a9..790b24d4a0c7 100644 --- a/lld/ELF/Relocations.cpp +++ b/lld/ELF/Relocations.cpp @@ -408,6 +408,14 @@ static bool isStaticLinkTimeConstant(RelExpr e, RelType type, const Symbol &sym, assert(absVal && relE); + // Allow R_PLT_PC (optimized to R_PC here) to a hidden undefined weak symbol + // in PIC mode. This is a little strange, but it allows us to link function + // calls to such symbols (e.g. glibc/stdlib/exit.c:__run_exit_handlers). + // Normally such a call will be guarded with a comparison, which will load a + // zero from the GOT. + if (sym.isUndefWeak()) + return true; + // We set the final symbols values for linker script defined symbols later. // They always can be computed as a link time constant. if (sym.scriptDefined) diff --git a/lld/test/ELF/weak-undef-hidden.s b/lld/test/ELF/weak-undef-hidden.s index 632e051ffdc1..2baad5738c36 100644 --- a/lld/test/ELF/weak-undef-hidden.s +++ b/lld/test/ELF/weak-undef-hidden.s @@ -2,6 +2,11 @@ // RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o // RUN: ld.lld %t.o -o %t.so -shared // RUN: llvm-readobj -r -S --section-data %t.so | FileCheck %s +// RUN: ld.lld %t.o -o %t -pie +// RUN: llvm-readobj -r -S --section-data %t | FileCheck %s + +/// This is usually guarded with a comparison. Don't report an error. +call g .data .weak g