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llvm-project/lld/test/ELF/relocation-relative-weak.s

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ELF: Forbid all relative relocations to absolute symbols in PIC, except for weak undefined. Weak undefined symbols resolve to the image base. This is a little strange, but it allows us to link function calls to such symbols. Normally such a call will be guarded with a comparison, which will load a zero from the GOT. There's one example of such a function call in crti.o in Linux's CRT. As part of this change, I also needed to make the synthetic start and end symbols image base relative in the case where their sections were empty, so that PC-relative references to those symbols would continue to work. Differential Revision: http://reviews.llvm.org/D19844 llvm-svn: 268350
2016-05-03 09:21:08 +08:00
# REQUIRES: x86
# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
# RUN: ld.lld %t.o -o %t -pie
# RUN: llvm-readobj -dyn-relocations %t | FileCheck %s
# CHECK: Dynamic Relocations {
[ELF] Simplify RelRo, TLS, NOBITS section ranks and make RW PT_LOAD start with RelRo Old: PT_LOAD(.data | PT_GNU_RELRO(.data.rel.ro .bss.rel.ro) | .bss) New: PT_LOAD(PT_GNU_RELRO(.data.rel.ro .bss.rel.ro) | .data .bss) The placement of | indicates page alignment caused by PT_GNU_RELRO. The new layout has simpler rules and saves space for many cases. Old size: roundup(.data) + roundup(.data.rel.ro) New size: roundup(.data.rel.ro + .bss.rel.ro) + .data Other advantages: * At runtime the 3 memory mappings decrease to 2. * start(PT_TLS) = start(PT_GNU_RELRO) = start(RW PT_LOAD). This simplifies binary manipulation tools. GNU strip before 2.31 discards PT_GNU_RELRO if its address is not equal to the start of its associated PT_LOAD. This has been fixed by https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=f2731e0c374e5323ce4cdae2bcc7b7fe22da1a6f But with this change, we will be compatible with GNU strip before 2.31 * Before, .got.plt (non-relro by default) was placed before .got (relro by default), which made it impossible to have _GLOBAL_OFFSET_TABLE_ (start of .got.plt on x86-64) equal to the end of .got (R_GOT*_FROM_END) (https://bugs.llvm.org/show_bug.cgi?id=36555). With the new ordering, we can improve on this regard if we'd like to. Reviewers: ruiu, espindola, pcc Subscribers: emaste, arichardson, llvm-commits, joerg, jdoerfert Differential Revision: https://reviews.llvm.org/D56828 llvm-svn: 356117
2019-03-14 11:47:45 +08:00
# CHECK-NEXT: 0x3018 R_X86_64_JUMP_SLOT w 0x0
ELF: Forbid all relative relocations to absolute symbols in PIC, except for weak undefined. Weak undefined symbols resolve to the image base. This is a little strange, but it allows us to link function calls to such symbols. Normally such a call will be guarded with a comparison, which will load a zero from the GOT. There's one example of such a function call in crti.o in Linux's CRT. As part of this change, I also needed to make the synthetic start and end symbols image base relative in the case where their sections were empty, so that PC-relative references to those symbols would continue to work. Differential Revision: http://reviews.llvm.org/D19844 llvm-svn: 268350
2016-05-03 09:21:08 +08:00
# CHECK-NEXT: }
.globl _start
_start:
.globl w
.weak w
call w@PLT