From 1a3f3a3fa17909aa22958a4f06b1a357a75b1bb4 Mon Sep 17 00:00:00 2001 From: Petr Hosek Date: Mon, 15 Feb 2021 23:41:16 -0800 Subject: [PATCH] [lld][ELF] __start_/__stop_ refs don't retain C-ident named group sections The special root semantics for identifier-named sections is meant specifically for the metadata sections. In the context of group semantics, where group members are always retained or discarded as a unit, it's natural not to have this semantics apply to a section in a group, otherwise we would never discard the group defeating the purpose of using the group in the first place. This change modifies the GC behavior so that __start_/__stop_ references don't retain C identifier named sections in section groups which allows for these groups to be collected. This matches the behavior of BFD ld. The only kind of existing case that might break is interdependent metadata sections that are all in a group together, but that group doesn't contain any other sections referenced by anything except implicit inclusion in a `__start_` and/or `__stop_`-referenced identifier-named section, but such cases should be unlikely. Differential Revision: https://reviews.llvm.org/D96753 --- lld/ELF/MarkLive.cpp | 2 +- lld/test/ELF/gc-sections-startstop.s | 47 ++++++++++++++++++++++++++++++++++++ lld/test/ELF/startstop-gccollect.s | 44 --------------------------------- 3 files changed, 48 insertions(+), 45 deletions(-) create mode 100644 lld/test/ELF/gc-sections-startstop.s delete mode 100644 lld/test/ELF/startstop-gccollect.s diff --git a/lld/ELF/MarkLive.cpp b/lld/ELF/MarkLive.cpp index 7d89338..92b968d 100644 --- a/lld/ELF/MarkLive.cpp +++ b/lld/ELF/MarkLive.cpp @@ -270,7 +270,7 @@ template void MarkLive::run() { if (isReserved(sec) || script->shouldKeep(sec)) { enqueue(sec, 0); - } else if (isValidCIdentifier(sec->name)) { + } else if (isValidCIdentifier(sec->name) && !sec->nextInSectionGroup) { cNamedSections[saver.save("__start_" + sec->name)].push_back(sec); cNamedSections[saver.save("__stop_" + sec->name)].push_back(sec); } diff --git a/lld/test/ELF/gc-sections-startstop.s b/lld/test/ELF/gc-sections-startstop.s new file mode 100644 index 0000000..20c244a --- /dev/null +++ b/lld/test/ELF/gc-sections-startstop.s @@ -0,0 +1,47 @@ +## Check that group members are retained or discarded as a unit, and +## sections whose names are C identifiers aren't considered roots if +## they're members of a group. + +# REQUIRES: x86 + +# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o +# RUN: ld.lld %t.o --gc-sections -o %t +# RUN: llvm-readelf -s %t | FileCheck %s + +# RUN: echo ".global __start___data; __start___data:" > %t2.s +# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %t2.s -o %t2.o +# RUN: ld.lld -shared %t2.o -o %t2.so +# RUN: ld.lld %t.o --gc-sections -o %t2 %t2.so +# RUN: llvm-readelf -s %t2 | FileCheck %s + +# CHECK: [[#%x,ADDR:]] {{.*}} __start___data +# CHECK: [[#ADDR + 8]] {{.*}} __stop___data +# CHECK: _start +# CHECK: f +# CHECK-NOT: g + +.weak __start___data +.weak __stop___data + +.section .text,"ax",@progbits +.global _start +_start: + .quad __start___data - . + .quad __stop___data - . + call f + +.section __data,"axG",@progbits,f +.quad 0 + +.section .text.f,"axG",@progbits,f +.global f +f: + nop + +.section __data,"axG",@progbits,g +.quad 0 + +.section .text.g,"axG",@progbits,g +.global g +g: + nop diff --git a/lld/test/ELF/startstop-gccollect.s b/lld/test/ELF/startstop-gccollect.s deleted file mode 100644 index 8017bca..0000000 --- a/lld/test/ELF/startstop-gccollect.s +++ /dev/null @@ -1,44 +0,0 @@ -# REQUIRES: x86 -# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t - -## Default run: sections foo and bar exist in output -# RUN: ld.lld %t -o %tout -# RUN: llvm-objdump -d %tout | FileCheck --check-prefix=DISASM %s - -## Check that foo and bar sections are not garbage collected, -## we do not want to reclaim sections if they are referred -## by __start_* and __stop_* symbols. -# RUN: ld.lld %t --gc-sections -o %tout -# RUN: llvm-objdump -d %tout | FileCheck --check-prefix=DISASM %s - -# RUN: echo ".global __start_foo; __start_foo:" > %t2.s -# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %t2.s -o %t2.o -# RUN: ld.lld -shared %t2.o -o %t2.so -# RUN: ld.lld %t --gc-sections -o %tout %t2.so -# RUN: llvm-objdump -d %tout | FileCheck --check-prefix=DISASM %s - -# DISASM: <_start>: -# DISASM-NEXT: callq {{.*}} <__start_foo> -# DISASM-NEXT: callq {{.*}} <__stop_bar> -# DISASM-EMPTY: -# DISASM-NEXT: Disassembly of section foo: -# DISASM-EMPTY: -# DISASM-NEXT: <__start_foo>: -# DISASM-NEXT: nop -# DISASM-EMPTY: -# DISASM-NEXT: Disassembly of section bar: -# DISASM-EMPTY: -# DISASM-NEXT: : -# DISASM-NEXT: nop - -.global _start -.text -_start: - callq __start_foo - callq __stop_bar - -.section foo,"ax" - nop - -.section bar,"ax" - nop -- 2.7.4