Commit 6fa1343b authored by Fangrui Song's avatar Fangrui Song
Browse files

[ELF] Resolve R_DTPREL in .debug_* referencing discarded symbols to -1 

The location of a TLS variable is encoded as a DW_OP_const4u/DW_OP_const8u
followed by a DW_OP_push_tls_address (or DW_OP_GNU_push_tls_address https://sourceware.org/bugzilla/show_bug.cgi?id=11616 ).

This change follows up to D81784 and makes relocations types generalized as
R_DTPREL (e.g. R_X86_64_DTPOFF{32,64}, R_PPC64_DTPREL64) use -1 as the
tombstone value as well. This works for both TLS Variant I and Variant II
architectures.

* arm: .long tls(tlsldo)   # not working currently (R_ARM_TLS_LDO32 is R_ABS)
* mips64: .dtpreldword tls+32768
* ppc64: .quad tls@DTPREL+0x8000
* riscv: neither GCC nor clang has implemented DW_AT_location. It is likely .long/.quad tls@dtprel+0x800
* x86-32: .long tls@DTPOFF
* x86-64: .long tls@DTPOFF; .quad tls@DTPOFF

tls has a non-negative st_value, so such relocations (st_value+addend)
never resolve to -1 in a normal (not discarded) case.

```
// clang -fuse-ld=lld -g -ffunction-sections a.c -Wl,--gc-sections
// foo and tls will be discarded by --gc-sections.
// DW_AT_location [DW_FORM_exprloc] (DW_OP_const8u 0xffffffffffffffff, DW_OP_GNU_push_tls_address)
thread_local int tls;
int foo() { return ++tls; }
int main() {}
```

Also, drop logic added in D26201 intended to address PR30793. It added a test
(gc-debuginfo-tls.s) using a non-SHF_ALLOC section and a local symbol, which
does not reflect the intended scenario: a relocation in a SHF_ALLOC section
referencing a discarded non-local symbol. For such a non .debug_* section, just
emit an error.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D82899
parent eb46137d

lld/ELF/InputSection.cpp

+5 −6
Original line number Diff line number Diff line
@@ -907,12 +907,7 @@ void InputSection::relocateNonAlloc(uint8_t *buf, ArrayRef<RelTy> rels) { 
      continue;

    }



    if (sym.isTls() && !Out::tlsPhdr) {

      target->relocateNoSym(bufLoc, type, 0);

      continue;

    }



    if (isDebug && type == target->symbolicRel) {

    if (isDebug && (type == target->symbolicRel || expr == R_DTPREL)) {

      // Resolve relocations in .debug_* referencing (discarded symbols or ICF

      // folded section symbols) to a tombstone value. Resolving to addend is

      // unsatisfactory because the result address range may collide with a
@@ -924,6 +919,10 @@ void InputSection::relocateNonAlloc(uint8_t *buf, ArrayRef<RelTy> rels) { 
      // to resolve an address attribute (which may have a non-zero addend) to

      // -1+addend (wrap around to a low address).

      //

      // R_DTPREL type relocations represent an offset into the dynamic thread

      // vector. The computed value is st_value plus a non-negative offset.

      // Negative values are invalid, so -1 can be used as the tombstone value.

      //

      // If the referenced symbol is discarded (made Undefined), or the

      // section defining the referenced symbol is garbage collected,

      // sym.getOutputSection() is nullptr. `ds->section->repl != ds->section`

lld/test/ELF/debug-dead-reloc-tls-arm.s

0 → 100644
+23 −0
Original line number Diff line number Diff line 
# REQUIRES: arm

## Test we resolve relocations referencing TLS symbols in .debug_* sections to

## a tombstone value if the referenced TLS symbol is discarded.



# RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %s -o %t.o

# RUN: ld.lld --gc-sections %t.o -o %t

# RUN: llvm-objdump -s %t | FileCheck %s



# CHECK:      Contents of section .debug_info:

## FIXME: Use ffffffff

# CHECK-NEXT:  0000 00000000



.globl _start

_start:

  bx lr



.section .tbss,"awT",%nobits

.globl tls

  .long 0



.section .debug_info

## R_ARM_TLS_LDO32

  .long tls(tlsldo)

lld/test/ELF/debug-dead-reloc-tls.s

0 → 100644
+32 −0
Original line number Diff line number Diff line 
# REQUIRES: x86

## Test we resolve relocations referencing TLS symbols in .debug_* sections to

## a tombstone value if the referenced TLS symbol is discarded.



# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o

# RUN: ld.lld --gc-sections %t.o -o %t

# RUN: llvm-objdump -s %t | FileCheck %s



# CHECK:      Contents of section .debug_info:

# CHECK-NEXT:  0000 ffffffff ffffffff ffffffff ffffffff

# CHECK-NEXT:  0010 ffffffff ffffffff



.globl _start

_start:

  ret



.section .tbss,"awT",@nobits

.globl global

local:

global:

  .quad 0



.section .debug_info

## On ppc64, .quad local@dtprel+0x8000 (st_value 0 is supposed to point to

## 0x8000 bytes past the start of ## the dynamic TLS vector. References usually

## have an addend of 0x8000). MIPS is similar. RISC-V uses 0x800.

  .quad local@dtpoff+0x8000

  .quad global@dtpoff+0x8000



## Many other architectures don't use an offset. GCC x86-64 uses a 32-bit value.

  .long global@dtpoff

  .long -1

lld/test/ELF/gc-debuginfo-tls.s

deleted100644 → 0
+0 −24
Original line number Diff line number Diff line 
# REQUIRES: x86

# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o

# RUN: ld.lld %t.o --gc-sections -shared -o %t1

# RUN: ld.lld %t.o -shared -o %t2

# RUN: llvm-readobj --symbols %t1 | FileCheck %s --check-prefix=GC

# RUN: llvm-readobj --symbols %t2 | FileCheck %s --check-prefix=NOGC



# NOGC:      Symbol {

# NOGC:        Name: patatino

# NOGC-NEXT:   Value: 0x0

# NOGC-NEXT:   Size: 0

# NOGC-NEXT:   Binding: Local

# NOGC-NEXT:   Type: TLS

# NOGC-NEXT:   Other: 0

# NOGC-NEXT:   Section: .tbss

# NOGC-NEXT: }



# GC-NOT: tbss



.section .tbss,"awT",@nobits

patatino:

  .long 0

  .section .noalloc,""

  .quad patatino

lld/test/ELF/gc-sections-tls.s

0 → 100644
+25 −0
Original line number Diff line number Diff line 
# REQUIRES: x86

# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o



## Relocation in a non .debug_* referencing a discarded TLS symbol is invalid.

## If we happen to have no PT_TLS, we will emit an error.

# RUN: not ld.lld %t.o --gc-sections -o /dev/null 2>&1 | FileCheck %s --check-prefix=ERR



# ERR: error: {{.*}}.o has an STT_TLS symbol but doesn't have an SHF_TLS section



## If we happen to have a PT_TLS, we will resolve the relocation to

## an arbitrary value (current implementation uses a negative value).

# RUN: echo '.section .tbss,"awT"; .globl root; root: .long 0' | \

# RUN:   llvm-mc -filetype=obj -triple=x86_64 - -o %t1.o

# RUN: ld.lld --gc-sections -u root %t.o %t1.o -o %t

# RUN: llvm-readelf -x .noalloc %t | FileCheck %s



# CHECK:      Hex dump of section '.noalloc':

# CHECK-NEXT: 0x00000000 {{[0-9a-f]+}} ffffffff



.section .tbss,"awT",@nobits

tls:

  .long 0



.section .noalloc,""

  .quad tls+0x8000