[lld][LoongArch] Support the R_LARCH_{ADD,SUB}_ULEB128 relocation types #81133

MQ-mengqing · 2024-02-08T13:28:56Z

For a label difference like .uleb128 A-B, MC generates a pair of R_LARCH_{ADD,SUB}_ULEB128 if A-B cannot be folded as a constant. GNU assembler generates a pair of relocations in more cases (when A or B is in a code section with linker relaxation). It is similar to RISCV.

R_LARCH_{ADD,SUB}_ULEB128 relocations are created by Clang and GCC in .gcc_except_table and other debug sections with linker relaxation enabled. On LoongArch, first read the buf and count the available space. Then add or sub the value. Finally truncate the expected value and fill it into the available space.

For a label difference like `.uleb128 A-B`, MC generates a pair of R_LARCH_{ADD,SUB}_ULEB128 if A-B cannot be folded as a constant. GNU assembler generates a pair of relocations in more cases (when A or B is in a code section with linker relaxation). It is similar to RISCV. R_LARCH_{ADD,SUB}_ULEB128 relocations are created by Clang and GCC in `.gcc_except_table` and other debug sections with linker relaxation enabled. On LoongArch, first read the buf and count the available space. Then add or sub the value. Finally truncate the expected value and fill it into the available space.

llvmbot · 2024-02-08T13:29:31Z

@llvm/pr-subscribers-lld

@llvm/pr-subscribers-lld-elf

Author: Jinyang He (MQ-mengqing)

Changes

For a label difference like .uleb128 A-B, MC generates a pair of R_LARCH_{ADD,SUB}_ULEB128 if A-B cannot be folded as a constant. GNU assembler generates a pair of relocations in more cases (when A or B is in a code section with linker relaxation). It is similar to RISCV.

R_LARCH_{ADD,SUB}_ULEB128 relocations are created by Clang and GCC in .gcc_except_table and other debug sections with linker relaxation enabled. On LoongArch, first read the buf and count the available space. Then add or sub the value. Finally truncate the expected value and fill it into the available space.

Full diff: https://github.com/llvm/llvm-project/pull/81133.diff

2 Files Affected:

(modified) lld/ELF/Arch/LoongArch.cpp (+25)
(added) lld/test/ELF/loongarch-reloc-leb128.s (+100)

diff --git a/lld/ELF/Arch/LoongArch.cpp b/lld/ELF/Arch/LoongArch.cpp
index 3e9d6e0e742008..e6a222c95ef348 100644
--- a/lld/ELF/Arch/LoongArch.cpp
+++ b/lld/ELF/Arch/LoongArch.cpp
@@ -11,6 +11,7 @@
 #include "Symbols.h"
 #include "SyntheticSections.h"
 #include "Target.h"
+#include "llvm/Support/LEB128.h"
 
 using namespace llvm;
 using namespace llvm::object;
@@ -153,6 +154,22 @@ static bool isJirl(uint32_t insn) {
   return (insn & 0xfc000000) == JIRL;
 }
 
+static void handleUleb128(uint8_t *loc, uint64_t val) {
+  const char *err = nullptr;
+  uint32_t count, maxcount = 1 + (config->is64 ? 64 : 32) / 7;
+  uint64_t mask = config->is64 ? -1 : -1;
+  uint64_t orig = decodeULEB128(loc, &count, nullptr, &err);
+  if (err)
+    fatal(getErrorLocation(loc) + "could not decode uleb128 value: " + err);
+  if (count > maxcount)
+    errorOrWarn(getErrorLocation(loc) + "extra space for uleb128");
+  else if (count < maxcount)
+    mask = (1 << 7 * count) - 1;
+
+  val = (orig + val) & mask;
+  encodeULEB128(val, loc, count);
+}
+
 LoongArch::LoongArch() {
   // The LoongArch ISA itself does not have a limit on page sizes. According to
   // the ISA manual, the PS (page size) field in MTLB entries and CSR.STLBPS is
@@ -394,11 +411,13 @@ RelExpr LoongArch::getRelExpr(const RelType type, const Symbol &s,
   case R_LARCH_ADD16:
   case R_LARCH_ADD32:
   case R_LARCH_ADD64:
+  case R_LARCH_ADD_ULEB128:
   case R_LARCH_SUB6:
   case R_LARCH_SUB8:
   case R_LARCH_SUB16:
   case R_LARCH_SUB32:
   case R_LARCH_SUB64:
+  case R_LARCH_SUB_ULEB128:
     // The LoongArch add/sub relocs behave like the RISCV counterparts; reuse
     // the RelExpr to avoid code duplication.
     return R_RISCV_ADD;
@@ -633,6 +652,9 @@ void LoongArch::relocate(uint8_t *loc, const Relocation &rel,
   case R_LARCH_ADD64:
     write64le(loc, read64le(loc) + val);
     return;
+  case R_LARCH_ADD_ULEB128:
+    handleUleb128(loc, val);
+    return;
   case R_LARCH_SUB6:
     *loc = (*loc & 0xc0) | ((*loc - val) & 0x3f);
     return;
@@ -648,6 +670,9 @@ void LoongArch::relocate(uint8_t *loc, const Relocation &rel,
   case R_LARCH_SUB64:
     write64le(loc, read64le(loc) - val);
     return;
+  case R_LARCH_SUB_ULEB128:
+    handleUleb128(loc, -val);
+    return;
 
   case R_LARCH_MARK_LA:
   case R_LARCH_MARK_PCREL:
diff --git a/lld/test/ELF/loongarch-reloc-leb128.s b/lld/test/ELF/loongarch-reloc-leb128.s
new file mode 100644
index 00000000000000..be81dcac58c48d
--- /dev/null
+++ b/lld/test/ELF/loongarch-reloc-leb128.s
@@ -0,0 +1,100 @@
+# REQUIRES: loongarch
+# RUN: rm -rf %t && split-file %s %t && cd %t
+
+# RUN: llvm-mc --filetype=obj --triple=loongarch64 --mattr=+relax a.s -o a.o
+# RUN: llvm-readobj -r -x .gcc_except_table -x .debug_rnglists -x .debug_loclists a.o | FileCheck %s --check-prefix=REL
+# RUN: ld.lld -shared --gc-sections --noinhibit-exec a.o -o a.so
+# RUN: llvm-readelf -x .gcc_except_table -x .debug_rnglists -x .debug_loclists a.so | FileCheck %s
+
+# RUN: llvm-mc --filetype=obj --triple=loongarch32 --mattr=+relax extraspace.s -o extraspace32.o
+# RUN: llvm-mc --filetype=obj --triple=loongarch64 --mattr=+relax extraspace.s -o extraspace64.o --defsym=size64=1
+# RUN: not ld.lld -shared extraspace32.o 2>&1 | FileCheck %s --check-prefix=ERROR
+# RUN: not ld.lld -shared extraspace64.o 2>&1 | FileCheck %s --check-prefix=ERROR
+# ERROR: error: {{.*}}.o:(.rodata+0x0): extra space for uleb128
+
+#--- a.s
+.cfi_startproc
+.cfi_lsda 0x1b,.LLSDA0
+.cfi_endproc
+
+.section .text.w,"axR"
+break 0; break 0; break 0; w1:
+  .p2align 4    # 4 bytes after relaxation
+w2: break 0
+
+.section .text.x,"ax"
+break 0; break 0; break 0; x1:
+  .p2align 4    # 4 bytes after relaxation
+x2: break 0
+
+.section .gcc_except_table,"a"
+.LLSDA0:
+.uleb128 w2-w1+116                   # initial value: 0x0080
+.uleb128 w1-w2+141                   # initial value: 0x0080
+.uleb128 w2-w1+16372                 # initial value: 0x008080
+.uleb128 w1-w2+16397                 # initial value: 0x008080
+.uleb128 w2-w1+2097140               # initial value: 0x00808080
+.uleb128 w1-w2+2097165               # initial value: 0x00808080
+
+.section .debug_rnglists
+.uleb128 w2-w1+116                   # initial value: 0x0080
+.uleb128 w1-w2+141                   # initial value: 0x0080
+.uleb128 w2-w1+16372                 # initial value: 0x008080
+.uleb128 w1-w2+16397                 # initial value: 0x008080
+.uleb128 w2-w1+2097140               # initial value: 0x00808080
+.uleb128 w1-w2+2097165               # initial value: 0x00808080
+
+.section .debug_loclists
+.uleb128 x2-x1                       # references discarded symbols
+
+# REL:      Section ({{.*}}) .rela.debug_rnglists {
+# REL-NEXT:   0x0 R_LARCH_ADD_ULEB128 w2 0x74
+# REL-NEXT:   0x0 R_LARCH_SUB_ULEB128 w1 0x0
+# REL-NEXT:   0x2 R_LARCH_ADD_ULEB128 w1 0x8D
+# REL-NEXT:   0x2 R_LARCH_SUB_ULEB128 w2 0x0
+# REL-NEXT:   0x4 R_LARCH_ADD_ULEB128 w2 0x3FF4
+# REL-NEXT:   0x4 R_LARCH_SUB_ULEB128 w1 0x0
+# REL-NEXT:   0x7 R_LARCH_ADD_ULEB128 w1 0x400D
+# REL-NEXT:   0x7 R_LARCH_SUB_ULEB128 w2 0x0
+# REL-NEXT:   0xA R_LARCH_ADD_ULEB128 w2 0x1FFFF4
+# REL-NEXT:   0xA R_LARCH_SUB_ULEB128 w1 0x0
+# REL-NEXT:   0xE R_LARCH_ADD_ULEB128 w1 0x20000D
+# REL-NEXT:   0xE R_LARCH_SUB_ULEB128 w2 0x0
+# REL-NEXT: }
+# REL:      Section ({{.*}}) .rela.debug_loclists {
+# REL-NEXT:   0x0 R_LARCH_ADD_ULEB128 x2 0x0
+# REL-NEXT:   0x0 R_LARCH_SUB_ULEB128 x1 0x0
+# REL-NEXT: }
+
+# REL:      Hex dump of section '.gcc_except_table':
+# REL-NEXT: 0x00000000 80008000 80800080 80008080 80008080
+# REL-NEXT: 0x00000010 8000
+# REL:      Hex dump of section '.debug_rnglists':
+# REL-NEXT: 0x00000000 80008000 80800080 80008080 80008080
+# REL-NEXT: 0x00000010 8000
+# REL:      Hex dump of section '.debug_loclists':
+# REL-NEXT: 0x00000000 00
+
+# CHECK: Hex dump of section '.gcc_except_table':
+# CHECK-NEXT: 0x[[#%x,]] f8008901 f8ff0089 8001f8ff ff008980 .
+# CHECK-NEXT: 0x[[#%x,]] 8001                                .
+# CHECK:      Hex dump of section '.debug_rnglists':
+# CHECK-NEXT: 0x00000000 f8008901 f8ff0089 8001f8ff ff008980 .
+# CHECK-NEXT: 0x00000010 8001                                .
+# CHECK:      Hex dump of section '.debug_loclists':
+# CHECK-NEXT: 0x00000000 00                                  .
+
+#--- extraspace.s
+.text
+w1:
+  la.pcrel $t0, w1
+w2:
+
+.rodata
+.reloc ., R_LARCH_ADD_ULEB128, w2
+.reloc ., R_LARCH_SUB_ULEB128, w1
+.fill 5, 1, 0x80
+.ifdef size64
+  .fill 5, 1, 0x80
+.endif
+.byte 0

MQ-mengqing · 2024-02-08T15:17:49Z

Add @SixWeining @MaskRay

lld/ELF/Arch/LoongArch.cpp

lld/test/ELF/loongarch-reloc-leb128.s

lld/ELF/Arch/LoongArch.cpp

lld/test/ELF/loongarch-reloc-leb128.s

lld/ELF/Arch/LoongArch.cpp

MaskRay

Thanks for reviewing the change, but please do not land.
For LoongArch linker changes and changes to the generic assembler code, there have been many instances in the past where more adjustments were needed and suggested by me. I appreciate that you wait on my input.

lld/ELF/Arch/LoongArch.cpp

SixWeining · 2024-02-21T05:27:57Z

Thanks for reviewing the change, but please do not land. For LoongArch linker changes and changes to the generic assembler code, there have been many instances in the past where more adjustments were needed and suggested by me. I appreciate that you wait on my input.

Thanks. No problem. I'll not land it until all review comments are addressed.

MaskRay · 2024-02-29T05:10:36Z

lld/ELF/Arch/LoongArch.cpp

+    mask = (1ULL << 7 * count) - 1;
+
+  val = (orig + val) & mask;
+  encodeULEB128(val, loc, count);


encodeULEB128((orig + val) & mask, loc, count); to avoid changing val

MaskRay · 2024-02-29T05:42:33Z

lld/ELF/Arch/LoongArch.cpp

+  uint64_t orig = decodeULEB128(loc, &count);
+  if (count > maxcount)
+    errorOrWarn(getErrorLocation(loc) + "extra space for uleb128");
+  uint64_t mask = config->is64 ? -1 : -1u;


UINT64_MAX can be used for 32-bit as well.

Perhaps count < maxcount ? (UINT64_C(1) << 7 * count) -1 : UINT64_MAX

If config->is64 == false, we expect mask is 0x0000'0000'ffff'ffff so that the results will not be broken by the high-32-bits. (In 32-bits ELF file the addr size is 32bits, so the content should not be overflowed.)

Are the relocations defined to be 32-bit on ELF32? It looks strange.

And there is no test checking the 32-bit behavior

I can add the same test as a.o for 32-bit.

uleb128(-1u) == 0x0f'ff'ff'ff'ff. Assuming it reserved 5 bytes space and we executing SUB -1 on 32-bit first, then without this mask we might get 0x7f'ff'ff'ff'ff, which [35:33] is 1. These relocations is relocated address, and address should be limited in 32-bit on ELF32. So we need this mask to avoid breaking the high-space.

I've checked x86, mips and riscv ELF32 by GUN AS, they all handle uleb128 as 64bits value. So it's my misunderstanding. I'll correct it.

MaskRay · 2024-02-29T05:44:49Z

lld/test/ELF/loongarch-reloc-leb128.s

+# RUN: llvm-mc --filetype=obj --triple=loongarch64 --mattr=+relax extraspace.s -o extraspace64.o --defsym=size64=1
+# RUN: not ld.lld -shared extraspace32.o 2>&1 | FileCheck %s --check-prefix=ERROR
+# RUN: not ld.lld -shared extraspace64.o 2>&1 | FileCheck %s --check-prefix=ERROR
+# ERROR: error: {{.*}}.o:(.rodata+0x0): extra space for uleb128


a.o since the filename is determined

MaskRay

.

MaskRay · 2024-03-05T05:20:27Z

Thanks for making .uleb128 consistently 64-bit

…es (llvm#81133) For a label difference like `.uleb128 A-B`, MC generates a pair of R_LARCH_{ADD,SUB}_ULEB128 if A-B cannot be folded as a constant. GNU assembler generates a pair of relocations in more cases (when A or B is in a code section with linker relaxation). It is similar to RISCV. R_LARCH_{ADD,SUB}_ULEB128 relocations are created by Clang and GCC in `.gcc_except_table` and other debug sections with linker relaxation enabled. On LoongArch, first read the buf and count the available space. Then add or sub the value. Finally truncate the expected value and fill it into the available space. (cherry picked from commit eaa9ef6)

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Address @SixWeining's comments

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Remove useless --noinhibit-exec

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Remove err parameter in decodeULEB128 and use 1ULL when shift

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Address @MaskRay's comments

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Add elf32 test and use 64bits uleb128 value

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[lld][LoongArch] Support the R_LARCH_{ADD,SUB}_ULEB128 relocation types #81133

[lld][LoongArch] Support the R_LARCH_{ADD,SUB}_ULEB128 relocation types #81133

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[lld][LoongArch] Support the R_LARCH_{ADD,SUB}_ULEB128 relocation types #81133

[lld][LoongArch] Support the R_LARCH_{ADD,SUB}_ULEB128 relocation types #81133

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