[ELF] - Relax checks for R_386_8/R_386_16 relocations.

[grimar]

Summary

This fixes PR36927.

The issue is next. Imagine we have -Ttext 0x7c and code below.

.code16
.global _start
_start:
movb $_start+0x83,%ah

So we have R_386_8 relocation and _start at 0x7C.
Addend is 0x83 == 131. We will sign extend it to 0xffffffffffffff83.

Now, 0xffffffffffffff83 + 0x7c gives us 0xFFFFFFFFFFFFFFFF.
Techically 0x83 + 0x7c == 0xFF, we do not exceed 1 byte value, but
currently LLD errors out, because we use checkUInt<8>.

Let's try to use checkInt<8> now and the following code to see if it can help (no):
main.s:
.byte foo

input.s:
.globl foo
.hidden foo
foo = 0xff

Here, foo is 0xFF. And addend is 0x0. Final value is 0x00000000000000FF.
Again, it fits one byte well, but with checkInt<8>,
we would error out it, so we can't use it.

What we want to do is to check that the result fits 1 byte well.
Patch changes the check to checkIntUInt to fix the issue.

[ruiu]

I believe what you are doing in this patch is correct, but the error message is inappropriate, as the range [-256, 255] is too large. As an error message, it doesn't make sense. -256 is not represenable in one byte.

[ruiu]

I think what you need to do is to use checkIntUInt<8>

[grimar]

• Using checkIntUInt.

[• espindola]

Maybe the problem is the we should have read the implicit addend as unsigned?

[• espindola]

test/ELF/i386-reloc8-reloc16-addend.s shows that we cannot just zero extend the addend. This patch is correct.

ELF/Arch/X86.cpp

 }
 
 void X86::relocateOne(uint8_t *Loc, RelType Type, uint64_t Val) const {
   switch (Type) {
   case R_386_8:
     // R_386_{PC,}{8,16} are not part of the i386 psABI, but they are
     // being used for some 16-bit programs such as boot loaders, so
     // we want to support them.
-    checkUInt(Loc, Val, 8, Type);
+    checkIntUInt(Loc, Val, 8, Type);
     *Loc = Val;
     break;
   case R_386_PC8:
     checkInt(Loc, Val, 8, Type);
     *Loc = Val;
     break;
   case R_386_16:
-    checkUInt(Loc, Val, 16, Type);
+    checkIntUInt(Loc, Val, 16, Type);
     write16le(Loc, Val);
     break;
   case R_386_PC16:
     // R_386_PC16 is normally used with 16 bit code. In that situation
     // the PC is 16 bits, just like the addend. This means that it can
     // point from any 16 bit address to any other if the possibility
     // of wrapping is included.
     // The only restriction we have to check then is that the destination

test/ELF/i386-reloc-16-large-addend.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=i386-pc-linux %s -o %t
+# RUN: ld.lld -Ttext 0x7c00 %t -o %t2
+# RUN: llvm-objdump -s %t2 | FileCheck %s
+
+# CHECK:       Contents of section .text:
+# CHECK-NEXT:  7c00 b800ff
+
+.code16
+.global _start
+_start:
+ movw $_start+0x8300,%ax

test/ELF/i386-reloc-16.s

 // REQUIRES: x86
 
 // RUN: llvm-mc -filetype=obj -triple=i386-pc-linux %S/Inputs/x86-64-reloc-16.s -o %t1
 // RUN: llvm-mc -filetype=obj -triple=i386-pc-linux %S/Inputs/x86-64-reloc-16-error.s -o %t2
 // RUN: llvm-mc -filetype=obj -triple=i386-pc-linux %s -o %t
 // RUN: ld.lld -shared %t %t1 -o %t3
 // RUN: llvm-objdump -s %t3 | FileCheck %s
 
 // CHECK:      Contents of section .text:
 // CHECK-NEXT:   1000 42
 
 // RUN: not ld.lld -shared %t %t2 -o %t4 2>&1 | FileCheck --check-prefix=ERROR %s
-// ERROR: relocation R_386_16 out of range: 65536 is not in [0, 65535]
+// ERROR: relocation R_386_16 out of range: 65536 is not in [-32768, 32767]
 
 .short foo

test/ELF/i386-reloc-8-large-addend.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=i386-pc-linux %s -o %t
+# RUN: ld.lld -Ttext 0x7c %t -o %t2
+# RUN: llvm-objdump -s %t2 | FileCheck %s
+
+# CHECK:       Contents of section .text:
+# CHECK-NEXT:  007c b4ff
+
+.code16
+.global _start
+_start:
+ movb $_start+0x83,%ah

test/ELF/i386-reloc-8.s

 // REQUIRES: x86
 
 // RUN: llvm-mc -filetype=obj -triple=i386-pc-linux %S/Inputs/i386-reloc-8.s -o %t1
 // RUN: llvm-mc -filetype=obj -triple=i386-pc-linux %S/Inputs/i386-reloc-8-error.s -o %t2
 // RUN: llvm-mc -filetype=obj -triple=i386-pc-linux %s -o %t
 // RUN: ld.lld -shared %t %t1 -o %t3
 // RUN: llvm-objdump -s %t3 | FileCheck %s
 
 // CHECK:      Contents of section .text:
 // CHECK-NEXT:   1000 ff
 
 // RUN: not ld.lld -shared %t %t2 -o %t4 2>&1 | FileCheck --check-prefix=ERROR %s
-// ERROR: relocation R_386_8 out of range: 256 is not in [0, 255]
+// ERROR: relocation R_386_8 out of range: 256 is not in [-128, 127]
 
 .byte foo