[ELF] - Allow relative relocations to a absolute value defined in linker script.

[grimar]

Summary

I faced next error during linking linux kernel:
arch/x86/entry/vdso/vclock_gettime.c:148: relocation R_X86_64_PC32 cannot refer to absolute symbol 'vvar_vsyscall_gtod_data' defined in (internal)

Symbol is defined in script:

 /*
	 * User/kernel shared data is before the vDSO.  This may be a little
	 * uglier than putting it after the vDSO, but it avoids issues with
	 * non-allocatable things that dangle past the end of the PT_LOAD
	 * segment.
	 */
 vvar_start = . - 2 * (1 << 12);
 vvar_page = vvar_start;
/*
<skipped>
 * These variables live in a page of kernel data that has an extra RO
 * mapping for userspace.  Each variable needs a unique offset within
 * that page; specify that offset with the DECLARE_VVAR macro.  (If
 * you mess up, the linker will catch it.)
 */
/* The kernel linker script defines its own magic to put vvars in the
 * right place.
 */
/* DECLARE_VVAR(offset, type, name) */
vvar_vsyscall_gtod_data = vvar_page + 128;

I was able to extract minimal reproducable using the original parts of code
for demonstation of their use case.

main.c:

// https://github.com/torvalds/linux/blob/d33d5a6c88fcd53fec329a1521010f1bc55fa191/arch/x86/include/asm/vgtod.h
// https://github.com/torvalds/linux/blob/5924bbecd0267d87c24110cbe2041b5075173a25/arch/x86/include/asm/vvar.h
// https://github.com/torvalds/linux/blob/d33d5a6c88fcd53fec329a1521010f1bc55fa191/arch/x86/entry/vdso/vclock_gettime.c

struct vsyscall_gtod_data {
	unsigned seq;
};

#define DECLARE_VVAR(offset, type, name)				\
	extern type vvar_ ## name __attribute__((visibility("hidden")));
DECLARE_VVAR(128, struct vsyscall_gtod_data, vsyscall_gtod_data)

#define VVAR(name) (vvar_ ## name)

#define gtod (&VVAR(vsyscall_gtod_data))

void foo() {
  unsigned seq = gtod->seq;
}

script.lds:

SECTIONS { vvar_vsyscall_gtod_data = 0x10000; }

Run:

gcc -shared main.c -S -o main_gcc.s
llvm-mc -filetype=obj -triple=x86_64-pc-linux main_gcc.s -o main.o
ld -shared main.o --script script.lds -o out.lld

bfd accepts this inputs fine, LLD fails with "cannot refer to absolute symbol..." error.
Patch allows to have such relocations.

[grimar]

D29332 prepared instead.

ELF/Relocations.cpp

static bool isStaticLinkTimeConstant(RelExpr E, uint32_t Type,
   // Another special case is MIPS _gp_disp symbol which represents offset
   // between start of a function and '_gp' value and defined as absolute just
   // to simplify the code.
   if (AbsVal && RelE) {
     if (Body.isUndefined() && !Body.isLocal() && Body.symbol()->isWeak())
       return true;
     if (&Body == ElfSym<ELFT>::MipsGpDisp)
       return true;
+    // Sometimes code has relocations to absolute symbol defined in linker script.
+    // Example is linux kernel. Hence we allow it.
+    if (!Body.File)
+      return true;
     error(S.getLocation(RelOff) + ": relocation " + toString(Type) +
           " cannot refer to absolute symbol '" + toString(Body) +
           "' defined in " + toString(Body.File));
     return true;
   }
 
   return Target->usesOnlyLowPageBits(Type);
 }

test/ELF/linkerscript/relative-absolute.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t1.o
+# RUN: echo "SECTIONS { vvar_vsyscall_gtod_data = 0x10000; }" > %t.script
+# RUN: ld.lld -shared %t1.o --script %t.script -o %t
+# RUN: llvm-objdump -d %t | FileCheck %s --check-prefix=DUMP
+# RUN: llvm-readobj -t %t | FileCheck %s --check-prefix=SYMBOL
+
+# DUMP:       Disassembly of section .text:
+# DUMP-NEXT:  foo:
+# DUMP-NEXT:   0: {{.*}} 65530(%rip), %eax
+
+# SYMBOL:     Symbol {
+# SYMBOL:        Name: vvar_vsyscall_gtod_data
+# SYMBOL-NEXT:   Value: 0x10000
+# SYMBOL-NEXT:   Size: 0
+# SYMBOL-NEXT:   Binding: Local
+# SYMBOL-NEXT:   Type: None
+# SYMBOL-NEXT:   Other [
+# SYMBOL-NEXT:     STV_HIDDEN
+# SYMBOL-NEXT:   ]
+# SYMBOL-NEXT:   Section: Absolute (0xFFF1)
+# SYMBOL-NEXT: }
+
+.text
+.globl foo
+.type foo, @function
+foo:
+ movl vvar_vsyscall_gtod_data(%rip), %eax
+
+.hidden vvar_vsyscall_gtod_data