[PATCH] D28857: [ELF] - Allow relative relocations to a absolute value defined in linker script.
George Rimar via Phabricator via llvm-commits
llvm-commits at lists.llvm.org
Wed Jan 18 05:43:57 PST 2017
grimar created this revision.
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.
https://reviews.llvm.org/D28857
Files:
ELF/Relocations.cpp
test/ELF/linkerscript/relative-absolute.s
Index: test/ELF/linkerscript/relative-absolute.s
===================================================================
--- test/ELF/linkerscript/relative-absolute.s
+++ test/ELF/linkerscript/relative-absolute.s
@@ -0,0 +1,30 @@
+# 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
Index: ELF/Relocations.cpp
===================================================================
--- ELF/Relocations.cpp
+++ ELF/Relocations.cpp
@@ -356,6 +356,10 @@
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));
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