[llvm-bugs] [Bug 40507] New: LLVM and binutils disagree about values of R_AARCH64_TLS_DTPMOD64 and R_AARCH64_TLS_DTPREL64

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=40507

            Bug ID: 40507
           Summary: LLVM and binutils disagree about values of
                    R_AARCH64_TLS_DTPMOD64 and R_AARCH64_TLS_DTPREL64
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: AArch64
          Assignee: unassignedbugs at nondot.org
          Reporter: rprichard at google.com
                CC: arnaud.degrandmaison at arm.com,
                    llvm-bugs at lists.llvm.org, peter.smith at linaro.org,
                    Ties.Stuij at arm.com

Both LLVM (ELFRelocs/AArch64.def) and the AArch64 ELF ABI spec (IHI0056B) list
the values of the R_AARCH64_TLS_DTPREL64 and R_AARCH64_TLS_DTPMOD64 relocations
as:
 - R_AARCH64_TLS_DTPREL64 == 1028
 - R_AARCH64_TLS_DTPMOD64 == 1029

binutils and glibc (and other projects) instead use these flipped values:
 - R_AARCH64_TLS_DTPMOD64 == 1028
 - R_AARCH64_TLS_DTPREL64 == 1029

The relocations are used in executables and shared objects, and they're
relocated by a dynamic loader. AFAIK, the effect on LLVM is minimal because
Clang and lld only implement the TLSDESC scheme for dynamic TLS accesses, which
use R_AARCH64_TLSDESC relocations instead of DTPMOD/DTPREL.

The disagreement is observable when using gcc's -mtls-dialect=trad mode with
llvm-readelf, though:

$ cat test.c
__thread int somevar;
int bump() {
  return ++somevar;
}

$ aarch64-linux-gnu-gcc-7 test.c -fpic -shared -mtls-dialect=trad

$ aarch64-linux-gnu-readelf -rW a.out | grep TLS
0000000000010fd0  0000000a00000404 R_AARCH64_TLS_DTPMOD64 0000000000000000
somevar + 0
0000000000010fd8  0000000a00000405 R_AARCH64_TLS_DTPREL64 0000000000000000
somevar + 0

$ llvm-readelf -r a.out | grep TLS
0000000000010fd0  0000000a00000404 R_AARCH64_TLS_DTPREL64 0000000000000000
somevar + 0
0000000000010fd8  0000000a00000405 R_AARCH64_TLS_DTPMOD64 0000000000000000
somevar + 0

musl is using the same relocation values as glibc.

FreeBSD recently added support for these two relocations using the glibc values
in https://reviews.freebsd.org/rS341738. It adds a comment:

    /*
     * !!! BEWARE !!!
     * ARM ELF ABI defines TLS_DTPMOD64 as 1029, and TLS_DTPREL64
     * as 1028. But actual bfd linker and the glibc RTLD linker
     * treats TLS_DTPMOD64 as 1028 and TLS_DTPREL64 1029.
     */
    case R_AARCH64_TLS_DTPREL64: /* efectively is TLS_DTPMOD64 */
        *where += (Elf_Addr)defobj->tlsindex;
        break;
    case R_AARCH64_TLS_DTPMOD64: /* efectively is TLS_DTPREL64 */
        *where += (Elf_Addr)(def->st_value + rela->r_addend);
        break;

FWIW: I'm adding arm64 ELF TLS support to Bionic/Android. For now, I've omitted
the DTPMOD/DTPREL relocations on arm64:
https://android-review.googlesource.com/c/platform/bionic/+/886116

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