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head/libexec/rtld-elf/amd64/reloc.c

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void *__tls_get_addr(tls_index *ti) void *__tls_get_addr(tls_index *ti)
{ {
Elf_Addr** segbase; Elf_Addr** segbase;
__asm __volatile("movq %%fs:0, %0" : "=r" (segbase)); __asm __volatile("movq %%fs:0, %0" : "=r" (segbase));
return tls_get_addr_common(&segbase[1], ti->ti_module, ti->ti_offset); return tls_get_addr_common(&segbase[1], ti->ti_module, ti->ti_offset);
} }
size_t
calculate_first_tls_offset(size_t size, size_t align, size_t offset)
{
size_t res;
res = roundup(size, align);
fbsd-phab_maskray.meUnsubmitted
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Considering a PT_TLS segment with p_vaddr == 4, p_align == 8 and p_memsz == 9, the computed offset is 20 (does that mean TP-20?)

LLD computed offset is -12 (matches musl libc)
https://github.com/llvm/llvm-project/blob/main/lld/ELF/InputSection.cpp#L665 The offset equals p_vaddr modulo p_align.

Local-exec TLS offsets are a protocol between the linker and the dynamic loader. Their values must match.

fbsd-phab_maskray.me: Considering a PT_TLS segment with p_vaddr == 4, p_align == 8 and p_memsz == 9, the computed…
kibAuthorUnsubmitted
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Nobody ever answered by query about the right formula. Please provide the algorithm that should be implemented by rtld.

kib: Nobody ever answered by query about the right formula. Please provide the algorithm that…
fbsd-phab_maskray.meUnsubmitted
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https://github.com/llvm/llvm-project/blob/main/lld/ELF/InputSection.cpp#L665 is the right formula.

fbsd-phab_maskray.me: https://github.com/llvm/llvm-project/blob/main/lld/ELF/InputSection.cpp#L665 is the right…
kibAuthorUnsubmitted
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20 mod 8 == 12 mod 8 == 4. Can you provide self-contained elf binary that demonstrates the problem you complain about?

It might be that the problem is not in the calculation, but in the alignment of the end of the tls segment (from which this offset is subtracted).

kib: 20 mod 8 == 12 mod 8 == 4. Can you provide self-contained elf binary that demonstrates the…
fbsd-phab_maskray.meUnsubmitted
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20 mod 8 == 12 mod 8 == 4.

This is not sufficient. For the local-exec TLS model, the linker and the dynamic loader must agree on the TP offset, otherwise if you take the address from an executable (local-exec) and take the address from a DF_STATIC_TLS shared object (initial-exec movq tls@GOTTPOFF(%rip), %rax), the two addresses may mismatch.

Can you provide self-contained elf binary that demonstrates the problem you complain about?

It might be that the problem is not in the calculation, but in the alignment of the end of the tls segment (from which this offset is subtracted).

LLD has a workaround to force p_vaddr%p_align = 0. You can delete the else if (Out::tlsPhdr && Out::tlsPhdr->firstSec == p->firstSec) branch in https://reviews.llvm.org/D64906 , then LLD can make p_vaddr%p_align != 0 if sh_addralign(.tdata) < sh_addralign(.tbss), e.g.

# REQUIRES: x86
# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o
# RUN: ld.lld %t.o -o %t
# RUN: llvm-objdump -d --no-show-raw-insn %t | FileCheck %s

# -alignTo(p_memsz, p_align) = -alignTo(4, 64) = -64

# CHECK: movl %fs:-64, %eax

  movl %fs:a@TPOFF, %eax

.section .tdata,"awT"
a:
.long 0

.section .tbss,"awT",@nobits
.balign 64

I don't spend time to craft an example to make the FreeBSD rtld inconsistent, but with appropriate sh_size(.tdata) you can.

I wrote most stuff in https://maskray.me/blog/2021-02-14-all-about-thread-local-storage

fbsd-phab_maskray.me: > 20 mod 8 == 12 mod 8 == 4. This is not sufficient. For the local-exec TLS model, the linker…
offset &= align - 1;
if (offset != 0)
res += align - offset;
return (res);
}
size_t
calculate_tls_offset(size_t prev_offset, size_t prev_size __unused, size_t size,
size_t align, size_t offset)
{
size_t res;
res = roundup(prev_offset + size, align);
offset &= align - 1;
if (offset != 0)
res += align - offset;
return (res);
}
size_t
calculate_tls_end(size_t off, size_t size __unused)
{
return (off);
}