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Message-ID: <CAAYN94YMKp71ixPTfcAhK0sMubtYTM4Yxoo5BpX9MW-N2aSWXA@mail.gmail.com>
Date: Mon, 6 Jul 2026 18:25:33 +0400
From: Andrey Smirnov <smirnov.andrey@...il.com>
To: musl@...ts.openwall.com
Subject: aarch64: TLSDESC to a NEEDED shared library resolves via
__tlsdesc_static to an offset outside the main thread's static TLS block
(SIGSEGV at startup)
We hit a rustc crash (1.96.1) compiled with muscl on aarch64 (it works
just fine on x86_64).
This is a report of our findings so far, but we might be missing something.
Summary: a thread-local in a NEEDED (startup) shared lib is accessed
via TLSDESC; musl wires the descriptor to __tlsdesc_static with tpoff
0x5f88, but that offset is past the main thread's allocated static TLS
block, so the access faults. Same program on x86_64/musl works because
it uses traditional GD (__tls_get_addr, dynamic TLS) rather than
TLSDESC.
Environment: musl 1.2.6, aarch64 (TLS variant I), dynamically linked;
crashing binary is rustc, thread-local in NEEDED librustc_driver-*.so
(PT_TLS memsz 0x5f90 ≈ 24 KiB); LLVM 22 (aarch64 defaults to TLSDESC).
Symptom + ptrace evidence: the faulting instruction is the standard
aarch64 TLSDESC sequence; at the fault: x0(tpoff)=0x5f88, x27(TP)=…,
si_addr = TP+0x5f88 (unmapped). Descriptor slot0 = &__tlsdesc_static,
slot1 = 0x5f88. /proc/self/maps shows only ~12 KiB mapped around TP,
so TP+0x5f88 (~24 KiB) is out of bounds.
x86_64 comparison — same symbol uses R_X86_64_DTPMOD64/DTPOFF64
(__tls_get_addr), never touches the static block → works.
There is no minimal C reproducer: a 32 KiB __thread in a NEEDED lib,
constructor access, dlopen, and initial-exec TLS each work in
isolation on this musl; only the real large librustc_driver triggers
it.
Question: is musl expected to place a large NEEDED lib's TLS in the
main-thread static block and resolve TLSDESC via __tlsdesc_static,
sized to cover it? Or must toolchains avoid
TLSDESC-to-large-NEEDED-lib on aarch64 (emit traditional GD like
x86_64)?
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