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Donenfeld" , "Peter Zijlstra (Intel)" , "Theodore Ts'o" , =?UTF-8?Q?Thomas_Wei=C3=9Fschuh?= , Xin Li , Andrew Cooper , Andy Lutomirski , Ard Biesheuvel , Borislav Petkov , Brian Gerst , Dave Hansen , Ingo Molnar , James Morse , Jarkko Sakkinen , Josh Poimboeuf , Kees Cook , Nam Cao , Oleg Nesterov , Perry Yuan , Thomas Gleixner , Thomas Huth , Uros Bizjak , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-sgx@vger.kernel.org, x86@kernel.org, Indu Bhagat , Claudiu Zissulescu-Ianculescu , Heiko Carstens , Vasily Gorbik References: <20260106211856.560186-6-hpa@zytor.com> <223707e2-3231-4037-bd1f-490ddf6aeeb6@linux.ibm.com> <29D95765-BD93-451D-8FD8-54250ADE1DEB@zytor.com> <0ee773dea213f6efbdac03009ee80f350b17c949.camel@xry111.site> Content-Language: en-US, sv-SE From: "H. Peter Anvin" In-Reply-To: <0ee773dea213f6efbdac03009ee80f350b17c949.camel@xry111.site> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 9E4B0140006 X-Stat-Signature: 44ofghb66waid8o5ktf9h6qi6datzwms X-Rspam-User: X-HE-Tag: 1770699260-970720 X-HE-Meta: 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 jCzg1IrX 7SWPb9FqSQOttvk/b4LLudj2GhnGgfpfDJo3J866GfiIEPYeRe3c+MFK/TTdwUte0P3n/OPMT9sPA3o8y7cK/w6Yht2HMRQ8Xp5LeWaCl9AK97jsRGItnKaoFIY6H0yiI4RZqUhWgOScJinls1c7GDRQepoq47hyXV6Np/5HWYztKoHqsr0U9jIpfB24fiBKoEwZiTIwL6AesIL4eAOnb/21T4PWObUjDBDiFbKa0l0qoIiLXvU5jarefs1I5LKLVeZEIIrhnqApWuoL7D/diWeXEe9mFiRN3mVhQNt/68YtDPS2GhOcP4+HFv3YkVRVfLZOjwZ6O9EGGCKwN0oNrPwy6Rg== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2026-02-09 20:45, Xi Ruoyao wrote: >> >> Oh, good grief... >> >> How does this possibly work on non-x86 platforms? > > On ARM64 the vdso does not have eh_frame_hdr at all, on LoongArch > eh_frame_hdr is empty (note that an ampty en_frame_hdr is actually buggy > and I'm trying to fix it), so _Unwind_Find_FDE returns NULL and libgcc > falls back to MD_FALLBACK_FRAME_STATE_FOR, which handles the sigreturn > trampoline using some machine-dependant logic. > > On RISC-V things are more theatrical: the sigreturn trampoline happens > to be at the beginning of the vdso .text section, so after subtracting 1 > from the PC, the result is out of the .text section and so not in any > FDE. Thus _Unwind_Find_FDE returns NULL and libgcc again falls back to > MD_FALLBACK_FRAME_STATE_FOR. > > If the RISC-V sigreturn trampoline was not the first in .text, > subtracting 1 would cause the PC to be in the FDE of the previous > function and then _Unwind_Find_FDE would return that FDE, then RISC-V > would have some big trouble. > > I've not taken a serious look at other architectures yet. > In other words, it is broken on pretty much all architectures and it ends up using the hacky fallbacks on every single platform... -hpa