linux-kernel - Re: [PATCH v4.1 06/10] x86/entry/vdso32: remove open-coded DWARF in sigreturn.S

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Message-ID: <bbd9ee1e-d604-43d6-9cb5-77e2972807ef@zytor.com>
Date: Mon, 9 Feb 2026 20:53:37 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Xi Ruoyao <xry111@...111.site>, Jens Remus <jremus@...ux.ibm.com>
Cc: "Jason A. Donenfeld" <Jason@...c4.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        "Theodore Ts'o" <tytso@....edu>,
        Thomas Weißschuh <thomas.weissschuh@...utronix.de>,
        Xin Li <xin@...or.com>, Andrew Cooper <andrew.cooper3@...rix.com>,
        Andy Lutomirski <luto@...nel.org>, Ard Biesheuvel <ardb@...nel.org>,
        Borislav Petkov <bp@...en8.de>, Brian Gerst <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, James Morse <james.morse@....com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>, Kees Cook <kees@...nel.org>,
        Nam Cao <namcao@...utronix.de>, Oleg Nesterov <oleg@...hat.com>,
        Perry Yuan <perry.yuan@....com>, Thomas Gleixner <tglx@...utronix.de>,
        Thomas Huth <thuth@...hat.com>, Uros Bizjak <ubizjak@...il.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-sgx@...r.kernel.org, x86@...nel.org,
        Indu Bhagat <indu.bhagat@...cle.com>,
        Claudiu Zissulescu-Ianculescu <claudiu.zissulescu-ianculescu@...cle.com>,
        Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>
Subject: Re: [PATCH v4.1 06/10] x86/entry/vdso32: remove open-coded DWARF in
 sigreturn.S

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