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

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Message-ID: <29D95765-BD93-451D-8FD8-54250ADE1DEB@zytor.com>
Date: Mon, 09 Feb 2026 20:15:46 -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 February 9, 2026 7:11:25 PM PST, Xi Ruoyao <xry111@...111.site> wrote:
>On Mon, 2026-02-02 at 19:57 -0800, H. Peter Anvin wrote:
>> That hack dates back from before the signal frame extension. It is no
>> longer necessary.
>
>Unfortunately at least it seems libgcc unwinder does not handle the
>signal frame extension properly.  The code reads:
>
>  fde = _Unwind_Find_FDE (context->ra + _Unwind_IsSignalFrame (context) - 1, 
>                          &context->bases);
>  if (fde == NULL)
>    {    
>#ifdef MD_FALLBACK_FRAME_STATE_FOR
>      /* Couldn't find frame unwind info for this function.  Try a
>         target-specific fallback mechanism.  This will necessarily
>         not provide a personality routine or LSDA.  */
>      return MD_FALLBACK_FRAME_STATE_FOR (context, fs); 
>#else
>      return _URC_END_OF_STACK;
>#endif
>    }    
>
>  fs->pc = context->bases.func;
>
>  cie = get_cie (fde);
>  insn = extract_cie_info (cie, context, fs); 
>
>Thus, it indeed attempts to avoid subtracting 1 for a signal frame, but
>... _Unwind_IsSignalFrame (context) actually extracts a flag in context
>which will only be raised up by extract_cie_info.
>
>Or am I missing something here?
>

Oh, good grief...

How does this possibly work on non-x86 platforms?