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

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Message-ID: <32ec2da7ad9b0c65f41b7c3e14fb6d8e1a5c1865.camel@xry111.site>
Date: Tue, 10 Feb 2026 12:56:39 +0800
From: Xi Ruoyao <xry111@...111.site>
To: "H. Peter Anvin" <hpa@...or.com>, 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 Mon, 2026-02-09 at 20:51 -0800, H. Peter Anvin wrote:
> On 2026-02-09 19:11, Xi Ruoyao 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?
> > 
> 
> Well, it looks like this might be a non-manifest bug on x86 *IN THIS CASE*,

But only if after the patch _Unwind_Find_FDE returns NULL here (instead
of a FDE of another function).  I've not verified it on x86.  If glibc
tests (specifically misc/tst-sigcontext-get_pc) work fine things should
be fine.

/* snip */

> Could you file a gcc bug on this?

Will do.


-- 
Xi Ruoyao <xry111@...111.site>