linux-kernel - Re: [PATCH 3/4] openrisc: Support floating point user api

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Date:   Tue, 27 Jun 2023 17:41:03 +0100
From:   Stafford Horne <shorne@...il.com>
To:     Szabolcs Nagy <nsz@...t70.net>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux OpenRISC <linux-openrisc@...r.kernel.org>,
        Jonas Bonn <jonas@...thpole.se>,
        Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>,
        Eric Biederman <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        linux-mm@...ck.org
Subject: Re: [PATCH 3/4] openrisc: Support floating point user api

On Mon, Jun 26, 2023 at 11:38:40PM +0200, Szabolcs Nagy wrote:
> * Stafford Horne <shorne@...il.com> [2023-04-18 17:58:12 +0100]:
> > Add support for handling floating point exceptions and forwarding the
> > SIGFPE signal to processes.  Also, add fpu state to sigcontext.
> > 
> > Signed-off-by: Stafford Horne <shorne@...il.com>
> > ---
> ...
> > --- a/arch/openrisc/include/uapi/asm/sigcontext.h
> > +++ b/arch/openrisc/include/uapi/asm/sigcontext.h
> > @@ -28,6 +28,7 @@
> >  
> >  struct sigcontext {
> >  	struct user_regs_struct regs;  /* needs to be first */
> > +	struct __or1k_fpu_state fpu;
> >  	unsigned long oldmask;
> >  };
> 
> this seems to break userspace abi.
> glibc and musl have or1k abi without this field.
> 
> either this is a new abi where binaries opt-in with some marking
> and then the base sigcontext should be unmodified,
> 
> or the fp state needs to be added to the signal frame in a way that
> does not break existing abi (e.g. end of the struct ?) and also
> advertise the new thing via a hwcap, otherwise userspace cannot
> make use of it.
> 
> unless i'm missing something.

I think you are right, I meant to look into this but it must have slipped
though.  Is this something causing you issues or did you just notice it?

I didn't run into issues when running the glibc test suite, but I may have
missed it.

Just moving this to the end of the sigcontext may be all that is needed.

-Stafford