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

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Message-ID: <20230626213840.GA1236108@port70.net>
Date:   Mon, 26 Jun 2023 23:38:40 +0200
From:   Szabolcs Nagy <nsz@...t70.net>
To:     Stafford Horne <shorne@...il.com>
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

* 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.