Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Feedback-ID: ieff94742:Fastmail
User-Agent: Cyrus-JMAP/3.7.0-alpha0-1015-gaf7d526680-fm-20220929.001-gaf7d5266
Mime-Version: 1.0
Message-Id: <40484dc2-8da3-486d-9c53-02ae23a50fbb@app.fastmail.com>
In-Reply-To: <20221003222133.20948-8-aliraza@bu.edu>
References: <20221003222133.20948-1-aliraza@bu.edu>
 <20221003222133.20948-8-aliraza@bu.edu>
Date:   Tue, 04 Oct 2022 10:34:12 -0700
From:   "Andy Lutomirski" <luto@kernel.org>
To:     "Ali Raza" <aliraza@bu.edu>,
        "Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>
Cc:     "Jonathan Corbet" <corbet@lwn.net>, masahiroy@kernel.org,
        michal.lkml@markovi.net,
        "Nick Desaulniers" <ndesaulniers@google.com>,
        "Thomas Gleixner" <tglx@linutronix.de>,
        "Ingo Molnar" <mingo@redhat.com>, "Borislav Petkov" <bp@alien8.de>,
        "Dave Hansen" <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        "Kees Cook" <keescook@chromium.org>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        "Al Viro" <viro@zeniv.linux.org.uk>,
        "Arnd Bergmann" <arnd@arndb.de>, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        "Steven Rostedt" <rostedt@goodmis.org>,
        "Ben Segall" <bsegall@google.com>, mgorman@suse.de,
        bristot@redhat.com, vschneid@redhat.com,
        "Paolo Bonzini" <pbonzini@redhat.com>, jpoimboe@kernel.org,
        linux-doc@vger.kernel.org, linux-kbuild@vger.kernel.org,
        linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
        linux-arch@vger.kernel.org,
        "the arch/x86 maintainers" <x86@kernel.org>, rjones@redhat.com,
        munsoner@bu.edu, tommyu@bu.edu, drepper@redhat.com,
        lwoodman@redhat.com, mboydmcse@gmail.com, okrieg@bu.edu,
        rmancuso@bu.edu
Subject: Re: [RFC UKL 07/10] x86/signal: Adjust signal handler register values and
 return frame
Content-Type: text/plain
Precedence: bulk


On Mon, Oct 3, 2022, at 3:21 PM, Ali Raza wrote:
> For a UKL thread, returning to a signal handler is not done with iret or
> sysret.  This means we need to adjust the way the return stack frame is
> handled for these threads.  When constructing the signal frame, we leave
> the previous frame in place because we will return to it from the signal
> handler.  We also leave space for pushing eflags and the return address.
> UKL threads will only use the __KERNEL_DS value in the ss register and 0xC3
> in the cs register.

This is unclear.  Are you taking about returning from the kernel fault code *to* the signal handler or are you talking about returning *from* the user signal hander to the user code that was running when the signal happened?

In any case, I don't see what this has to do with iret or sysret.  Surely UKL can use a sigreturn() just like regular Linux.

The part where a UKL thread has permission to return to a CPL0 context should be a separate patch.

--Andy