linux-kernel - Re: [RFC UKL 07/10] x86/signal: Adjust signal handler register values and return frame

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Message-Id: <40484dc2-8da3-486d-9c53-02ae23a50fbb@app.fastmail.com>
Date:   Tue, 04 Oct 2022 10:34:12 -0700
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Ali Raza" <aliraza@...edu>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Cc:     "Jonathan Corbet" <corbet@....net>, masahiroy@...nel.org,
        michal.lkml@...kovi.net,
        "Nick Desaulniers" <ndesaulniers@...gle.com>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...hat.com>, "Borislav Petkov" <bp@...en8.de>,
        "Dave Hansen" <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Kees Cook" <keescook@...omium.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        "Al Viro" <viro@...iv.linux.org.uk>,
        "Arnd Bergmann" <arnd@...db.de>, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        "Steven Rostedt" <rostedt@...dmis.org>,
        "Ben Segall" <bsegall@...gle.com>, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com,
        "Paolo Bonzini" <pbonzini@...hat.com>, jpoimboe@...nel.org,
        linux-doc@...r.kernel.org, linux-kbuild@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-arch@...r.kernel.org,
        "the arch/x86 maintainers" <x86@...nel.org>, rjones@...hat.com,
        munsoner@...edu, tommyu@...edu, drepper@...hat.com,
        lwoodman@...hat.com, mboydmcse@...il.com, okrieg@...edu,
        rmancuso@...edu
Subject: Re: [RFC UKL 07/10] x86/signal: Adjust signal handler register values and
 return frame



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