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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sun, 31 Jan 2021 15:55:54 -0800
Message-ID: <CAHk-=wgOp10DO9jtMC=B=RoTLWe7MFTS5pH4JeZ78-tbqTY1vw@mail.gmail.com>
Subject: Re: [REGRESSION] x86/entry: TIF_SINGLESTEP handling is still broken
To:     Kyle Huey <me@kylehuey.com>
Cc:     Andy Lutomirski <luto@amacapital.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andy Lutomirski <luto@kernel.org>,
        Gabriel Krisman Bertazi <krisman@collabora.com>,
        open list <linux-kernel@vger.kernel.org>,
        "Robert O'Callahan" <rocallahan@gmail.com>
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On Sun, Jan 31, 2021 at 3:35 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> I wonder if the simple solution is to just
>
>  (a) always set one of the SYSCALL_WORK_EXIT bits on the child in
> ptrace (exactly to catch the child on system call exit)
>
>  (b) basically revert 299155244770 ("entry: Drop usage of TIF flags in
> the generic syscall code") and have the syscall exit code check the
> TIF_SINGLESTEP flag

Actually, (b) looks unnecessary - as long as we get to
syscall_exit_work(), the current code will work fine.

So maybe just add a dummy SYSCALL_WORK_SYSCALL_EXIT_TRAP, and set that
flag whenever a singestep is requested for a process that is currently
in a system call?

IOW, make it a very explicit "do TF for system calls", rather than the
old code that was doing so implicitly and not very obviously. Hmm?

                    Linus