Re: what's papered over by set_fs(USER_DS) in amd64 signal delivery?

[Linus Torvalds]

On Fri, Sep 24, 2010 at 8:52 AM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>        What the hell is going on in amd64 handle_signal()?  We do
>
> #ifdef CONFIG_X86_64
>        /*
>         * This has nothing to do with segment registers,
>         * despite the name.  This magic affects uaccess.h
>         * macros' behavior.  Reset it to the normal setting.
>         */
>        set_fs(USER_DS);
> #endif

I _think_ it is historical, and probably relates to just restoring all
the user mode state at signal delivery to a known state. IOW, I think
it really does go hand-in-hand with  the whole "clear bits in the
eflags register" thing.

x86-64 has historically had some left-over crap that we already
cleaned up in 32-bit mode, for the simple reason that the original
x86-64 code was forked from an earlier base, and then hacked up
somewhat. So I think this "#ifdef CONFIG_X86_64" is just a case of
that.

But maybe we should have a WARN_ON_ONCE() to verify it, rather than
just kill it outright.

                                         Linus
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