linux-kernel - Re: [patch V2.1 05/20] x86/extable: Rework the exception table mechanics

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Date:   Tue, 7 Sep 2021 13:37:45 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Tony Luck <tony.luck@...el.com>,
        Song Liu <songliubraving@...com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Peter Ziljstra <peterz@...radead.org>
Subject: Re: [patch V2.1 05/20] x86/extable: Rework the exception table mechanics

On Tue, Sep 7, 2021 at 1:24 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> Rework the exception fixup mechanics by storing a fixup type number instead
> of the handler address and invoke the proper handler for each fixup
> type. Also teach the extable sort to leave the type field alone.

I see why you do this, but it does make the exception handlers less
flexible. It used to be that you could just make up any exception
handler without having to inform some centralized place about it.

But I guess that's also a downside, and maybe we don't want the extra
flexibility.

The bpf case was an example where somebody was able to add their own
specialized handler, and didn't need to serialize with anybody else.

So I don't hate this, I'm just ambivalent about it. On the one hand I
like the more strict format, on the other hand it's kind of sad.

The other reaction I have is that the handler type is now wasteful and
pointless. I think you could make it be about which *section* the
exception thing is in, and not need that extra ".long" argument to
hold a couple of bits of data.

So _ASM_EXTABLE_TYPE() could instead do something like this:

  # define _ASM_EXTABLE_TYPE(from, to, type)                     \
          .pushsection "__ex_table." #type ,"a" ;                         \
          .balign 4 ;                                             \
          .long (from) - . ;                                      \
          .long (to) - . ;                                        \
          .popsection

and we'd not have any 'type' field in there at all, because the type
would be determined by the section is is in.

Hmm?

                Linus