linux-kernel - Re: Why do kprobes and uprobes singlestep?

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Message-ID: <20210302202806.GB21871@redhat.com>
Date:   Tue, 2 Mar 2021 21:28:07 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S. Miller" <davem@...emloft.net>, X86 ML <x86@...nel.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Subject: Re: Why do kprobes and uprobes singlestep?

forgot to add Srikar, sorry for resend...

On 03/01, Andy Lutomirski wrote:
>
> On Mon, Mar 1, 2021 at 8:51 AM Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > But I guess this has nothing to do with uprobes, they do not single-step
> > in kernel mode, right?
>
> They single-step user code, though, and the code that makes this work
> is quite ugly.  Single-stepping on x86 is a mess.

But this doesn't really differ from, say, gdb doing si ? OK, except uprobes
have to hook DIE_DEBUG. Nevermind...

> > > Uprobes seem to single-step user code for no discernable reason.
> > > (They want to trap after executing an out of line instruction, AFAICT.
> > > Surely INT3 or even CALL after the out-of-line insn would work as well
> > > or better.)
> >
> > Uprobes use single-step from the very beginning, probably because this
> > is the most simple and "standard" way to implement xol.
> >
> > And please note that CALL/JMP/etc emulation was added much later to fix the
> > problems with non-canonical addresses, and this emulation it still incomplete.
>
> Is there something like a uprobe test suite?

Afaik, no.

> How maintained /

Add Srikar who sent the initial implementation. I can only say that I am glad that
./scripts/get_maintainer.pl no longer mentions me ;) I did some changes (including
emulation) but a) this was a long ago and b) only because I was forced^W asked to
fix the numerous bugs in this code.

> actively used is uprobe?

I have no idea, sorry ;)

Oleg.