linux-kernel - Re: [PATCH v8 3/6] Uprobes: Support SDT markers having reference count (semaphore)

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Message-ID: <20180813145121.GD28360@redhat.com>
Date:   Mon, 13 Aug 2018 16:51:22 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
Cc:     Song Liu <liu.song.a23@...il.com>, srikar@...ux.vnet.ibm.com,
        Steven Rostedt <rostedt@...dmis.org>, mhiramat@...nel.org,
        Peter Zijlstra <peterz@...radead.org>, mingo@...hat.com,
        acme@...nel.org, alexander.shishkin@...ux.intel.com,
        jolsa@...hat.com, namhyung@...nel.org,
        open list <linux-kernel@...r.kernel.org>,
        ananth@...ux.vnet.ibm.com,
        Alexis Berlemont <alexis.berlemont@...il.com>,
        naveen.n.rao@...ux.vnet.ibm.com,
        linux-arm-kernel@...ts.infradead.org, linux-mips@...ux-mips.org,
        linux@...linux.org.uk, ralf@...ux-mips.org, paul.burton@...s.com
Subject: Re: [PATCH v8 3/6] Uprobes: Support SDT markers having reference
 count (semaphore)

On 08/13, Oleg Nesterov wrote:
>
> On 08/13, Ravi Bangoria wrote:
> >
> > > But damn, process creation (exec) is trivial. We could add a new uprobe_exec()
> > > hook and avoid delayed_uprobe_install() in uprobe_mmap().
> >
> > I'm sorry. I didn't get this.
>
> Sorry for confusion...
>
> I meant, if only exec*( could race with _register(), we could add another uprobe
> hook which updates all (delayed) counters before return to user-mode.
>
> > > Afaics, the really problematic case is dlopen() which can race with _register()
> > > too, right?
> >
> > dlopen() should internally use mmap() right? So what is the problem here? Can
> > you please elaborate.
>
> What I tried to say is that we can't avoid uprobe_mmap()->delayed_uprobe_install()
> because dlopen() can race with _register() too, just like exec.

I'm afraid I added even more confusion... so let me clarify although I am sure you
understand this better than me ;)

of course, the main reason why we can't avoid uprobe_mmap()->delayed_uprobe_install()
is not the race with _register(), the main reason is that dlopen() can mmap the code
first, then mmap the counter we need to increment.

Again, just like exec, but exec is "atomic" in that it can't return to user-mode until
evrything is done, so we could add another uprobe hook and avoid delayed_uprobe_install
in uprobe_mmap().

Just in case my previous email was not clear.

Oleg.