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

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Date:   Mon, 13 Aug 2018 13:50:19 +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, Ravi Bangoria wrote:
>
> On 08/11/2018 01:27 PM, Song Liu wrote:
> >> +
> >> +static void delayed_uprobe_delete(struct delayed_uprobe *du)
> >> +{
> >> +       if (!du)
> >> +               return;
> > Do we really need this check?
>
> Not necessary though, but I would still like to keep it for a safety.

Heh. I tried to ignore all minor problems in this version, but now that Song
mentioned this unnecessary check...

Personally I really dislike the checks like this one.

	- It can confuse the reader who will try to understand the purpose

	- it can hide a bug if delayed_uprobe_delete(du) is actually called
	  with du == NULL.

IMO, you should either remove it and let the kernel crash (to notice the
problem), or turn it into

	if (WARN_ON(!du))
		return;

which is self-documented and reports the problem without kernel crash.

> >> +       rc_vma = find_ref_ctr_vma(uprobe, mm);
> >> +
> >> +       if (rc_vma) {
> >> +               rc_vaddr = offset_to_vaddr(rc_vma, uprobe->ref_ctr_offset);
> >> +               ret = __update_ref_ctr(mm, rc_vaddr, is_register ? 1 : -1);
> >> +
> >> +               if (is_register)
> >> +                       return ret;
> >> +       }
> > Mixing __update_ref_ctr() here and delayed_uprobe_add() in the same
> > function is a little confusing (at least for me). How about we always use
> > delayed uprobe for uprobe_mmap() and use non-delayed in other case(s)?
>
>
> No. delayed_uprobe_add() is needed for uprobe_register() case to handle race
> between uprobe_register() and process creation.

Yes.

But damn, process creation (exec) is trivial. We could add a new uprobe_exec()
hook and avoid delayed_uprobe_install() in uprobe_mmap().

Afaics, the really problematic case is dlopen() which can race with _register()
too, right?

Oleg.