linux-kernel - Re: perfmon trouble

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Message-ID: <20180611200601.GW30522@ZenIV.linux.org.uk>
Date:   Mon, 11 Jun 2018 21:06:01 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Stephane Eranian <eranian@...gle.com>
Cc:     willy@...radead.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        hch@...radead.org, LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, "Luck, Tony" <tony.luck@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>, linux-ia64@...r.kernel.org
Subject: Re: perfmon trouble

On Mon, Jun 11, 2018 at 11:51:34AM -0700, Stephane Eranian wrote:

> Thanks Al for the careful analysis. If I understand correctly, the
> problem seems to be that on
> execve the mapping of the sampling buffer is not destroyed and then on
> close, the kernel
> unmaps whatever the new binary had in that address range. The fix
> would be to ensure
> the mmap is destroyed on execve.

mmap *is* destroyed on execve; it's just that perfmon doesn't notice and
then tries to destroy it again on close, nevermind that it's a different
process, different mapping, etc.

> My problem is that I do not have IA64 hw anymore, so whatever the fix,
> I will not be able
> to test this. In the meantime, I agree with Matthew, simply disable
> PERFMON support.

Out of curiosity - what happened to never-went-into-mainline rewrite
of perfmon circa 2008?  perfmon2 sourceforget site has a collection
of those, up to 2.6.29; AFAICS, they did have a somewhat saner syscall
interface (no auto-mmap on creation, no auto-munmap attempts on close).
By this point it obviously would've been too late to resurrect anyway,
but I wonder what happened with it...