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From: Yann Ylavic <ylavic.dev@gmail.com>
Date: Thu, 2 Feb 2017 23:32:56 +0100
Message-ID: <CAKQ1sVMXRG2n2CuNiOWWW6oha+HEsTMC3bnOjNvR4WL4G_PvVw@mail.gmail.com>
Subject: Re: httpd 2.4.25, mpm_event, ssl: segfaults
To: httpd-dev <dev@httpd.apache.org>
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archived-at: Thu, 02 Feb 2017 22:33:03 -0000

On Thu, Feb 2, 2017 at 11:19 PM, Jacob Champion <champion.p@gmail.com> wrote:
>
> Idle thoughts: "Cannot access memory" in this case could be a red herring,
> if Niklas' gdb can't peer into mmap'd memory spaces [1]. It seems reasonable
> that the data in question could be mmap'd, given the nice round address and
> 4 MiB length (equal to APR_MMAP_LIMIT).
>
> That doesn't mean we're looking in the wrong place, though, since SIGBUS can
> also be generated by an out-of-bounds access to an mmap'd region.

Right, looks like the memory has been unmapped though (SIGBUS) before
being (re)used.
Does "EnableMMAP off" help or produce another backtrace?

>
> Niklas, what version of APR are you using? Are you serving large (> 4 MiB)
> static files? I have not been able to reproduce so far (Ubuntu 16.04, httpd
> 2.4.25 + mod_ssl + mpm_event).

The original file bucket comes from mod_cache, and indeed looks larger than 4MB.
If it were (htcache)cleaned while being served, SIGBUS shouldn't
happen still since we hold an fd (and reference) on it...


Regards,
Yann.