From: Luke Diamand <ldiamand@roku.com>
To: Mark Wielaard <mark@klomp.org>,
Florian Weimer <fweimer@redhat.com>,
Luke Diamand via Elfutils-devel <elfutils-devel@sourceware.org>
Subject: Re: [EXTERNAL] Re: Hitting g dwfl->lookup_elts limit in report_r_debug, so not all modules show up and backtracing fails
Date: Fri, 12 May 2023 17:55:46 +0100 [thread overview]
Message-ID: <3a148590-1dbe-22a5-e98e-797b173c2a34@roku.com> (raw)
In-Reply-To: <20756d5ff06a0541e51e2f862312f107a60d525d.camel@klomp.org>
On 08/05/2023 17:35, Mark Wielaard wrote:
> Hi Florian, Hi Luke,
>
> On Tue, 2023-05-02 at 09:57 +0200, Florian Weimer via Elfutils-devel
> wrote:
>> * Luke Diamand via Elfutils-devel:
>>
>>> I've got a few cores where report_r_debug() in link_map.c fails to
>>> find all of the modules - for example I had libc.so missing. This
>>> obviously meant that elfutils could not backtrace my core.
>>>
>>> It seems to be related to this code:
>>>
>>> /* There can't be more elements in the link_map list than there are
>>> segments. DWFL->lookup_elts is probably twice that number, so it
>>> is certainly above the upper bound. If we iterate too many times,
>>> there must be a loop in the pointers due to link_map clobberation. */
>>> size_t iterations = 0;
>>>
>>> while (next != 0 && ++iterations < dwfl->lookup_elts)
>>>
>>> I've changed this to just keep going until it reaches
>>> dwfl->lookup_elts*5, which seems to "fix" it, but I feel there must be
>>> a better fix!
>>>
>>> The most recent core I saw with this had lookup_elts=36, and hit 109
>>> iterations of the loop and then backtraced just fine.
>>
>> It's probably another fallout from -z separate-code, which tends to
>> create four LOAD segments. The magic number 5 sounds about right, as
>> gold also has -z text-unlikely-segment, which might result in creating
>> that number of load segments (but I haven't tried).
>
> Wow, that had never occurred to me. Thanks.
>
> Luke does the binary/libraries from which your core file was generated
> contain multiple PT_LOAD segments?
>
> We could add something like:
>
> diff --git a/libdwfl/link_map.c b/libdwfl/link_map.c
> index 06d85eb6..76f23354 100644
> --- a/libdwfl/link_map.c
> +++ b/libdwfl/link_map.c
> @@ -331,11 +331,17 @@ report_r_debug (uint_fast8_t elfclass, uint_fast8_t elfdata,
> int result = 0;
>
> /* There can't be more elements in the link_map list than there are
> - segments. DWFL->lookup_elts is probably twice that number, so it
> - is certainly above the upper bound. If we iterate too many times,
> - there must be a loop in the pointers due to link_map clobberation. */
> + segments. A segment is created for each PT_LOAD and there can be
> + up to 5 per module (-z separate-code, tends to create four LOAD
> + segments, gold has -z text-unlikely-segment, which might result
> + in creating that number of load segments) DWFL->lookup_elts is
> + probably twice the number of modules, so that multiplied by max
> + PT_LOADs is certainly above the upper bound. If we iterate too
> + many times, there must be a loop in the pointers due to link_map
> + clobberation. */
> +#define MAX_PT_LOAD 5
> size_t iterations = 0;
> - while (next != 0 && ++iterations < dwfl->lookup_elts)
> + while (next != 0 && ++iterations < dwfl->lookup_elts * MAX_PT_LOAD)
> {
> if (read_addrs (&memory_closure, elfclass, elfdata,
> &buffer, &buffer_available, next, &read_vaddr,
>
> Does that sound reasonable?
Sorry - I did not see this until just after sending in my patch!
Let me try it with this change and I will re-roll it.
Luke
prev parent reply other threads:[~2023-05-12 16:55 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-04-25 19:00 Luke Diamand
2023-05-02 7:57 ` Florian Weimer
2023-05-08 16:35 ` Mark Wielaard
2023-05-12 16:55 ` Luke Diamand [this message]
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