Re: [Libunwind-devel] Minimum Linux Version requirements for libunwind?

Hi Luke,

If I do a nm on libc.so.1 I see that it also has references to uclibc… we have both libc and uclibc on our system. I don’t know if that means that the libc library is really uclibc or if the libc is just augmenting its functionality by linking to the uclibc. It looks like its using uclibc to pull capabilities such as bcopy and other low-level functions from it.

I have no idea on how to determine what type of stack backtracing is being used. All this stuff is a bit beyond my knowledge and understanding. I will see if I can figure it out.

Thanks for your help.

John

From: Luke Diamand <address@hidden>
Sent: Tuesday, May 22, 2018 3:03 PM
To: John Knight <address@hidden>
Cc: address@hidden
Subject: Re: [Libunwind-devel] Minimum Linux Version requirements for libunwind?

On 22 May 2018 at 20:14, John Knight <address@hidden> wrote:
> Thanks Luke for your response. This is good information.
>
> According to the man page for backtrace, it indicates that "backtrace(), backtrace_symbols(), and backtrace_symbols_fd() are provided in glibc since version 2.1". Sorry for any confusion on this.
> Unfortunately, my glibc is apparently older than 2.1. I am not sure exactly how to tell how old... the usual method to try to run the libc.so.1 to get the version number results in a segmentation fault. Ldd --version is not understood by ldd as well. I also tried to use the gnu_get_libc_version() function that is supposed to always work, but it does not exist on my system. I think my glibc is very old... libc-so.1 vs libc-so.6 I am seeing commonly referenced. Sigh.

Or maybe not glibc at all? uclibc or musl?

The release history glibc suggests that if you have glibc2.1, then
it's over fifteen years old!

> So to summarize, my linux kernel is 3.14.77, glibc is old (version ????), and gcc is 4.8.3.
>
> Anyhow, I am being unsuccessful at getting libunwind working too. It prints one stack backtrace step for __clone + 80 and then exits. I know there is one function call from the signal handler to get to the backtrace() function, so this looks bogus. I just need a backtrace capability that works. This is an embedded linux environment and I am restricted as to software updates I can do, so I think I am between a rock and a hard place.

Do you know what stack backtracing method you are using?

- might be frame pointer enabled (so -fno-omit-frame-pointer turned
on), but people tend not to use that as it's a bit slower and larger
- might be using c++ unwind tables, but then you either need to be
compiling c++ code with exceptions enabled, or, if using c, add
-fexceptions or -funwind-tables/-fasynchronous-unwind-tables along
with some linker flags
- using the DWARF debug information, but then you need to give
libunwind access to the debug image, and you need to build libunwind
with support for it

You're probably not using the last of those.

>
> John
>
>
> -----Original Message-----
> From: Luke Diamand <address@hidden>
> Sent: Tuesday, May 22, 2018 1:50 AM
> To: John Knight <address@hidden>
> Cc: address@hidden
> Subject: Re: [Libunwind-devel] Minimum Linux Version requirements for libunwind?
>
> On 22 May 2018 at 00:55, John Knight <address@hidden> wrote:
>> Hi,
>>
>>
>>
>> I am trying to get libunwind to work on an ARM processor running an
>> older version of linx (version 3.14.77). I am wondering if there are
>> any particular requirements of the Linux kernel or supporting
>> libraries that would prohibit me from running on this older kernel?
>> If so, what is the minimum kernel version required for libunwind?
>
> I don't think it has any special kernel dependencies unless you're going to a really old kernel version (?). I'm using libunwind with 3.10.
>
>>
>> This kernel does NOT have support of backtrace()… my understanding is
>> that support came with glibc in version 3.2 which had kernel support
>> for it. The lack of a backtrace has led me to try and use libunwind
>> as an alternative to give me a stack backtrace when my program SEG faults.
>
> I didn't think backtrace was a kernel feature - doesn't it live in glibc? And uses unwind tables generated by gcc/g++ together with the unwinding code in libstdc++ (as per the ABI).
>
> If you're using glibc's backtrace() with unwind tables rather than frame pointer unwinding then you'll need a gcc that's new enough to generate it (and enable generation). That's been mostly there for a decade or more, but I think a bug was fixed recently (gcc 4.9.4/5.2
> timeframe) for some obscure corner-cases.
>
> But back to your question, I think the oldest kernel version I've used libunwind with is 3.0 with glibc=2.18 and gcc=4.9.
>
> Luke
>
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From:	John Knight
Subject:	Re: [Libunwind-devel] Minimum Linux Version requirements for libunwind?
Date:	Tue, 22 May 2018 22:12:58 +0000