Re: Recursive SIGSEGV question

[Jonny Grant <jg@jguk.org>]

On 19/03/2019 22:05, Florian Weimer wrote:
> * Jonny Grant:
> 
>> Wanted to ask opinion about the following.
>>
>> Compiling with g++ 8.2.0 and saw the following. The program was in a
>> recursive function call (bug). My test case is attached, although could
>> not reproduce exactly same backtrace.
>>
>> I had a look at https://github.com/lattera/glibc/blob/master/malloc/malloc.c
>>
>> Is there an issue in _int_malloc? or was it most likely just out of
>> memory? Do out of memory issues normally show up as SIGSEGV? I had
>> expected some sort of "out of memory"
> 
> This isn't really a GCC question, _int_malloc looks like something
> that would be part of glibc.
> 
>> This is the log from own software (not attached) :-
>>
>> Program terminated with signal SIGSEGV, Segmentation fault.
>> #0  0x00007faa0e37b30e in _int_malloc (av=av@entry=0x7fa980000020,
>>       bytes=bytes@entry=45) at malloc.c:3557
>> 3557	malloc.c: No such file or directory.
>> [Current thread is 1 (Thread 0x7fa997860700 (LWP 20571))]
>> (gdb) bt
>> #0  0x00007faa0e37b30e in _int_malloc (av=av@entry=0x7fa980000020,
>>       bytes=bytes@entry=45) at malloc.c:3557
>> #1  0x00007faa0e37e2ed in __GI___libc_malloc (bytes=45) at malloc.c:3065
>> #2  0x00007faa0eba21a8 in operator new(unsigned long) ()
>>      from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
> 
> How does hit go on after that?  Where does the fault actually happen?
> 
> See:
> 
> (gdb) print $_siginfo._sifields._sigfault
> 
> Usually that's heap corruption.  For example, the application might
> have overrun a buffer overwritten some internal malloc data
> structures.
> 
> If you can reproduce it at will, valgrind is a great diagnostic tool
> for such issues.
> 
>> I tried to create a test case, but got slightly different messages, they
>> actually vary. Is there a gdb bug if the same program has different
>> backtraces?
>> GNU gdb (Ubuntu 8.1-0ubuntu3) 8.1.0.20180409-git
>>
>> Core was generated by `./loop'.
>> Program terminated with signal SIGSEGV, Segmentation fault.
>> #0  0x00007fc10dee51e7 in void std::__cxx11::basic_string<char,
>> std::char_traits<char>, std::allocator<char>
>>   >::_M_construct<char*>(char*, char*, std::forward_iterator_tag) ()
>>      from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
>> (gdb) bt
>> #0  0x00007fc10dee51e7 in void std::__cxx11::basic_string<char,
>> std::char_traits<char>, std::allocator<char>
>>   >::_M_construct<char*>(char*, char*, std::forward_iterator_tag) ()
>>      from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
>> #1  0x00005592fbb669d7 in func (f="a", g=0) at loop.cpp:7
>> #2  0x00005592fbb669e8 in func (f="a", g=0) at loop.cpp:7
>> #3  0x00005592fbb669e8 in func (f="a", g=0) at loop.cpp:7
> 
> This looks like a very different thing.  Due to the deep recursion,
> the code faults when accessing the guard page below the stack.
> 

Thanks for your reply Florian.

I guess I was just expecting GCC to generate code that avoided 
overrunning the stack (or heap) and exiting gracefully. I don't know if 
that is gcc, glibc, or kernel. Or if it's just down the program!

I'll look into this a bit more.

Jonny