Re: Recursive SIGSEGV question

[Florian Weimer <fw@deneb.enyo.de>]

* 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.