using -fsplit-stack

[mathieu lacage <mathieu.lacage@cutebugs.net>]

hi,

I would like to use -fsplit-stack in a small application of mine so I
tried a simple test program to see if it can be used to build binaries
that can have an unlimited stack space (since the stack is supposed to
be allocated on the heap on demand, I naively expect it to be limited by
the heap allocator) and run and be debugged on a normal linux
installation.

$ cat > test.c << EOF
#include <stdio.h>
int main(int argc, char *argv[])
{
  char buffer[10000000];
  printf("hello %p\n", &buffer);
  return 0;
}
EOF

$ gcc -fsplit-stack ./test.c -o test
$ ./test
hello 0x7fbecfd58960
Segmentation fault (core dumped)

Obviously, the same test will also segfault without -fsplit-stack but
now, it segfaults with an interesting backtrace:

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7de3af5 in do_lookup_x (new_hash=2090266759,
old_hash=0x7ffff6e7b110, result=0x7ffff6e7b0e0, scope=0x7ffff7ffe5a0,
i=0, flags=1, 
    skip=0x0, undef_map=0x7ffff7ffa9a8) at dl-lookup.c:82
82	dl-lookup.c: No such file or directory.
(gdb) bt
#0  0x00007ffff7de3af5 in do_lookup_x (new_hash=2090266759,
old_hash=0x7ffff6e7b110, result=0x7ffff6e7b0e0, scope=0x7ffff7ffe5a0,
i=0, flags=1, 
    skip=0x0, undef_map=0x7ffff7ffa9a8) at dl-lookup.c:82
#1  0x00007ffff7de4533 in _dl_lookup_symbol_x (undef_name=0x7ffff7816f25
"free", undef_map=0x7ffff7ffa9a8, ref=0x7ffff6e7b170, 
    symbol_scope=0x7ffff7ffad00, version=0x7ffff7fd9180, type_class=1,
flags=1, skip_map=0x0) at dl-lookup.c:739
#2  0x00007ffff7de8784 in _dl_fixup (l=<optimized out>,
reloc_arg=<optimized out>) at ../elf/dl-runtime.c:119
'#3  0x00007ffff7def245 in _dl_runtime_resolve ()
at ../sysdeps/x86_64/dl-trampoline.S:42
#4  0x00007ffff784dac4 in _IO_vfprintf_internal (s=<optimized out>,
format=<optimized out>, ap=<optimized out>) at vfprintf.c:2052
#5  0x00007ffff78588d9 in __printf (format=<optimized out>) at
printf.c:35
#6  0x0000000000400d21 in main ()

It looks like printf cannot find free anymore. Strange.

Because I can imagine the myriad of ways in which this could fail and
lead to this kind of strange hard-to-debug problem, here are a couple of
questions about stack-split itself:

1) I have disassembled the generated code and I see one symbol that
appears to be related to this:__morestack
Is there documentation somewhere on the exact semantics that is supposed
to be provided by this function ? Are there other functions that the
stack-split code will potentially call ?

2) What is the story about interaction with code that is not compiled
with stack-split code ? i.e., can I call non-stack-split code from
stack-split code and vice-versa ?

I read the gcc documentation:

     When code compiled with `-fsplit-stack' calls code compiled
     without `-fsplit-stack', there may not be much stack space
     available for the latter code to run.  If compiling all code,
     including library code, with `-fsplit-stack' is not an option,
     then the linker can fix up these calls so that the code compiled
     without `-fsplit-stack' always has a large stack.  Support for
     this is implemented in the gold linker in GNU binutils release 2.21
     and later.

I think that this means that the gold linker will check that every
library that a binary links to was compiled with split-stack and if not
change the generated code to do something differen. I think that it also
means that GNU linker does not do this and that I need to link my binary
myself with gold. I tried to do this actually but I have not yet been
able to find the right combination of options to make gold find the
binaries that contain __morestack, printf, and __stack_check_fail (yeah,
I am using a dumb ubuntu box)

gold -lc ./test.o -o test
gold: error: ./test.o: could not convert call to '__morestack' to
'__morestack_non_split'
./test.o:test.c:function main: error: undefined reference to
'__morestack'

But, anyway, I can't see how that link-time code change would work for
libraries. Say, binary A links to B and C. A and B  are stack-split. C
is not. What is the linker going to do ?. 

What happens at runtime ? Does the loader perform a similar check ? (if
so, which loader/version ?


3) gdb: Does gcc generate the right kind of dwarf information for gdb to
understand how to unwind a stack-split stack ?

4) Any data on CPU/memory performance for using -fsplit-stack ? i.e.,
what would be the downsides of compiling everything with this feature
on ?

Well, anyway, at this point, I welcome feedback/ideas of any kind :)

thanks for this pretty cool feature,
Mathieu