Open MPI logo

Open MPI Development Mailing List Archives

  |   Home   |   Support   |   FAQ   |   all Development mailing list

Subject: Re: [OMPI devel] Possible buffer overrun bug in opal_free_list_grow, called by MPI::Init
From: Stephan Kramer (stephan.kramer_at_[hidden])
Date: 2008-10-16 19:29:08


George Bosilca wrote:
> I did investigate this issue for about 3 hours yesterday. Neither
> valgrind nor efence report any errors on my cluster. I'm using debian
> unstable with gcc-4.1.2. Adding printfs doesn't shows the same output
> as you, all addresses are in the correct range. I went over the code
> manually, and to be honest I cannot imagine how this might happens IF
> the compiler is doing what it is supposed to do.
>
> I run out of options on this one. If you can debug it and figure out
> what's the problem there I'll be happy to hear.
>
> george.
Hi George,

Thanks a lot for your effort of looking into this. I think I've come a
bit further with this. The reproducibility may in fact have to do with
32bit/64 bit differences.
I think the culprit is line 105 of opal_free_list.c:

    mod = (intptr_t)ptr % CACHE_LINE_SIZE;
    if(mod != 0) {
        ptr += (CACHE_LINE_SIZE - mod);
    }

As intptr_t casts to a signed integer, for 32 bit with addresses above
0x7fff ffff the outcome of mod will be negative. Thus ptr will be
increased with more than CACHE_LINE_SIZE, which is not accounted for in
the allocated buffer size in line 93, and a buffer overrun will appear
in the subsequent element loop. This is confirmed with the output of
some debugging statements I've pasted below. Also I haven't come across
the same bug on 64bit machines.

I guess this should be uintptr_t instead?

Cheers
Stephan Kramer

The debugging output:

mpidebug: num_elements = 1, flist->fl_elem_size = 40
mpidebug: sizeof(opal_list_item_t) = 16
mpidebug: allocating 184
mpidebug: allocated at memory address 0xb2d29f48
mpidebug: mod = -40, CACHE_LINE_SIZE = 128

and at point of the buffer overrun/efence segfault in gdb:

(gdb) print item
$23 = (opal_free_list_item_t *) 0xb2d2a000

which is exactly at (over) the end of the buffer: 0xb2d2a000=0xb2d29f48
+ 184

>
> On Oct 14, 2008, at 11:03 AM, Stephan Kramer wrote:
>
>> Would someone mind having another look at the bug reported below? I'm
>> hitting exactly the same problem with Debian Unstable, openmpi
>> 1.2.7~rc2. Both valgrind and efence are indispensable tools for any
>> developper, where both may catch errors the other won't report.
>> Electric fence is especially good at catching buffer overruns as it
>> protects the beginning and end of each allocated buffer. The original
>> bug report shows an undeniable buffer overrun in MPI::Init, i.e. the
>> attached patch prints out exactly the address it's trying to access
>> which is over the end of the buffer. Any help would be much appreciated
>>
>> Stephan Kramer
>>
>>
>>> Patrick,
>>>
>>> I'm unable to reproduce the buffer overrun with the latest trunk. I
>>> run valgrind (with the memchekcer tool) on a regular basis on the
>>> trunk, and I never noticed anything like that. Moreover, I went over
>>> the code, and I cannot imagine how we can overrun the buffer in the
>>> code you pinpointed.
>>>
>>> Thanks,
>>> george.
>>>
>>> On Aug 23, 2008, at 7:57 PM, Patrick Farrell wrote:
>>>
>>> > Hi,
>>> >
>>> > I think I have found a buffer overrun in a function
>>> > called by MPI::Init, though explanations of why I am
>>> > wrong are welcome.
>>> >
>>> > I am using the openmpi included in Ubuntu Hardy,
>>> > version 1.2.5, though I have inspected the latest trunk by eye
>>> > and I don't believe the relevant code has changed.
>>> >
>>> > I was trying to use Electric Fence, a memory debugging library,
>>> > to debug a suspected buffer overrun in my own program.
>>> > Electric Fence works by replacing malloc/free in such
>>> > a way that bounds violation errors issue a segfault.
>>> > While running my program under Electric Fence, I found
>>> > that I got a segfault issued at:
>>> >
>>> > 0xb5cdd334 in opal_free_list_grow (flist=0xb2b46a50, num_elements=1)
>>> > at class/opal_free_list.c:113
>>> > 113 OBJ_CONSTRUCT_INTERNAL(item, flist->fl_elem_class);
>>> > (gdb) bt
>>> > #0 0xb5cdd334 in opal_free_list_grow (flist=0xb2b46a50,
>>> > num_elements=1) at class/opal_free_list.c:113
>>> > #1 0xb5cdd479 in opal_free_list_init (flist=0xb2b46a50,
>>> > elem_size=56, elem_class=0xb2b46e20, num_elements_to_alloc=73,
>>> > max_elements_to_alloc=-1, num_elements_per_alloc=1) at class/
>>> > opal_free_list.c:78
>>> > #2 0xb2b381aa in ompi_osc_pt2pt_component_init
>>> > (enable_progress_threads=false, enable_mpi_threads=false) at
>>> > osc_pt2pt_component.c:173
>>> > #3 0xb792b67c in ompi_osc_base_find_available
>>> > (enable_progress_threads=false, enable_mpi_threads=false) at base/
>>> > osc_base_open.c:84
>>> > #4 0xb78e6abe in ompi_mpi_init (argc=5, argv=0xbfd61f84,
>>> > requested=0, provided=0xbfd61e78) at runtime/ompi_mpi_init.c:411
>>> > #5 0xb7911a87 in PMPI_Init (argc=0xbfd61f00, argv=0xbfd61f04) at
>>> > pinit.c:71
>>> > #6 0x0811ca6c in MPI::Init ()
>>> > #7 0x08118b8a in main ()
>>> >
>>> > To investigate further, I replaced the OBJ_CONSTRUCT_INTERNAL
>>> > macro with its definition in opal/class/opal_object.h, and ran it
>>> > again.
>>> > It appears that the invalid memory access is happening
>>> > on the instruction
>>> >
>>> > ((opal_object_t *) (item))->obj_class = (flist->fl_elem_class);
>>> >
>>> > Investigating further, I modified the source to opal_free_list
>>> > with the attached patch. It adds a few debugging printfs to
>>> > diagnose exactly what the code is doing. The output of the debugging
>>> > statements are:
>>> >
>>> > mpidebug: allocating 216
>>> > mpidebug: allocated at memory address 0xb62bdf28
>>> > mpidebug: accessing address 0xb62be000
>>> > [segfault]
>>> >
>>> > Now, 0xb62be000 - 0xb62bdf28 = 216, which is
>>> > the size of the buffer allocated, and so I think
>>> > this is a buffer overrun.
>>> >
>>> > Steps to reproduce:
>>> >
>>> > a) Install Electric Fence
>>> > b) Compile the following program
>>> >
>>> > #include <stdlib.h>
>>> > #include <unistd.h>
>>> >
>>> > #include <mpi.h>
>>> >
>>> > int main(int argc, char **argv)
>>> > {
>>> > MPI::Init(argc, argv);
>>> > MPI::Finalize();
>>> >
>>> > return 0;
>>> > }
>>> >
>>> > with
>>> >
>>> > mpiCC -o test ./test.cpp
>>> >
>>> > c) gdb ./test
>>> > d) set environment LD_PRELOAD /usr/lib/libefence.so.0.0
>>> > e) run
>>> >
>>> > Hope this helps,
>>> >
>>> > Patrick Farrell
>>> >
>>> > --
>>> > Patrick Farrell
>>> > PhD student
>>> > Imperial College London
>>> > --- openmpi-1.2.5/opal/class/opal_free_list.c 2008-08-23
>>> > 18:35:03.000000000 +0100
>>> > +++ openmpi-1.2.5-modified/opal/class/opal_free_list.c 2008-08-23
>>> > 18:31:47.000000000 +0100
>>> > @@ -90,9 +90,12 @@
>>> > if (flist->fl_max_to_alloc > 0 && flist->fl_num_allocated +
>>> > num_elements > flist->fl_max_to_alloc)
>>> > return OPAL_ERR_TEMP_OUT_OF_RESOURCE;
>>> >
>>> > + fprintf(stderr, "mpidebug: allocating %d\n", (num_elements *
>>> > flist->fl_elem_size) + sizeof(opal_list_item_t) + CACHE_LINE_SIZE);
>>> > alloc_ptr = (unsigned char *)malloc((num_elements * flist-
>>> > >fl_elem_size) +
>>> > sizeof(opal_list_item_t) +
>>> > CACHE_LINE_SIZE);
>>> > + fprintf(stderr, "mpidebug: allocated at memory address %p\n",
>>> > alloc_ptr);
>>> > +
>>> > if(NULL == alloc_ptr)
>>> > return OPAL_ERR_TEMP_OUT_OF_RESOURCE;
>>> >
>>> > @@ -110,7 +113,16 @@
>>> > for(i=0; i<num_elements; i++) {
>>> > opal_free_list_item_t* item = (opal_free_list_item_t*)ptr;
>>> > if (NULL != flist->fl_elem_class) {
>>> > - OBJ_CONSTRUCT_INTERNAL(item, flist->fl_elem_class);
>>> > + do {
>>> > + if (0 == (flist->fl_elem_class)->cls_initialized) {
>>> > + opal_class_initialize((flist->fl_elem_class));
>>> > + }
>>> > + fprintf(stderr, "mpidebug: accessing address %p\n",
>>> > &((opal_object_t *) (item))->obj_class);
>>> > + ((opal_object_t *) (item))->obj_class = (flist-
>>> > >fl_elem_class);
>>> > + fprintf(stderr, "mpidebug: accessing address %p\n",
>>> > &((opal_object_t *) (item))->obj_reference_count);
>>> > + ((opal_object_t *) (item))->obj_reference_count = 1;
>>> > + opal_obj_run_constructors((opal_object_t *) (item));
>>> > + } while (0);
>>> > }
>>> > opal_list_append(&(flist->super), &(item->super));
>>> > ptr += flist->fl_elem_size;
>>> > @@ -119,5 +131,3 @@
>>> > return OPAL_SUCCESS;
>>> > }
>>> >
>>> > -
>>> > -
>>> > _______________________________________________
>>> > devel mailing list
>>> > devel_at_[hidden]
>>> > http://www.open-mpi.org/mailman/listinfo.cgi/devel
>>>
>> _______________________________________________
>> devel mailing list
>> devel_at_[hidden]
>> http://www.open-mpi.org/mailman/listinfo.cgi/devel
>
> _______________________________________________
> devel mailing list
> devel_at_[hidden]
> http://www.open-mpi.org/mailman/listinfo.cgi/devel
>