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: George Bosilca (bosilca_at_[hidden])
Date: 2008-10-21 13:26:25


Stephan,

The fix was committed in the trunk (revision 19778). Fixes for the
1.2, as well as the 1.3 are pending.

   Thanks for your help,
     george.

On Oct 20, 2008, at 5:43 PM, George Bosilca wrote:

> Stephen,
>
> I think you're completely right, and that I had a wrong
> understanding of the modulus operator. Based on my memory, I was
> pretty sure that the modulus is ALWAYS positive. Now, even Wikipedia
> seems to contradict me :) They have a pretty good definition of %
> based on the programming language (http://en.wikipedia.org/wiki/Modulo_operation
> ).
>
> I will apply your patch to all places where we use modulus in Open
> MPI. Thanks for your help on this issue.
>
> Thanks,
> george.
>
> On Oct 19, 2008, at 1:43 PM, Stephan Kramer wrote:
>
>> George Bosilca wrote:
>>> Stephan,
>>>
>>> You might be right. intptr_t is a signed type, which allows the
>>> result of % to be potentially negative. However, on the other
>>> side, mod is defined as a size_t which [based on my memory] is
>>> definitively unsigned as it represent a size.
>>>
>>> Did you try to apply your patch to Open MPI ? If yes does it
>>> resolve the issue ?
>>>
>>> george.
>> Yes, I have applied the patch intptr_t -> uintptr_t and it does
>> resolve the issue.
>>
>> I think the way this works, I'm not a C programmer myself, is:
>> - the outcome of the % is a signed and negative number, say -x
>> - this number gets wrapped in the assignment to the signed integer
>> mod: UINT_MAX+1-x
>> - in the subtraction CACHE_LINE_SIZE-mod, the result is wrapped
>> around again, giving CACHE_LINE_SIZE+x
>>
>> Cheers
>> Stephan
>>>
>>> On Oct 16, 2008, at 7:29 PM, Stephan Kramer wrote:
>>>
>>>> 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
>>>>>
>>>>
>>>> _______________________________________________
>>>> 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
>
> _______________________________________________
> devel mailing list
> devel_at_[hidden]
> http://www.open-mpi.org/mailman/listinfo.cgi/devel



  • application/pkcs7-signature attachment: smime.p7s