Re: [B.A.T.M.A.N.] [PATCH] [batman] Add padding around allocation debugger structures

[Sven Eckelmann <sven.eckelmann@gmx.de>]

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On Monday 01 June 2009 20:03:43 Nathan Wharton wrote:
> I had to copy the patches out of the e-mail.
>
> Here is the back trace:
> #0  list_add_tail (new=0x29bf0, head=0x298c9) at list-batman.c:68
> #1  0x0000ee7c in _hna_global_add (orig_node=0x29f80,
> hna_element=0x29ba8) at hna.c:371
> #2  0x0000f160 in hna_global_add (orig_node=0x29f80, new_hna=<value
> optimized out>, new_hna_len=<value optimized out>)
>     at hna.c:529
> #3  0x000099c8 in update_routes (orig_node=0x29f80,
> neigh_node=0x2a080, hna_recv_buff=0xbead1591 "\n\002\001",
>     hna_buff_len=10) at batman.c:377
> #4  0x0000c730 in update_orig (orig_node=0x29f80, in=0xbead157f,
> neigh=167772673, if_incoming=0x27678,
>     hna_recv_buff=0xbead1591 "\n\002\001", hna_buff_len=-16723,
> is_duplicate=0 '\0', curr_time=3199014207)
>     at originator.c:227
> #5  0x0000a7e0 in batman () at batman.c:956
> #6  0x000148d4 in main (argc=14, argv=0xbead1e14) at posix/posix.c:629
>
> Looks like debugMalloc didn't return an aligned value for head.  I'll
> step through that and see what I see.
Ok, I think I see the problem. The malloc returned a valid aligned adress. 
list_add_tail will get a pointer to an element in hna_global_entry. This 
structure is packed and all operations on it should be non-alignment safe. If 
you look at it further you will notice that orig_list is at position 9 
(assuming 4 bytes for a pointer) - which will not be aligned to 4 bytes of 
course.....
And here comes the problem: the compiler will only do the safe operations on 
non-aligned data if it knows that it is not alignent. Since a cast is done by 
calling list_add_tail it will not know that this parameter is not aligned and 
the non-alignment bug will occur.

So my question to marek: Is it really needed to have "struct hna_global_entry" 
packed in hna.h:57? If not then we should remove it and this problem should be 
gone. And what is with "struct hna_element".

Thank you for your work, Nathan :)

Regards,
	Sven

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