Fri 10 Oct 2008 02:21:56 PM UTC, original submission:
I don't know if this is a duplicate for #23309. Since we have a detailed error description here (thanks to Ed), this one is easy to solve.
Ed Sutter described it on lwip-users, here's the mail:
Ok, I've spoken to a few folks off-line about this, and have narrowed it
down a bit further. There's either a bug or I'm not configuring something
properly...
I'm running LWIP 1.3.0 on a Blackfin, which is picky about accessing longs
on long-word boundaries. I have MEM_ALIGNMENT set to 4. My network generates
occasional fragmented IP packets, and it causes the function
ip_reass_chain_frag_into_datagram_and_validate() to crash.
Here's the flow...
The packet is received and buffered up using pbuf_alloc() (with each payload
aligned on a 4-byte boundary) then that set of pbufs is passed to ethernet_input().
The ethernet_input() function then sees that it is IP, increments the payload
pointer by 14 (size of ethernet header) and passes the pbufs to ip_input().
Note that at this point, the payload pointer is no longer aligned on a 4-byte
boundary because the original 4-byte-aligned payload pointer is incremented
by 14.
Next, ip_input() sees that the packet is a fragment, so it calls ip_reass()
which then calls ip_reass_chain_frag_into_datagram_and_validate(). About 15
lines from the top, this function overlays a "helper" structure pointer on
the payload area (which is no longer 4-byte aligned). The helper structure's
first member is a pointer which is initialized to NULL, this causes the crash
because loading the pointer is a 4-byte-wide access, but the pointer is not on
a 4-byte aligned address.
This is only seen on an alignment-sensitive processor when an fragment is
received. I made a simple change to the helper structure to fix this (see
below); however, I would like to make sure that there isn't some other
configuration parameter that I should be setting to avoid this.
I changed this...
struct ip_reass_helper {
struct pbuf *next_pbuf; <<<< this member access is not aligned
u16_t start;
u16_t end;
};
to this...
struct ip_reass_helper {
PACK_STRUCT_FIELD(u16_t start);
PACK_STRUCT_FIELD(struct pbuf *next_pbuf); <<<< now it is
PACK_STRUCT_FIELD(u16_t end);
};
and my crash has disappeared. Putting the 'start' member above the next_pbuf
pointer will naturally align the pointer, which as far as I can tell will
always be mis-aligned by 2 bytes when MEM_ALIGNMENT is set to 4.
Is this a valid fix or am I just configuring something incorrectly to cause
this crash?
Sorry for the verbosity!
Ed
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