Ethereal-dev: Re: [Ethereal-dev] Ethereal crashes while thereal works for my decoder
> I just started finding my way to write decoders, what I don't understand
> is, when I finish load my amf data, the GUI window crashes. If I use
> thereal -r, it works fine. Attached my core dump, and Here's my
> dissector:
>
> dissect_amf(const u_char *pd, int offset, frame_data *fd, proto_tree
> *tree)
> {
> struct amf_hdr *dp;
> int len;
> char to_slot[255];
> char from_slot[255];
> char to_msa[255];
> char to_mae[255];
> char from_msa[255];
> char from_mae[255];
> char priority[255];
> char length[255];
>
> dp = (struct amf_hdr *)&pd[offset];
As Gilbert notes, casting "&pd[offset]" to a pointer to a particular
structure, and then dereferencing the pointer, isn't a good idea, as
there is no guarantee that "&pd[offset]" is properly aligned, and if any
of those fields are integral values longer than one byte, and aren't
aligned on the natural boundary, the attempt to refer to them will cause
a crash on a number of processors.
In addition, if any of the fields in the structure are longer than one
byte long, the *byte order* of those fields will matter, and you'll want
to refer to them in the byte order they have in the *protocol*, not in
the byte order on the machine on which you're running Ethereal, as
Ethereal runs on machines of both byte orders, and so there is a
reasonable chance that Ethereal will get incorrect values for those
fields on some machines.
As he also noted, you should be using tvbuffs. I would suggest that you
do something such as
/*
* Offsets of fields in an AMF header.
*/
#define FROM_SLOT 0
#define TO_SLOT 2
#define FROM_MSA 4
and so on, #defining values for the offsets within a structure (the
above code is assuming that those fields are all 2-byte fields; you'd
obviously put the correct offset values in there), and do stuff such as
to_slot_val = tvb_get_ntohs(tvb, TO_SLOT);
to fetch the value (that code assumes that the "to_slot" field is a
2-byte big-endian quantity; use "tvb_get_guint8()" if it's 1-byte,
"tvb_get_letohs()" if it's 2-byte little-endian, "tvb_get_ntohl()" if
it's 4-byte big-endian, and "tvb_get_letohl()" if it's little-endian.
