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Re: KPI, mbuf, strange mbuf and mbuf_len strange definition
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Re: KPI, mbuf, strange mbuf and mbuf_len strange definition


  • Subject: Re: KPI, mbuf, strange mbuf and mbuf_len strange definition
  • From: Vincent Lubet <email@hidden>
  • Date: Thu, 8 Dec 2005 11:42:32 -0800

Stéphane,

On Dec 8, 2005, at 11:25 AM, Stephane Sudre wrote:

Could there be a problem with the declaration of mbuf_len?

size_t mbuf_len(mbuf_t mbuf);

size_t is said to be a __darwin_size_t

__darwin_size_t is said to be __SIZE_TYPE__

__SIZE_TYPE__ is said to be long unsigned int (from what I've found with google).

The length field should have been unsigned in the first place but that's not how it was in BSD. Because the detail of mbuf structure is now opaque to the KPI, this inconsistency should not be a problem for kernel extensions.


My problem is that the m_hdr struct is stating (in the xnu source code):

struct m_hdr {
	struct	mbuf *mh_next;		/* next buffer in chain */
	struct	mbuf *mh_nextpkt;	/* next chain in queue/record */
	long	mh_len;			/* amount of data in this mbuf */

so mh_len is signed if I'm not mistaken.

This is a bit "funny" when you're trying to deal with this kind of weird mbuf chain :

p *((struct mbuf *) 0x3d1fd100)
$3 = {
  m_hdr = {
    mh_next = 0x3d221700,
    mh_nextpkt = 0x0,
    mh_len = 0,
    mh_data = 0x3d1fd13b "@\002\001?\b>\006l!E",
    mh_type = 0,
    mh_flags = 0
  },
  ....

p *((struct mbuf *) 0x3d221700)
$5 = {
  m_hdr = {
    mh_next = 0x3d221600,
    mh_nextpkt = 0xfffffff7,
    mh_len = -1,
    mh_data = 0xffffffff <Address 0xffffffff out of bounds>,
    mh_type = 0,
    mh_flags = -1
  },

and you're testing if (mbuf_len( ) < 0) ...

Two questions: Why are you testing for a negative size and how did you get this mbuf chain of free mbufs?


Vincent _______________________________________________
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References: 
 >KPI, mbuf, strange mbuf and mbuf_len strange definition (From: Stephane Sudre <email@hidden>)

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