Re: Ancillary data and 32/64 bit processes
Re: Ancillary data and 32/64 bit processes
- Subject: Re: Ancillary data and 32/64 bit processes
- From: Michael Tuexen <email@hidden>
- Date: Wed, 6 Feb 2008 12:44:42 +0100
Hi Terry,
thanks for the note. We will work around this bug in our code and I will
add a release note that people using CMSG_ macros in 64bit applications
should update to 10.6.0 or a newer version of XCode when it becomes
available.
Thanks for letting me know.
Best regards
Michael
On Feb 6, 2008, at 1:14 AM, Terry Lambert wrote:
In my experience, header fixes are typically only released with an
XCode/SDK release or a full OS release, rather than in software
updates.
-- Terry
On Feb 4, 2008, at 11:16 PM, Michael Tuexen wrote:
Hi Terry,
it would be very nice if your fix and an appropriate fix
for the CMSG_* macros could go into some update of
Mac OS X 10.5.?.
Best regards
Michael
On Feb 4, 2008, at 10:33 PM, Terry Lambert wrote:
On Feb 4, 2008, at 12:20 PM, Michael Tuexen wrote:
Hi Kevin,
see my comments in-line.
Best regards
Michael
On Feb 4, 2008, at 7:18 AM, Kevin Van Vechten wrote:
On Feb 3, 2008, at 11:15 AM, Michael Tuexen wrote:
I habe a problem using ancillary data on 64-bit processes...
The macro CMSG_DATA ist defined in /usr/include/sys/socket.h as
#define CMSG_DATA(cmsg) ((unsigned char *)(cmsg) + \
__DARWIN_ALIGN(sizeof(struct
cmsghdr)))
__DARWIN_ALIGN behaves different on 32-bit processes and 64-bit
processes.
Therefore the offset of the CMSG_DATA is 12 bytes on 32-bit
processes
and 16 byte on 64-bit processes. However, the kernel ignores
this difference
and puts the CMSG_DATA at a 12 byte offset.
User processes may be 32-bit or 64-bit, but the kernel always
executes in a 32-bit address space.
Yes, that is the cause of the problem.
Therefore programs using ancillary data work, when compiled for
32-bit,
but do not work when compiled for 64-bit.
I can provide a simple UDP discard server which shows this bug.
Is this a known bug?
Please file a bug <http://developer.apple.com/bugreporter/>.
Even if it's a known issue, additional reports help us gauge the
overall impact of the issue. (A quick check didn't find any
issues reported against CMSG_DATA or DARWIN_ALIGN.)
Done, Bug ID# 5723161
Should it be fixed by changing the CMSG_DATA
macros to behave the same on 32-bit and 64-bit processes or
should
the kernel be changed to adjust the ancillary data?
I suspect the former, since that would preserve the binary
compatibility expectations of 64-bit processes with today's
kernel.
OK, sounds fine.
This problem is crucial for our SCTP NKE, which uses ancillary
data a lot...
As a workaround, you could define your own CMSG_DATA macro.
Yes, for the code we write... To enable a programmer to use the
macros we need
to patch /usr/include/sys/socket.h...
#pragma pack(4)
struct cmsghdr {
socklen_t cmsg_len; /* [XSI] data byte count,
including hdr */
int cmsg_level; /* [XSI] originating protocol
*/
int cmsg_type; /* [XSI] protocol-specific
type */
/* followed by unsigned char cmsg_data[]; */
};
#pragma pack()
You never have an array of cmsghdr, due to cmsg_data[]. If this
were strict C99 code, it wouldn't compile in CodeWarrior, which
doesn't grok correct C99 syntax, but cmsg_data[] would be there as
an unsized array for use with __offsetof() (probably named
__cmsg_data[] to keep it out of the POSIX namespace and make it
visible to the macros that POSIX requires).
If you don't do the packing, then the offset of the header will
differ between the 64 bit user space and the 32 bit kernel space,
and you will effectively start 4 bytes late in the 64 bit code,
which will still cause you problems.
-- Terry
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