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#62093 - 22/01/2002 19:14 In-kernel data munging?
wfaulk
carpal tunnel

Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
Hey, Mark (or anyone else who might know) -- I've been working on getting my VNC server into the kernel, and I'm noticing something very weird. In the userland app, I was reading a big chunk of data off the wire directly into a struct that was aligned properly. Now that I'm porting that to a kernel service, it seems that the data is not coming in the same order. Does that make any sense to anyone, or have I totally screwed something up somewhere that I can't find?
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Bitt Faulk

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#62094 - 22/01/2002 21:38 Re: In-kernel data munging? [Re: wfaulk]
mlord
carpal tunnel

Registered: 29/08/2000
Posts: 13628
Loc: Canada
Something must be buggered.. what you report doesn't register with me at all.. Do note, that the arm-gcc seems to screw up royally when declaring variables in "misaligned" sequence, especially for "local" static variables, as in:

int f(void)
{
static char localbuf[21]; // odd-sized static
static long last_jiffies = 0; // this gets misaligned
...
}

The above code will run, but data will not end up in the right places due to the misalignment of "last_jiffies".

I've been bitten by this several times..

Cheers


-ml


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#62095 - 23/01/2002 06:26 Re: In-kernel data munging? [Re: mlord]
wfaulk
carpal tunnel

Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
Nope. I've definitely got it aligned properly. Even got padding variables in the structure. I'll have to map it out and see where I've gone wrong. Thanks.
_________________________
Bitt Faulk

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#62096 - 23/01/2002 09:43 Re: In-kernel data munging? [Re: mlord]
wfaulk
carpal tunnel

Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
I think I figured it out, and as you (and I, really) suspected, it did have to do with alignment, but in a way I've never experienced. I was reading one byte off the wire when many more were pending. I then tried to read a big chunk (something like 20 bytes). For some reason, it skipped a byte between reading the single byte and reading the chunk, as if reads from the network can only occur in 16-bit hunks. If that's the case, I wasn't aware of it, and have never encountered it before. Anyway, the data I was expecting was offset by 8 bits, but because of some htons()-type stuff, it was a little opaque to me at first. Anyway, I think I've got that solved. Now onto further bugs....
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Bitt Faulk

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#62097 - 23/01/2002 10:56 Re: In-kernel data munging? [Re: wfaulk]
wfaulk
carpal tunnel

Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
Now it's skipping past other data for reasons I can't figure. This time it's skipping past a whole 16 bits. Is there some sort of long-word alignment, too?

Edit: Nope. I've verified that it can read odd numbers of 16-bit blocks (at least in other cases occurring milliseconds before). I've even tried changing it so that it's no longer MSG_PEEKing at the first byte, but reading two bytes and holding the second in a buffer for later use. Still didn't fix it. And I'm watching the data fly across the wire with a packet snooper, so it's not like I'm expecting data that's not really being sent. This is being a royal pain.

Edit edit: actually, looking at it some more, I can read in odd bytes in other places as well. This makes no sense.


Edited by wfaulk (23/01/2002 11:33)
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Bitt Faulk

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#62098 - 23/01/2002 21:02 Re: In-kernel data munging? [Re: wfaulk]
mlord
carpal tunnel

Registered: 29/08/2000
Posts: 13628
Loc: Canada
????

For network I/O, most apps can just use sockets. And for an example of socket-level I/O (TCP), have a glance at kftp.c::ksock_rw() from the Hijack patches.

Simple, robust, and no weird stuff..

????

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#62099 - 24/01/2002 05:36 Re: In-kernel data munging? [Re: mlord]
wfaulk
carpal tunnel

Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
Well, that's the weird thing, because I'm using your ksock_rw() and absolutely no other socket calls (besides the ones to open the connection, which I stole from your kftpd.c). I even modified it to printk() about all the bytes it sends and receives and it isn't showing it reading anything else -- it just skips over those bytes.

I don't suppose you'd want to take a look at it, would you?
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Bitt Faulk

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#62100 - 24/01/2002 07:33 Re: In-kernel data munging? [Re: wfaulk]
mlord
carpal tunnel

Registered: 29/08/2000
Posts: 13628
Loc: Canada
Send it on over, and I'll have a peek. If I like it, I might even integrate it..

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