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SubjectRe: access_ok inside kernelspace -- part II --
Date

Ges anyone download the sample code to test this behavior,
or does anyone know why this happens??

Vasili

On Thu, 21 Oct 1999, Vasili Goutas wrote:
>Hi,
>
>I'm sorry contacting you again for the same thing, but I get some strange
>thinks by using the get_fs()/set_fs() makros.
>
>I tryed it out it in the driver I developed for the company I
>work for, by using set_fs()/get_fs() to be able to call sys_open(),
>sys_write(), sys_read() and sys_close.
>At the beginning all seems ok, but after unloading at some time I get
>segmantation faults. The seq. faults came from processes like sendmail or
>cron, and I wandered why.
>I started a test running top on one shell and unloading the driver on an other,
>and I got the seg. faults faster (and every time from the top process) then
>without running top.
>I have to say, that my driver is programmed in C++ and I created a Class for
>FileAccess. To be sure that it has nothing to do with C++ I wrote a litle
>driver which does nothing exept file access and memory
>allocation/deallocation if special ioctl commands are called, in the form of
>init_module:
> fs=get_fs();
> set_fs(get_ds());
> printk("orig. fs.seg = 0x%X, new fs.seg = 0x%X\n", fs.seg, get_fs().seg);
> /* User space is now the kernel */
>
> fd = sys_open(logfile, flags, mode);
> printk("sys_open(%s, 0x%X, 0x%X) returned %i\n", logfile, flags, mode, fd);
>
> if(fd >= 0)
> {
> err = sys_write (fd, writebuf, blasize);
> printk("sys_write(%i, %s, %i) returned %i\n", fd, writebuf, blasize, err);
> err = sys_close(fd);
> printk("sys_close(%i) returned %i\n", fd, err);
> }
>
> set_fs(fs);
>
>
>cleanup_module:
> fs=get_fs();
> set_fs(get_ds());
> /* User space is now the kernel */
> printk("orig. fs.seg = 0x%X, new fs.seg = 0x%X\n", fs.seg, get_fs().seg);
> fd = sys_open(logfile, flags, mode);
> printk("sys_open(%s, 0x%X, 0x%X) returned %i\n", logfile, flags, mode, fd);
> if(fd)
> {
> err = sys_read(fd, readbuffer, 256);
> printk("sys_read(%i, ...) returned %i\n", fd, err);
> printk("readbuffer = %s\n", readbuffer);
>
> err = sys_close(fd);
> printk("sys_close(%i) returned %i\n", fd, err);
> }
> /* Put it back again */
> set_fs(fs);
>
>
>ioctl:
> switch(cmd){
> case TEST_ALLOC:
> testpointer=(char*)vmalloc((unsigned long)arg);//, GFP_KERNEL);
> printk("testpointer = 0x%X\n");
> (unsigned long)rc = (unsigned long)testpointer;
> break;
> case TEST_FREE:
> vfree(testpointer);
> printk("testpointer free\n");
> (unsigned long)rc = (unsigned long)testpointer;
> break;
> default:
> printk("unknown command\n");
> rc -1;
> break;
> }
> return rc;
>
>and a litle program which calls the ioctl commands.
>
>If I insmod/remove the driver nothing special hapens.
>Only some file operations are done.
>Oct 21 14:56:16 debug kernel: init
>Oct 21 14:56:16 debug kernel: orig. fs.seg = 0xC0000000, new fs.seg = 0xFFFFFFFF
>Oct 21 14:56:16 debug kernel: sys_open(/tmp/driverlog.txt, 0x442, 0x1C0) returned 4
>Oct 21 14:56:16 debug kernel: sys_write(4, bla bla from kernel driver
>Oct 21 14:56:16 debug kernel: , 27) returned 27
>Oct 21 14:56:16 debug kernel: sys_close(4) returned 0
>Oct 21 14:56:31 debug kernel: cleanup
>Oct 21 14:56:31 debug kernel: orig. fs.seg = 0xC0000000, new fs.seg = 0xFFFFFFFF
>Oct 21 14:56:31 debug kernel: sys_open(/tmp/driverlog.txt, 0x0, 0x0) returned 3
>Oct 21 14:56:31 debug kernel: sys_read(3, ...) returned 81
>Oct 21 14:56:31 debug kernel: readbuffer = bla bla from kernel driver
>Oct 21 14:56:31 debug kernel: bla bla from kernel driver
>Oct 21 14:56:31 debug kernel: bla bla from kernel driver
>Oct 21 14:56:31 debug kernel:
>Oct 21 14:56:31 debug kernel: sys_close(3) returned 0
>
>
>If I insmod it and run the program, I get a seg. fault realy fast.
>After that the machine is dead at all and can only powered off.
>Oct 21 15:04:50 debug kernel: init
>Oct 21 15:04:50 debug kernel: orig. fs.seg = 0xC0000000, new fs.seg = 0xFFFFFFFF
>Oct 21 15:04:50 debug kernel: sys_open(/tmp/driverlog.txt, 0x442, 0x1C0) returned 4
>Oct 21 15:04:50 debug kernel: sys_write(4, bla bla from kernel driver
>Oct 21 15:04:50 debug kernel: , 27) returned 27
>Oct 21 15:04:50 debug kernel: sys_close(4) returned 0
>Oct 21 15:04:59 debug kernel: Unable to handle kernel paging request at virtual address ca758b84
>Oct 21 15:04:59 debug kernel: current->tss.cr3 = 03062000, %cr3 = 03062000
>Oct 21 15:04:59 debug kernel: *pde = 00000000
>Oct 21 15:04:59 debug kernel: Oops: 0000
>Oct 21 15:04:59 debug kernel: CPU: 0
>
>
>An other problem is that the file descriptor cannot be createt ones and used in
>different functions. I tryed getting the file descr. in init_module with
>sys_open() stored it and reused it in cleanup_module() by sys_read() and
>sys_close(), but I get back -EBADF
>in sys_read the lines returning that are
> ret = -EBADF;
> file = fget(fd);
> if (!file)
> goto bad_file;
>
>If the file descr. is created inside the function it needs, all file accesses
>works fine.
>
>I created a patch for the sysbols which have to be exported and put it together
>with the source of the simple driver and the testprogram on our ftp server.
>You can download it from ftp.ratio.de/linux-file-access/ if you want.
>
>On my first posting 'access_ok inside kernel' people discussed the use of
>sys_xxx() calls insted of calling __syscallX.
>I would use the __syscall functions if I knew how. If someone can tell me how I
>would try to use them, but I think there have to be used the
>get_fs()/set_fs() makros too, so it will be the same problem.
>
>
>I hope anybody will help me getting access to the filesystem within the
>kernel.
>
>TIA
>
>Vasili
>
>
>On Mon, 18 Oct 1999, you wrote:
>>> I patched the 2.2.12 kernel to be able to write to the file system within my
>>> driver.
>>> I do this by patching the copy_to/from_user functions like this
>>
>>You don't need to do this
>>
>>> If someone has written such a function, or knows which already implemented
>>> function of Linux does that, please inform me.
>>
>>
>> mm_segment_t fs;
>>
>> fs=get_fs();
>> set_fs(get_ds());
>>
>> /* User space is now the kernel */
>>
>>
>> /* Put it back again */
>>
>> set_fs(fs);
>--
>Vasilios Goutas
>Software-Development
>RATIO Entwicklungen GmbH
>Phone:+49-(0)40-369007-0 Admiralittstr. 59
>Fax:+49-(0)40-369007-25 20459 Hamburg
>Email: mailto:vgo@ratio.de
>
>
>-
>To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
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>Please read the FAQ at http://www.tux.org/lkml/
--
Vasilios Goutas
Software-Development
RATIO Entwicklungen GmbH
Phone:+49-(0)40-369007-0 Admiralittstr. 59
Fax:+49-(0)40-369007-25 20459 Hamburg
Email: mailto:vgo@ratio.de


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