f_op->read seems to be always NULL since Linux 4.1

f_op->read seems to be always NULL since Linux 4.1

[Andreas Hartmann]

Hello!

Given is a module like the following snippet running fine w/ Linux 4.0
and ext4 fs - but doesn't work w/ Linux 4.1 because f->f_op->read is not
defined any more (= NULL). Is this the intended behavior now?

vfs_read(f, buf, 128, &f->f_pos) works fine.


module.c
--------------------------------------------------------------------
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/fs.h>
#include <asm/uaccess.h>

int init_module(void)
{
    struct file *f;
    char buf[128];
    mm_segment_t fs;
    int i;
    int len=128;

    for(i=0;i<len;i++)
        buf[i] = 0;

    printk(KERN_INFO "My module is loaded\n");

    f = filp_open("/etc/fedora-release", O_RDONLY, 0);
    if(f == NULL)
        printk(KERN_ALERT "filp_open error!!.\n");
    else{
        fs = get_fs();
        set_fs(get_ds());

        if (f->f_op->read) {
            f->f_op->read(f, buf, len, &f->f_pos);
            printk(KERN_INFO "buf:%s\n",buf);
            }
        else {
            printk(KERN_INFO "No read method\n");
            }

        set_fs(fs);

    }
    filp_close(f,NULL);
    return 0;
}

void cleanup_module(void)
{
    printk(KERN_INFO "My module is unloaded\n");
}
-----------------------------------------------------------

Makefile:
-----------------------------------------------------------
obj-m += module.o

all:
        make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules

clean:
        make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean
------------------------------------------------------------



Regards,
Andreas

Re: f_op->read seems to be always NULL since Linux 4.1

[Richard Weinberger]

On Sat, Jun 27, 2015 at 7:32 PM, Andreas Hartmann
<andihartmann@01019freenet.de> wrote:
> Hello!
>
> Given is a module like the following snippet running fine w/ Linux 4.0
> and ext4 fs - but doesn't work w/ Linux 4.1 because f->f_op->read is not
> defined any more (= NULL). Is this the intended behavior now?

See __vfs_read().
Your module most not rely on such internals.

-- 
Thanks,
//richard

Re: f_op->read seems to be always NULL since Linux 4.1

[Andreas Hartmann]

On Sat, Jun 27, 2015 at 8:10 PM, Richard Weinberger wrote:
> On Sat, Jun 27, 2015 at 7:32 PM, Andreas Hartmann
> <andihartmann@01019freenet.de> wrote:
[...]
> See __vfs_read().
> Your module most not rely on such internals.

Thanks for your hint to the function which exists since 3.19.

Is there a site out there which lists all relevant changes done for each 
kernel version and the recommendations how to correctly handle them?


Kind regards,
Andreas

Re: f_op->read seems to be always NULL since Linux 4.1

[Richard Weinberger]

Am 28.06.2015 um 08:36 schrieb Andreas Hartmann:
> On Sat, Jun 27, 2015 at 8:10 PM, Richard Weinberger wrote:
>> On Sat, Jun 27, 2015 at 7:32 PM, Andreas Hartmann
>> <andihartmann@01019freenet.de> wrote:
> [...]
>> See __vfs_read().
>> Your module most not rely on such internals.
> 
> Thanks for your hint to the function which exists since 3.19.
> 
> Is there a site out there which lists all relevant changes done for each kernel version and the recommendations how to correctly handle them?

There is no such site.
The only way do deal with the ever changing in-kernel ABI is
bringing your code mainline.

Thanks,
//richard

Re: f_op->read seems to be always NULL since Linux 4.1

[Al Viro]

On Sun, Jun 28, 2015 at 08:36:18AM +0200, Andreas Hartmann wrote:
> On Sat, Jun 27, 2015 at 8:10 PM, Richard Weinberger wrote:
> >On Sat, Jun 27, 2015 at 7:32 PM, Andreas Hartmann
> ><andihartmann@01019freenet.de> wrote:
> [...]
> >See __vfs_read().
> >Your module most not rely on such internals.
> 
> Thanks for your hint to the function which exists since 3.19.
> 
> Is there a site out there which lists all relevant changes done for
> each kernel version and the recommendations how to correctly handle
> them?

localhost.  It's in Documentation/filesystems/porting in the kernel
source.  To quote the relevant entry (not far from the end - they
are in chronological order):

[mandatory]
        never call ->read() and ->write() directly; use __vfs_{read,write} or
        wrappers; instead of checking for ->write or ->read being NULL, look for
        FMODE_CAN_{WRITE,READ} in file->f_mode.

Sometimes TFM to R _is_ in the natural place...