----- Original Message -----
> From: "Cyril Hrubis" <chru...@suse.cz>
> To: "Jan Stancek" <jstan...@redhat.com>
> Cc: ltp-list@lists.sourceforge.net
> Sent: Wednesday, 9 September, 2015 3:09:34 PM
> Subject: Re: [LTP] [PATCH] mprotect04: fix powerpc crash when copying
> exec_func
>
> Hi!
> > It doesn't seem reliable:
> >
> > open("/dev/null", O_WRONLY|O_CREAT, 0644) = 3
> > write(3, NULL, 1) = 1
> > close(3) = 0
> >
> > I'm seeing this on RHEL6.7 kernel, I haven't tried latest upstream yet.
>
> Ah, you are right, the memory is not touched in the write_null()
> function at all.
Right, I got confused by "access_ok(VERIFY_READ, buf, count)"
in vfs_write(), but after reading code and comments it's clear it
only checks if range _may_ be valid:
* access_ok: - Checks if a user space pointer is valid
...
* Returns true (nonzero) if the memory block may be valid, false (zero)
* if it is definitely invalid.
*
* Note that, depending on architecture, this function probably just
* checks that the pointer is in the user space range - after calling
* this function, memory access functions may still return -EFAULT.
Regards,
Jan
>
> drivers/char/mem.c:
>
> static ssize_t write_null(struct file *file, const char __user *buf,
> size_t count, loff_t *ppos)
> {
> return count;
> }
>
> --
> Cyril Hrubis
> chru...@suse.cz
>
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