Re: PROBLEM: fanotify_mark EFAULT on x86

["Paweł Jasiak" <pawel@jasiak.xyz>]

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On 01/11/20, Matthew Wilcox wrote:
> On Sun, Nov 01, 2020 at 10:27:38PM +0100, Paweł Jasiak wrote:
> > I am trying to run examples from man fanotify.7 but fanotify_mark always
> > fail with errno = EFAULT.
> > 
> > fanotify_mark declaration is
> > 
> > SYSCALL_DEFINE5(fanotify_mark, int, fanotify_fd, unsigned int, flags,
> > 			      __u64, mask, int, dfd,
> > 			      const char  __user *, pathname)
> 
> Don't worry about that.  You aren't calling the SYSCALL, you're calling
> glibc and glibc is turning it into a syscall.
> 
> extern int fanotify_mark (int __fanotify_fd, unsigned int __flags,
>                           uint64_t __mask, int __dfd, const char *__pathname)
> 
> > When 
> > 
> > fanotify_mark(4, FAN_MARK_ADD | FAN_MARK_ONLYDIR,
> >               FAN_CREATE | FAN_ONDIR, AT_FDCWD, 0xdeadc0de)
> 
> The last argument is supposed to be a pointer to a string.  I'm guessing
> there's no string at 0xdeadc0de.

You are right but it's not a problem. 0xdeadc0de is just a _well
known_ address here only for debug purpose.

pathname inside kernel should be a pointer to string located in
user space at 0xdeadc0de but it is equal to 0xffffff9c which is
AT_FDCWD.

If you call

fanotify_mark(fd, FAN_MARK_ADD | FAN_MARK_ONLYDIR, FAN_CREATE |
              FAN_ONDIR, AT_FDCWD, argv[1]);

from example with *valid* pointer at argv[1] you still get EFAULT
because pathname is equal to AT_FDCWD in kernel space -- last argument
is not used.

In my example in user space we have
    fanotify_fd = 4
    flags       = 0x9
    mask        = 0x40000100
    dfd         = 0xffffff9c
    pathname    = 0xdeadc0de

and in kernel space we have
    fanotify_fd = 4
    flags       = 0x9
    mask        = 0x40000100
    dfd         = 0
    pathname    = 0xffffff9c

So all arguments after __u64 mask are shifted by one.

It looks similar to https://lists.linux.it/pipermail/ltp/2020-June/017436.html

-- 

Paweł Jasiak

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