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Date:   Sun, 1 Nov 2020 21:38:45 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     =?utf-8?B?UGF3ZcWC?= Jasiak <pawel@jasiak.xyz>
Cc:     linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        jack@suse.cz
Subject: Re: PROBLEM: fanotify_mark EFAULT on x86
Message-ID: <20201101213845.GH27442@casper.infradead.org>
References: <20201101212738.GA16924@gmail.com>
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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.