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[185.56.211.78]) by smtp.gmail.com with ESMTPSA id z26sm1999353ljm.121.2020.11.01.14.28.01 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 01 Nov 2020 14:28:01 -0800 (PST) Date: Sun, 1 Nov 2020 23:27:59 +0100 From: =?utf-8?B?UGF3ZcWC?= Jasiak To: Matthew Wilcox Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, jack@suse.cz Subject: Re: PROBLEM: fanotify_mark EFAULT on x86 Message-ID: <20201101222759.GA25654@gmail.com> References: <20201101212738.GA16924@gmail.com> <20201101213845.GH27442@casper.infradead.org> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="BXVAT5kNtrzKuDFl" Content-Disposition: inline In-Reply-To: <20201101213845.GH27442@casper.infradead.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --BXVAT5kNtrzKuDFl Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On 01/11/20, Matthew Wilcox wrote: > On Sun, Nov 01, 2020 at 10:27:38PM +0100, Pawe=C5=82 Jasiak wrote: > > I am trying to run examples from man fanotify.7 but fanotify_mark always > > fail with errno =3D EFAULT. > >=20 > > fanotify_mark declaration is > >=20 > > SYSCALL_DEFINE5(fanotify_mark, int, fanotify_fd, unsigned int, flags, > > __u64, mask, int, dfd, > > const char __user *, pathname) >=20 > Don't worry about that. You aren't calling the SYSCALL, you're calling > glibc and glibc is turning it into a syscall. >=20 > extern int fanotify_mark (int __fanotify_fd, unsigned int __flags, > uint64_t __mask, int __dfd, const char *__pathn= ame) >=20 > > When=20 > >=20 > > fanotify_mark(4, FAN_MARK_ADD | FAN_MARK_ONLYDIR, > > FAN_CREATE | FAN_ONDIR, AT_FDCWD, 0xdeadc0de) >=20 > 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]); =66rom 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 =3D 4 flags =3D 0x9 mask =3D 0x40000100 dfd =3D 0xffffff9c pathname =3D 0xdeadc0de and in kernel space we have fanotify_fd =3D 4 flags =3D 0x9 mask =3D 0x40000100 dfd =3D 0 pathname =3D 0xffffff9c So all arguments after __u64 mask are shifted by one. It looks similar to https://lists.linux.it/pipermail/ltp/2020-June/017436.h= tml --=20 Pawe=C5=82 Jasiak --BXVAT5kNtrzKuDFl Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAABCgAdFiEENIqkLxDcCMlLMdi0FzfmQudzVTgFAl+fNmwACgkQFzfmQudz VTjRZhAA4fB7nZEsrAZKgFmixIZd5ROhKQIcstuQhSmfyve0iAWRyOS7XzxtNG98 u2qsUkddRorMb2RCp/Tns6zGrUsFQCv+Yy74Td/w91r/GjCHCDdVCq4bU9q1tlsj DGKLCYLVxzFr5h41/xn3TJ9FSMwZI4y+b08FcUKNdclA/jzT1RTzoNPo1q4iSTlI baF4Hx/kalIM7F7qV2zGgXnhTd45uDMJv8UAUN1Ky7h6wHCjZ/DzcBbn1WWGWmyw zJYSybXbAUhBj8VJUHuuubB9zDUWAcohBQ2JmwjxawIgDHmgJ3FLqNx3hkoY8WC6 G7f4gTZbqR7vWbgSEHsM9lg6vVH4VOSaSxvtwJOlB80wc6fTBi/cC/PyUBIP/KDI 8wl7003QMJI4UcrbojE+0QhWxFPRdH364qdGDlcb7B2/q9ckYuy/r5Oe10jVt/k+ f6lvGbXmveehYSgD5KWmF0LjrqtGL0m1EbxaeeJcTimcGCdBHJwxPDQ5tFXk7SM2 hkm2Xl++Jhk7gOrO4dYxrebgOkwYpyoMAPtwHZ2P4GGMdJUOM2FB8Jy2bEvMeeF4 eGsblO2Zy6V5KHRRNq8jt03jc24XDS7OJoiA5vN3RHV58UgLFfeVz6QemuBsInwL 9RC/ebRA8GBZXL6L8mVftY0olWjItiwgDVHwlLceUY/pIjPa1is= =Dvh3 -----END PGP SIGNATURE----- --BXVAT5kNtrzKuDFl--