From mboxrd@z Thu Jan 1 00:00:00 1970 From: Amir Goldstein Subject: Re: [RFC PATCH v2] fs/xattr: add *at family syscalls Date: Mon, 15 May 2023 17:20:46 +0300 Message-ID: References: <20230511150802.737477-1-cgzones@googlemail.com> <20230515-kopfgeld-umkurven-f27be4b68a26@brauner> <20230515-banal-vergab-a7abb53169b5@brauner> Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20221208; t=1684160458; x=1686752458; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:from:to:cc:subject:date :message-id:reply-to; bh=ppVyFGTleG0uV5qrwyuyzbwOGiTPSdwv0HSD6ZTUdHA=; b=Vqpf0hY1OSXAfbyCkLaVdobIWRo+Ylc5gQ58cvxZijNGVqy8NZM26ONjavWlaIj8hl sxcsZRJDABMIpaHJtnlARir32khiKKlG4bvjIOHJe/Hk4C1CDX2jn84SrtqCxoWMNarW q/2i5Z5oBi4g1qbcXj+EK8G5Xzmh0sC/0gV89Jbq8olaU7IMMr+WDbD7r7gR1VtmK7DU uGvxAodrVsfFlTwT21Qxe6t5xmtzIrWAFCYy8Zknj8Rek+hZGWboK5eXK2+WiI9rirFq 6m/16Or+wveWdYVe/dB93HEJAWHq6dgKyfGyLvKPRjhzudSVn5kFdi/E9rW75B2A1HJV hI6w== In-Reply-To: <20230515-banal-vergab-a7abb53169b5@brauner> List-ID: Content-Type: text/plain; charset="utf-8" To: Christian Brauner Cc: =?UTF-8?Q?Christian_G=C3=B6ttsche?= , x86@kernel.org, linux-alpha@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-ia64@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-sh@vger.kernel.org, sparclinux@vger.kernel.org, linux-fsdevel@vger.kernel.org, audit@vger.kernel.org, linux-arch@vger.kernel.org, linux-api@vger.kernel.org, linux-security-module@vger.kernel.org, selinux@vger.kernel.org, Jan Kara , jlayton@kernel.org, cyphar@cyphar.com, Arnd Bergmann On Mon, May 15, 2023 at 4:52=E2=80=AFPM Christian Brauner wrote: > > On Mon, May 15, 2023 at 04:04:21PM +0300, Amir Goldstein wrote: > > On Mon, May 15, 2023 at 1:33=E2=80=AFPM Christian Brauner wrote: > > > > > > On Thu, May 11, 2023 at 05:08:02PM +0200, Christian G=C3=B6ttsche wro= te: > > > > Add the four syscalls setxattrat(), getxattrat(), listxattrat() and > > > > removexattrat(). Those can be used to operate on extended attribut= es, > > > > especially security related ones, either relative to a pinned direc= tory > > > > or on a file descriptor without read access, avoiding a > > > > /proc//fd/ detour, requiring a mounted procfs. > > > > > > > > One use case will be setfiles(8) setting SELinux file contexts > > > > ("security.selinux") without race conditions. > > > > > > > > Add XATTR flags to the private namespace of AT_* flags. > > > > > > > > Use the do_{name}at() pattern from fs/open.c. > > > > > > > > Use a single flag parameter for extended attribute flags (currently > > > > XATTR_CREATE and XATTR_REPLACE) and *at() flags to not exceed six > > > > syscall arguments in setxattrat(). > > > > > > > > Previous approach ("f*xattr: allow O_PATH descriptors"): https://lo= re.kernel.org/all/20220607153139.35588-1-cgzones@googlemail.com/ > > > > v1 discussion: https://lore.kernel.org/all/20220830152858.14866-2-c= gzones@googlemail.com/ > > > > > > > > Signed-off-by: Christian G=C3=B6ttsche > > > > CC: x86@kernel.org > > > > CC: linux-alpha@vger.kernel.org > > > > CC: linux-kernel@vger.kernel.org > > > > CC: linux-arm-kernel@lists.infradead.org > > > > CC: linux-ia64@vger.kernel.org > > > > CC: linux-m68k@lists.linux-m68k.org > > > > CC: linux-mips@vger.kernel.org > > > > CC: linux-parisc@vger.kernel.org > > > > CC: linuxppc-dev@lists.ozlabs.org > > > > CC: linux-s390@vger.kernel.org > > > > CC: linux-sh@vger.kernel.org > > > > CC: sparclinux@vger.kernel.org > > > > CC: linux-fsdevel@vger.kernel.org > > > > CC: audit@vger.kernel.org > > > > CC: linux-arch@vger.kernel.org > > > > CC: linux-api@vger.kernel.org > > > > CC: linux-security-module@vger.kernel.org > > > > CC: selinux@vger.kernel.org > > > > --- > > > > > > Fwiw, your header doesn't let me see who the mail was directly sent t= o > > > so I'm only able to reply to lists which is a bit pointless... > > > > > > > v2: > > > > - squash syscall introduction and wire up commits > > > > - add AT_XATTR_CREATE and AT_XATTR_REPLACE constants > > > > > > > +#define AT_XATTR_CREATE 0x1 /* setxattrat(2): set= value, fail if attr already exists */ > > > > +#define AT_XATTR_REPLACE 0x2 /* setxattrat(2): set value, = fail if attr does not exist */ > > > > > > We really shouldn't waste any AT_* flags for this. Otherwise we'll ru= n > > > out of them rather quickly. Two weeks ago we added another AT_* flag > > > which is up for merging for v6.5 iirc and I've glimpsed another AT_* > > > flag proposal in one of the talks at last weeks Vancouver conference > > > extravaganza. > > > > > > Even if we reuse 0x200 for AT_XATTR_CREATE (like we did for AT_EACCES= S > > > and AT_REMOVEDIR) we still need another bit for AT_XATTR_REPLACE. > > > > > > Plus, this is really ugly since AT_XATTR_{CREATE,REPLACE} really isn'= t > > > in any way related to lookup and we're mixing it in with lookup > > > modifying flags. > > > > > > So my proposal for {g,s}etxattrat() would be: > > > > > > struct xattr_args { > > > __aligned_u64 value; > > > __u32 size; > > > __u32 cmd; > > > }; > > > > > > So everything's nicely 64bit aligned in the struct. Use the @cmd memb= er > > > to set either XATTR_REPLACE or XATTR_CREATE and treat it as a proper > > > enum and not as a flag argument like the old calls did. > > > > > > So then we'd have: > > > > > > setxattrat(int dfd, const char *path, const char __user *name, > > > struct xattr_args __user *args, size_t size, unsigned int = flags) > > > getxattrat(int dfd, const char *path, const char __user *name, > > > struct xattr_args __user *args, size_t size, unsigned int = flags) > > > > > > The current in-kernel struct xattr_ctx would be renamed to struct > > > kernel_xattr_args and then we do the usual copy_struct_from_user() > > > dance: > > > > > > struct xattr_args args; > > > err =3D copy_struct_from_user(&args, sizeof(args), uargs, usize); > > > > > > and then go on to handle value/size for setxattrat()/getxattrat() > > > accordingly. > > > > > > getxattr()/setxattr() aren't meaningfully filterable by seccomp alrea= dy > > > so there's not point in not using a struct. > > > > > > If that isn't very appealing then another option is to add a new flag > > > namespace just for setxattrat() similar to fspick() and move_mount() > > > duplicating the needed lookup modifying flags. > > > Thoughts? > > > > Here is a thought: I am not sure if I am sorry we did not discuss this = API > > issue in LSFMM or happy that we did not waste our time on this... :-/ > > > > I must say that I dislike redefined flag namespace like FSPICK_* > > just as much as I dislike overloading the AT_* namespace and TBH, > > I am not crazy about avoiding this problem with xattr_args either. > > > > A more sane solution IMO could have been: > > - Use lower word of flags for generic AT_ flags > > - Use the upper word of flags for syscall specific flags > > We'd have 16 lower bits for AT_* flags and upper 16 bits for non-AT_* > flags. That might be ok but it isn't great because if we ever extend > AT_* flags into the upper 16 bits that are generally useful for all > AT_* flag taking system calls we'd not be able to use them. And at the > rate people keep suggesting new AT_* flags that issue might arise > quicker than we might think. > > And we really don't want 64 bit flag arguments because of 32 bit > architectures as that gets really ugly to handle cleanly (Arnd has > talked a lot about issues in this area before). > > > > > So if it were up to me, I would vote starting this practice: > > > > + /* Start of syscall specific range */ > > + #define AT_XATTR_CREATE 0x10000 /* setxattrat(2): set > > value, fail if attr already exists */ > > + #define AT_XATTR_REPLACE 0x20000 /* setxattrat(2): set > > value, fail if attr does not exist */ > > > > Which coincidentally happens to be inline with my AT_HANDLE_FID patch..= . > > This is different though. The reason AT_HANDLE_FID is acceptable is > because we need the ability to extend an existing system call and we're > reusing a bit that is already used in two other system calls. So we > avoid adding a new system call just to add another flag argument and > we're also not using up an additional AT_* bit. This makes it bearable > imho. But here we're talking about new system calls where we can avoid > this problem arising in the first place. > > > > > Sure, we will have some special cases like MOVE_MOUNT_* and > > legacy pollution to the lower AT_ flags word, but as a generic solution > > for syscalls that need the common AT_ lookup flags and just a few > > private flags, that seems like the lesser evil to me. > > It is fine to do this in some cases but we shouldn't encourage mixing > distinct flag namespaces let alone advertising this as a generic > solution imho. The AT_XATTR_* flags aren't even flags they behave like > an enum. OK. I see your point. Also, wrt struct xattr_args, there is sort of a precedent with XFS_IOC_ATTRMULTI_BY_HANDLE ioctl, struct xfs_attr_multiop and flags XFS_IOC_ATTR_{CREATE,REPLACE}. Just a nit, I would use xattr_args field names that are the same as setxattr() arg names, so s/cmd/flags. Thanks, Amir.