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[100.75.92.58]) by sea.source.kernel.org (Postfix) with ESMTP id 2DDBF43E49; Wed, 21 Jan 2026 15:18:08 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 77DE6C4CEF1; Wed, 21 Jan 2026 15:18:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1769008688; bh=lVd2HMezJty6ZGKnQstQUJq6N9cBFPNbkoSEaWx1RuA=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=B+ezZ3eHebukNxQ6ds+XmXcbtpAbul4RxfzF2gl2V0PL3vPeVlmPvLopg28DyXtVG lpf2JaBI8sNw1dFsN8GPcqDAz7cYJ/MpBb+OtxwzbVJgiWMBA2Q+JSCGyhP4YvCaBo ilNzxYSfRO7p3/tG2bVtFnYiXos9Eaatfy1mbbfCyLpbOKZuRROlpaJXrVgLkkdX0a CTR/LYmZiV7OkBcyVFpaj8hTAJNEynK4JvlewuSGrLYoM33buuxi1wpNurTzes/TMW MPkzA2GX0mT5cHUkrvhXRmb1DdKItrpiTFcGvmh9OYN18yY4F+TcIhepWsNRkk5CIk gdVHjbd9xB82g== Message-ID: <3210d04fa2c0b1f4312d10506cac30586cb49a3c.camel@kernel.org> Subject: Re: [PATCH 00/29] fs: require filesystems to explicitly opt-in to nfsd export support From: Jeff Layton To: Christoph Hellwig Cc: NeilBrown , Christian Brauner , Amir Goldstein , Alexander Viro , Chuck Lever , Olga Kornievskaia , Dai Ngo , Tom Talpey , Hugh Dickins , Baolin Wang , Andrew Morton , Theodore Ts'o , Andreas Dilger , Jan Kara , Gao Xiang , Chao Yu , Yue Hu , Jeffle Xu , Sandeep Dhavale , Hongbo Li , Chunhai Guo , Carlos Maiolino , Ilya Dryomov , Alex Markuze , Viacheslav Dubeyko , Chris Mason , David Sterba , Luis de Bethencourt , Salah Triki , Phillip Lougher , Steve French , Paulo Alcantara , Ronnie Sahlberg , Shyam Prasad N , Bharath SM , Miklos Szeredi , Mike Marshall , Martin Brandenburg , Mark Fasheh , Joel Becker , Joseph Qi , Konstantin Komarov , Ryusuke Konishi , Trond Myklebust , Anna Schumaker , Dave Kleikamp , David Woodhouse , Richard Weinberger , Jan Kara , Andreas Gruenbacher , OGAWA Hirofumi , Jaegeuk Kim , linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-ext4@vger.kernel.org, linux-erofs@lists.ozlabs.org, linux-xfs@vger.kernel.org, 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Evolution 3.58.2 (3.58.2-1.fc43) MIME-Version: 1.0 X-Rspamd-Queue-Id: 72ADA180008 X-Stat-Signature: h8z8duprww39esnbpsjpg69kq953b5wi X-Rspam-User: X-Rspamd-Server: rspam06 X-HE-Tag: 1769008689-619129 X-HE-Meta: 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9a9yGS4q0cnflvLiUa2f0w6hxfoOCYBGpMlObIXifiuzWBnJoAEJRvZrByadPcdWwffRTJDoyMG9rL3ke41f75qAxbo4PKU/m9gt0InURqL5ITd1hL6jwVbYN1vYzF1xrL87xVtUrp3VvgZQzn9ceiND/33GWvPHIPNQ8kL5AyWDVNOnLd1MUmRgtPbF7LAuJTQ+g9DrQwmqgu/7O0EbdfSqVanfr/4O1NupKHhYh7lp+NL7wZ78p94xaJIzh++9vmr6Sz02fl30NotziXjicx3yi1g== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, 2026-01-21 at 06:47 -0800, Christoph Hellwig wrote: > On Wed, Jan 21, 2026 at 09:27:38AM -0500, Jeff Layton wrote: > > Using your definitions, stability is not a problem for Linux > > filesystems. The filehandles generally don't change after they have > > been established. >=20 > fat seems to be an exception as far as the 'real' file systems go. > And it did sound to me like some of the synthetic ones had similar > issues. >=20 Not sure what we can do about FAT without changing the filehandle format in some fashion. The export ops just use generic_encode_ino32_fh, and FAT doesn't have stable inode numbers. The "nostale" ops seem sane enough but it looks like they only work with the fs in r/o mode. ...and therein lies a problem. We can't reasonably stop exporting FAT (even with all of its issues), and it in no way meets the definition of persistent or unique handles. > > > We'll still need a stable handles flag, and expose it to userspace > > > to avoid applications being tricked into using broken non-stable > > > file handles. We should have caught that when they were added, but > > > didn't unfortunately. > > >=20 > >=20 > > If we assume he meant "unique handles" flag, then I think we're all > > mostly in agreement here. As far as this patchset goes: what if we > > were to just rename EXPORT_OP_STABLE_HANDLES to > > EXPORT_OP_UNIQUE_HANDLES (and clean up the documentation), since that's > > the main issue for existing filesystems. It would be fairly simple to > > advertise handle uniqueness using statx or something. >=20 > Unique seems to also only capture part of it, but I could absolutely > live with it, if the documentation includes all aspecs. But maybe > use persistent as in the nfs spec? The spec also has the concept of uniqueness. There is an attribute for that: 5.8.1.10. Attribute 9: unique_handles TRUE, if two distinct filehandles are guaranteed to refer to two differ= ent file system objects. So, the NFSv4 spec does allow for non-unique handles (oh, the humanity). Persistence has more to do with being non-volatile, AFAICT: FH4_PERSISTENT The value of FH4_PERSISTENT is used to indicate a persistent filehandle, which is valid until the object is removed from the file system. The server will not return NFS4ERR_FHEXPIRED for this filehandle. FH4_PERSISTENT is defined as a value in which none of the bits specified below are set. In this case, the filesystems we're most concerned about do not provide uniqueness, but do provide persistence. > >=20 > > Alternately, instead of denying access to these filesystems, we could > > just fix these filesystems to create unique handles (a'la random > > i_generation value or something similar). That should mostly prevent > > filehandles from being reusable across a reboot on these filesystems. >=20 > Do we even want to provide access to them? >=20 The point would be that there would be no need to flag them, since all filehandles would then meet the technical definition of unique and persistent (modulo FAT of course). > > That would leave cgroupfs and the like exportable via nfsd, but as you > > point out, we can't deny export by userland servers. If people want to > > do this kind of crazy stuff, maybe we shouldn't deny them after all. >=20 > I think Amirs patch would take care of that. Although userland nfs > servers or other storage applications using the handle syscalls would > still see them. Then again fixing the problem that some handles > did not fulfill the long standing (but not documented well enough) > semantics probably is a good fix on it's own. Agreed. We should try to ensure uniqueness and persistence in all filehandles both for nfsd and userland applications. --=20 Jeff Layton