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b=ljtTrKrzjwh9bmTgB/C4QRv4lPNCmDXb0oGrnMVLkXCeEPfpYMq2XwmiskAtwQlsK oR+9Ko1Pl3b/OoYpR0JTmvi3u4DF0d6V6KQt841p42E0GJwXEsp4zexEhStpCZGKot pkJLfSa1M3AMrCCthqht9MFhgLn+Yh9p8Octt6Ghr3uXvq/2JhHMONBnt+SmQKq34t I08bZ2ixHGY9RHmh/bJlC1bBT/o22sClyAftWhJVYpDvJ2AVldTnb++uYWhQYb014k kzsT0ppwzb8VRvjDyZBpKuGuVZJ8s3MbvTiItwdFL05JRwkjFnFrqSpfLfwR7+VZN8 /LmgWCZM8tKSQ== Message-ID: From: sashiko-bot@kernel.org Subject: Re: [PATCH RFC 36/38] selftests/proc: check that faulting sysctl accesses don't hold up removal Reply-To: sashiko-reviews@lists.linux.dev To: "Christian Brauner" Cc: "Linus Torvalds" , "Jann Horn" , "Josef Bacik" , "Chris Mason" , "Jeff Layton" , NeilBrown , "Eric Dumazet" , "Thomas Gleixner" , "Al Viro" , "Jonathan Corbet" , "Christoph Hellwig" , "Greg Kroah-Hartman" , "Joel Granados" , "Kees Cook" , "Jan Kara" , "Jan Kara" , linux-s390@vger.kernel.org, linux-scsi@vger.kernel.org, coreteam@netfilter.org, kexec@lists.infradead.org, linux-pci@vger.kernel.org, linux-sound@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-hwmon@vger.kernel.org, linux-mm@kvack.org, linux-usb@vger.kernel.org, samba-technical@lists.samba.org, sparclinux@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-serial@vger.kernel.org, "Takashi Iwai" , linux-input@vger.kernel.org, linux-raid@vger.kernel.org, linux-ext4@vger.kernel.org, linux-modules@vger.kernel.org, "Chuck Lever" , tytso@mit.edu, dmitry.torokhov@gmail.com, "Petr Pavlu" , "Anna Schumaker" , yukuai@fygo.io, "Heiko Carstens" , "Vasily Gorbik" , "Alexander Gordeev" , "Christian Borntraeger" In-Reply-To: <20261102-work-procfs-bugfixes-v1-36-59422ed3e465@kernel.org> References: <20261102-work-procfs-bugfixes-v1-0-59422ed3e465@kernel.org> <20261102-work-procfs-bugfixes-v1-36-59422ed3e465@kernel.org> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Date: Tue, 03 Nov 2026 14:48:40 +0000 X-Rspamd-Server: rspam06 X-Stat-Signature: tw4f9ucbfphd8w7rkq3x3i8nqj4xmozk X-Spam-Status: No, score=-10.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS autolearn=unavailable autolearn_force=no version=4.0.1 X-Rspamd-Queue-Id: 05568C0003 X-Rspam-User: X-HE-Tag: 1793717323-155002 X-HE-Meta: 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 oVY3rXdv 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 X-Archived-By: Thank you for your contribution! Sashiko AI review found 1 potential issue(= s) to consider: - [Medium] selftests/proc: getdents test faults in dir_emit_dots() before e= ntry lookup -- commit fc0ab3f791b6ab04b43d13e015f97882092d54fc Author: Christian Brauner selftests/proc: check that faulting sysctl accesses don't hold up removal This commit adds a proc selftest using userfaultfd and a tun device in a new network namespace to verify that page faults during read(), write(), and getdents64() on /proc/sys do not block sysctl table unregistration. > diff --git a/tools/testing/selftests/proc/proc-remove-slow-sysctl.c b/too= ls/testing/selftests/proc/proc-remove-slow-sysctl.c > new file mode 100644 > index 0000000000000..e1b1fd19a07ab > --- /dev/null > +++ b/tools/testing/selftests/proc/proc-remove-slow-sysctl.c > @@ -0,0 +1,213 @@ [ ... ] > +TEST_F(proc_sys_slow, getdents) > +{ > + struct worker w =3D { > + .op =3D SYSCTL_GETDENTS, > + .len =3D BUF_PAGES * self->psz, > + }; > + > + w.fd =3D open(CONF_PATH, O_RDONLY | O_DIRECTORY | O_CLOEXEC); > + ASSERT_GE(w.fd, 0); > + stall_and_remove(_metadata, self, &w); > + > + /* What was looked up before the removal still arrives, then nothing. */ > + EXPECT_GT(w.ret, 0); [Severity: Medium] Does TEST_F(proc_sys_slow, getdents) in tools/testing/selftests/proc/proc-remove-slow-sysctl.c actually fault on a looked-up sysctl entry, or does it fault inside dir_emit_dots() before any entry is looked up? In FIXTURE_SETUP(proc_sys_slow), only the first page of self->buf is registered with userfaultfd. When TEST_F(proc_sys_slow, getdents) opens CONF_PATH, w.fd starts at file position 0 and is passed directly to stall_and_remove() -> work_thread() -> getdents64(). In fs/proc/proc_sysctl.c:proc_sys_readdir(), dir_emit_dots() runs before sysctl_dir_next(): fs/proc/proc_sysctl.c:proc_sys_readdir() { ... if (!dir_emit_dots(file, ctx)) return 0; if (cursor && cursor->pos =3D=3D ctx->pos) { err =3D sysctl_dir_next(ctl_dir, &d, cursor->name, cursor->len); } else { loff_t pos =3D 2; err =3D sysctl_dir_next(ctl_dir, &d, NULL, 0); ... } Because ctx->pos is 0, dir_emit_dots() writes "." at the start of self->buf and triggers the single userfaultfd page fault before sysctl_dir_next() is called and while d.head is still NULL. While work_thread() is sleeping in dir_emit_dots(), stall_and_remove() clos= es self->tun (which unregisters CONF_PATH and marks ctl_dir->header as unregistering) and resolves the fault on the first page with UFFDIO_ZEROPAG= E. When work_thread() resumes, dir_emit_dots() finishes emitting "." and "..", sysctl_dir_next() immediately returns -ENOENT, and getdents64() returns the 48 bytes from the dot entries without ever entering the entry-emission loop. Should TEST_F(proc_sys_slow, getdents) advance w.fd past "." and ".." first (for example with lseek(w.fd, 2, SEEK_SET)) before calling stall_and_remove= () so the fault occurs in dir_emit() after sysctl_dir_next() and proc_sys_fill_dirent() have looked up an entry? --=20 Sashiko AI review =C2=B7 https://sashiko.dev/#/patchset/20261102-work-procf= s-bugfixes-v1-0-59422ed3e465@kernel.org?part=3D36