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Bruce Fields" , Arnd Bergmann , David Howells , Shuah Khan , Shuah Khan Subject: [PATCH v9 03/10] open: O_EMPTYPATH: procfs-less file descriptor re-opening Date: Sun, 7 Jul 2019 00:57:30 +1000 Message-Id: <20190706145737.5299-4-cyphar@cyphar.com> In-Reply-To: <20190706145737.5299-1-cyphar@cyphar.com> References: <20190706145737.5299-1-cyphar@cyphar.com> MIME-Version: 1.0 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20190706_075917_264089_EC5B2883 X-CRM114-Status: GOOD ( 19.35 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-ia64@vger.kernel.org, linux-sh@vger.kernel.org, Alexei Starovoitov , Oleg Nesterov , linux-kselftest@vger.kernel.org, sparclinux@vger.kernel.org, linux-arch@vger.kernel.org, linux-s390@vger.kernel.org, Tycho Andersen , Aleksa Sarai , linux-arm-kernel@lists.infradead.org, linux-mips@vger.kernel.org, linux-xtensa@linux-xtensa.org, Kees Cook , Jann Horn , linuxppc-dev@lists.ozlabs.org, Aleksa Sarai , Andy Lutomirski , David Drysdale , Christian Brauner , linux-parisc@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-api@vger.kernel.org, Chanho Min , linux-kernel@vger.kernel.org, Eric Biederman , linux-alpha@vger.kernel.org, linux-fsdevel@vger.kernel.org, Andrew Morton , Linus Torvalds , containers@lists.linux-foundation.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org Userspace has made use of /proc/self/fd very liberally to allow for descriptors to be re-opened. There are a wide variety of uses for this feature, but it has always required constructing a pathname and could not be done without procfs mounted. The obvious solution for this is to extend openat(2) to have an AT_EMPTY_PATH-equivalent -- O_EMPTYPATH. Now that descriptor re-opening has been made safe through the new magic-link resolution restrictions, we can replicate these restrictions for O_EMPTYPATH. In particular, we only allow "upgrading" the file descriptor if the corresponding FMODE_PATH_* bit is set (or the FMODE_{READ,WRITE} cases for non-O_PATH file descriptors). When doing openat(O_EMPTYPATH|O_PATH), O_PATH takes precedence and O_EMPTYPATH is ignored. Very few users ever have a need to O_PATH re-open an existing file descriptor, and so accommodating them at the expense of further complicating O_PATH makes little sense. Ultimately, if users ask for this we can always add RESOLVE_EMPTY_PATH to resolveat(2) in the future. Signed-off-by: Aleksa Sarai --- fs/fcntl.c | 2 +- fs/namei.c | 27 +++++++++++++++++++++++++++ fs/open.c | 7 ++++++- include/linux/fcntl.h | 2 +- include/uapi/asm-generic/fcntl.h | 5 +++++ 5 files changed, 40 insertions(+), 3 deletions(-) diff --git a/fs/fcntl.c b/fs/fcntl.c index 3d40771e8e7c..4cf05a2fd162 100644 --- a/fs/fcntl.c +++ b/fs/fcntl.c @@ -1031,7 +1031,7 @@ static int __init fcntl_init(void) * Exceptions: O_NONBLOCK is a two bit define on parisc; O_NDELAY * is defined as O_NONBLOCK on some platforms and not on others. */ - BUILD_BUG_ON(21 - 1 /* for O_RDONLY being 0 */ != + BUILD_BUG_ON(22 - 1 /* for O_RDONLY being 0 */ != HWEIGHT32( (VALID_OPEN_FLAGS & ~(O_NONBLOCK | O_NDELAY)) | __FMODE_EXEC | __FMODE_NONOTIFY)); diff --git a/fs/namei.c b/fs/namei.c index 4ec6168762db..4895717d2760 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -3605,6 +3605,31 @@ static int do_o_path(struct nameidata *nd, unsigned flags, struct file *file) return error; } +static int do_o_emptypath(struct nameidata *nd, struct file *newfile) +{ + int error; + struct fd f; + + /* We don't support AT_FDCWD since O_PATH cannot be set here. */ + f = fdget_raw(nd->dfd); + if (!f.file) + return -EBADF; + + /* Update opath_mask as though we went through trailing_symlink(). */ + if (!(f.file->f_mode & (FMODE_READ | FMODE_PATH_READ))) + nd->opath_mask &= ~FMODE_PATH_READ; + if (!(f.file->f_mode & (FMODE_WRITE | FMODE_PATH_WRITE))) + nd->opath_mask &= ~FMODE_PATH_WRITE; + + /* Obey the same restrictions as magic-links. */ + error = may_open_magiclink(f.file->f_mode, nd->acc_mode); + if (!error) + error = vfs_open(&f.file->f_path, newfile); + + fdput(f); + return error; +} + static struct file *path_openat(struct nameidata *nd, const struct open_flags *op, unsigned flags) { @@ -3620,6 +3645,8 @@ static struct file *path_openat(struct nameidata *nd, if (unlikely(file->f_flags & __O_TMPFILE)) { error = do_tmpfile(nd, flags, op, file); + } else if (unlikely(file->f_flags & O_EMPTYPATH)) { + error = do_o_emptypath(nd, file); } else if (unlikely(file->f_flags & O_PATH)) { error = do_o_path(nd, flags, file); } else { diff --git a/fs/open.c b/fs/open.c index ab20eae39df7..bdca45528524 100644 --- a/fs/open.c +++ b/fs/open.c @@ -996,6 +996,8 @@ static inline int build_open_flags(int flags, umode_t mode, struct open_flags *o lookup_flags |= LOOKUP_DIRECTORY; if (!(flags & O_NOFOLLOW)) lookup_flags |= LOOKUP_FOLLOW; + if (flags & O_EMPTYPATH) + lookup_flags |= LOOKUP_EMPTY; op->lookup_flags = lookup_flags; return 0; } @@ -1057,14 +1059,17 @@ long do_sys_open(int dfd, const char __user *filename, int flags, umode_t mode) { struct open_flags op; int fd = build_open_flags(flags, mode, &op); + int empty = 0; struct filename *tmp; if (fd) return fd; - tmp = getname(filename); + tmp = getname_flags(filename, op.lookup_flags, &empty); if (IS_ERR(tmp)) return PTR_ERR(tmp); + if (!empty) + op.open_flag &= ~O_EMPTYPATH; fd = get_unused_fd_flags(flags); if (fd >= 0) { diff --git a/include/linux/fcntl.h b/include/linux/fcntl.h index d019df946cb2..2868ae6c8fc1 100644 --- a/include/linux/fcntl.h +++ b/include/linux/fcntl.h @@ -9,7 +9,7 @@ (O_RDONLY | O_WRONLY | O_RDWR | O_CREAT | O_EXCL | O_NOCTTY | O_TRUNC | \ O_APPEND | O_NDELAY | O_NONBLOCK | O_NDELAY | __O_SYNC | O_DSYNC | \ FASYNC | O_DIRECT | O_LARGEFILE | O_DIRECTORY | O_NOFOLLOW | \ - O_NOATIME | O_CLOEXEC | O_PATH | __O_TMPFILE) + O_NOATIME | O_CLOEXEC | O_PATH | __O_TMPFILE | O_EMPTYPATH) #ifndef force_o_largefile #define force_o_largefile() (!IS_ENABLED(CONFIG_ARCH_32BIT_OFF_T)) diff --git a/include/uapi/asm-generic/fcntl.h b/include/uapi/asm-generic/fcntl.h index 9dc0bf0c5a6e..307f7c414a51 100644 --- a/include/uapi/asm-generic/fcntl.h +++ b/include/uapi/asm-generic/fcntl.h @@ -89,6 +89,11 @@ #define __O_TMPFILE 020000000 #endif +#ifndef O_EMPTYPATH +#define O_EMPTYPATH 040000000 +#endif + + /* a horrid kludge trying to make sure that this will fail on old kernels */ #define O_TMPFILE (__O_TMPFILE | O_DIRECTORY) #define O_TMPFILE_MASK (__O_TMPFILE | O_DIRECTORY | O_CREAT) -- 2.22.0 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel