From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from albireo.enyo.de (albireo.enyo.de [37.24.231.21]) by sourceware.org (Postfix) with ESMTPS id 63FB13858D35 for ; Mon, 4 May 2020 11:16:03 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 63FB13858D35 Authentication-Results: sourceware.org; dmarc=none (p=none dis=none) header.from=deneb.enyo.de Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=fw@deneb.enyo.de Received: from [172.17.203.2] (helo=deneb.enyo.de) by albireo.enyo.de with esmtps (TLS1.3:ECDHE_RSA_AES_256_GCM_SHA384:256) id 1jVZ4r-00030e-SP; Mon, 04 May 2020 11:16:01 +0000 Received: from fw by deneb.enyo.de with local (Exim 4.92) (envelope-from ) id 1jVZ4r-0002YI-Oe; Mon, 04 May 2020 13:16:01 +0200 From: Florian Weimer To: "Michael Kerrisk \(man-pages\)" Cc: libc-alpha Subject: Re: [PATCH] manual: Add Descriptor-Relative Access section References: <87d07p9v73.fsf@mid.deneb.enyo.de> <87zhaoxnt4.fsf@mid.deneb.enyo.de> <65c7dd0a-bfc8-7a23-6503-1f813947e5fa@gmail.com> <87wo5suku5.fsf@mid.deneb.enyo.de> Date: Mon, 04 May 2020 13:16:01 +0200 In-Reply-To: (Michael Kerrisk's message of "Mon, 4 May 2020 12:02:53 +0200") Message-ID: <87lfm8uf0u.fsf@mid.deneb.enyo.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Spam-Status: No, score=-22.4 required=5.0 tests=BAYES_00, GIT_PATCH_0, GIT_PATCH_1, GIT_PATCH_2, GIT_PATCH_3, KAM_DMARC_STATUS, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org X-BeenThere: libc-alpha@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 04 May 2020 11:16:06 -0000 * Michael Kerrisk: >> +@item >> +If a progrem wishes to access a directory tree which is being modified >> +concurrently, perhaps even by a different user on the system, the >> +program typically must avoid following symbolic links. With POSIX >> +interfaces, this can be done using the @code{O_NOFOLLOW} flag >> +(@pxref{Open-time Flags}) or the @code{AT_SYMLINK_FOLLOW} flag >> +(described below), but these flags affect only the final component of a >> +file name (the basename). Symbolic links in the parent directory part >> +are still followed. Therefore, without directory-relative access, it is >> +necessary to use the @code{fchdir} function to change the working >> +directory (@pxref{Working Directory}) and use the basename for file >> +system access. As explained before, this is not thread-safe. Keeping a >> +file descriptor of the directory is also required to be able to return > > Maybe better: s/of/that refers to/ > >> +to it later, so descriptor-based access is a natural fit. > > s/descriptor-relativebased/descriptor-relative/ ? Thanks, we have now: [=E2=80=A6] directory (@pxref{Working Directory}) and use the basename for file system access. As explained before, this is not thread-safe. Keeping a file descriptor that refers to the directory is also required to be able to return to it later, so descriptor-relative access is a natural fit. 8<------------------------------------------------------------------8< And document the functions openat, openat64, fstatat, fstatat64. (The safety assessment for fstatat was already obsolete because current glibc assumes kernel support for the underlying system call.) ----- manual/filesys.texi | 187 ++++++++++++++++++++++++++++++++++++++++++++++++= +--- manual/llio.texi | 28 ++++++++ manual/startup.texi | 7 +- 3 files changed, 210 insertions(+), 12 deletions(-) diff --git a/manual/filesys.texi b/manual/filesys.texi index 73e630842e..5a642d5c04 100644 --- a/manual/filesys.texi +++ b/manual/filesys.texi @@ -15,6 +15,7 @@ access permissions and modification times. @menu * Working Directory:: This is used to resolve relative file names. +* Descriptor-Relative Access:: Ways to control file name lookup. * Accessing Directories:: Finding out what files a directory contains. * Working with Directory Trees:: Apply actions to all files or a selectable @@ -206,6 +207,148 @@ An I/O error occurred. @end table @end deftypefun =20 +@node Descriptor-Relative Access +@section Descriptor-Relative Access +@cindex file name lookup based on descriptors +@cindex pathname resolution based on descriptors +@cindex descriptor-based file name resolution +@cindex @code{@dots{}at} functions + +Many functions that accept file names have @code{@dots{}at} variants +which accept a file descriptor and a file name argument instead of just +a file name argument. For example, @code{fstatat} is the +descriptor-based variant of the @code{fstat} function. Most such +functions also accept an additional flags argument which changes the +behavior of the file name lookup based on the @code{AT_@dots{}} flags +specified. + +There are several reasons to use descriptor-relative access: + +@itemize @bullet +@item +The working directory is a process-wide resource, so individual threads +cannot change it without affecting other threads in the process. +Explicitly specifying the directory against which relative paths are +resolved can be a thread-safe alternative to changing the working +directory. + +@item +If a progrem wishes to access a directory tree which is being modified +concurrently, perhaps even by a different user on the system, the +program typically must avoid following symbolic links. With POSIX +interfaces, this can be done using the @code{O_NOFOLLOW} flag +(@pxref{Open-time Flags}) or the @code{AT_SYMLINK_FOLLOW} flag +(described below), but these flags affect only the final component of a +file name (the basename). Symbolic links in the parent directory part +are still followed. Therefore, without directory-relative access, it is +necessary to use the @code{fchdir} function to change the working +directory (@pxref{Working Directory}) and use the basename for file +system access. As explained before, this is not thread-safe. Keeping a +file descriptor that refers to the directory is also required to be able +to return to it later, so descriptor-relative access is a natural fit. + +@item +Listing directory contents using the @code{readdir} or @code{readdir64} +functions (@pxref{Reading/Closing Directory}) does not provide full file +name paths. Using @code{@dots{}at} functions, it is possible to use the +file names directly, without having to construct such full paths. + +@item +Additional flags available with some of the @code{@dots{}at} functions +provide access to functionality which is not available otherwise. +@end itemize + +The file descriptor used by these @code{@dots{}at} functions has the +following uses: + +@itemize @bullet +@item +It can be a file descriptor referring to a directory. Such a descriptor +can be created explicitly using the @code{open} function, with or +without the @code{O_DIRECTORY} flag. @xref{Opening and Closing Files}. +Or it can be created implicitly by @code{opendir} and retrieved using +the @code{dirfd} function. @xref{Opening a Directory}. + +If a directory descriptor is used with one of the @code{@dots{}at} +functions, a relative file name argument is resolved relative to the +directory referred to by the directory descriptor, just as if that +directory were the current working directory. Absolute file name +arguments (starting with @samp{/}) are resolved against the file system +root, and the descriptor argument is effectively ignored for the +purposes of file name lookup. + +This means that file name lookup is not constrained to the directory of +the descriptor. For example, it is possible to access a file +@file{example} in the parent directory using a file name argument +@code{"../example"}, or in the root directory using @code{"/example"}. + +@item +@vindex @code{AT_FDCWD} +The special value @code{AT_FDCWD}. This means that the current working +directory is used for the lookup if the file name is a relative. For +@code{@dots{}at} functions with an @code{AT_@dots{}} flags argument, +this provides a shortcut to use those flags with regular (not +descriptor-based) file name lookups. + +@item +An arbitrary file descriptor, along with an empty string @code{""} as +the file name argument, and the @code{AT_EMPTY_PATH} flag. In this case, +the operation uses the file descriptor directly, without further +file name resolution. On Linux, this allows operations on descriptors +opened with the @code{O_PATH} flag. For regular descriptors (without +@code{O_PATH}), the same functionality is also available through the +plain descriptor-based functions (for example, @code{fstat} instead of +@code{fstatat}). + +This is a GNU extension. +@end itemize + +@cindex file name resolution flags +@cindex @code{AT_*} file name resolution flags +The flags argument in @code{@dots{}at} functions can be a combination of +the following flags, defined in @file{fcntl.h}. Not all such functions +support all flags. Some of the functions (such as @code{openat}) +completely lack an argument for the @code{AT_*} flags. + +In the flag descriptions below, the @dfn{effective basename} refers to +the final component (basename) of the full path constructed from the +descriptor and file name arguments, using file name lookup, as described +above. + +@vtable @code +@item AT_EMPTY_PATH +This flag is used with an empty file name @code{""} and a descriptor +which does not necessarily refer to a directory. It is most useful with +@code{O_PATH} descriptors, as described above. This flag is a GNU +extension. + +@item AT_NO_AUTOMOUNT +If the effective basename refers to a potential file system mount point +controlled by an auto-mounting service, the operation does not trigger +auto-mounting and refers to the unmounted mount point instead. +@xref{Mount-Unmount-Remount}. If a file system has already been mounted +at the effective basename, the operation applies to the mounted file +system, not the underlying file system that was mounted over. This flag +is a GNU extension. + +@item AT_SYMLINK_FOLLOW +If the effective basename is a symbolic link, the operation follows the +symbolic link and operates on its target. (For most functions, this is +the default behavior.) + +@item AT_SYMLINK_NOFOLLOW +If the effective basename is a symbolic link, the operation operates on +the symbolic link, without following it. The difference in behavior +enabled by this flag is similar to the difference between the +@code{lstat} and @code{stat} functions, or the behavior activated by the +@code{O_NOFOLLOW} flag (@pxref{Open-time Flags}). Even with the +@code{AT_SYMLINK_NOFOLLOW} flag present, symbolic links in a non-final +component of the file name are still followed. +@end vtable + +There is no relationship between these flags and the type argument to +the @code{getauxval} function (with @code{AT_@dots{}} constants defined +in @file{elf.h}). =20 @node Accessing Directories @section Accessing Directories @@ -1250,10 +1393,11 @@ A hardware error occurred while trying to read or w= rite the to filesystem. =20 The @code{linkat} function is analogous to the @code{link} function, except that it identifies its source and target using a combination of a -file descriptor (referring to a directory) and a pathname. If a -pathnames is not absolute, it is resolved relative to the corresponding -file descriptor. The special file descriptor @code{AT_FDCWD} denotes -the current directory. +file descriptor (referring to a directory) and a file name. +@xref{Descriptor-Relative Access}. For @code{linkat}, if a file name is +not absolute, it is resolved relative to the corresponding file +descriptor. As usual, the special value @code{AT_FDCWD} denotes the +current directory. =20 The @var{flags} argument is a combination of the following flags: =20 @@ -2091,9 +2235,36 @@ function is available under the name @code{fstat} an= d so transparently replaces the interface for small files on 32-bit machines. @end deftypefun =20 -@c fstatat will call alloca and snprintf if the syscall is not -@c available. -@c @safety{@mtsafe{}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}} +@deftypefun int fstatat (int @var{filedes}, const char *@var{filename}, st= ruct stat *@var{buf}, int @var{flags}) +@standards{POSIX.1, sys/stat.h} +@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} +This function is a descriptor-relative version of the @code{fstat} +function above. @xref{Descriptor-Relative Access}. The @var{flags} +argument can contain a combination of the flags @code{AT_EMPTY_PATH}, +@code{AT_NO_AUTOMOUNT}, @code{AT_SYMLINK_NOFOLLOW}. + +Compared to @code{fstat}, the following additional error conditions can +occur: + +@table @code +@item EBADF +The @var{filedes} argument is not a valid file descriptor. + +@item EINVAL +The @var{flags} argument is not valid. + +@item ENOTDIR +The descriptor @var{filedes} is not associated with a directory, and +@var{filename} is a relative file name. +@end table +@end deftypefun + +@deftypefun int fstatat64 (int @var{filedes}, const char *@var{filename}, = struct stat64 *@var{buf}, int @var{flags}) +@standards{GNU, sys/stat.h} +@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} +This function is the large-file variant of @code{fstatat}, similar to +how @code{fstat64} is the variant of @code{fstat}. +@end deftypefun =20 @deftypefun int lstat (const char *@var{filename}, struct stat *@var{buf}) @standards{BSD, sys/stat.h} @@ -3582,12 +3753,10 @@ The @code{mkdtemp} function comes from OpenBSD. @c fchmodat @c fchownat @c futimesat -@c fstatat (there's a commented-out safety assessment for this one) @c statx @c mkdirat @c mkfifoat @c name_to_handle_at -@c openat @c open_by_handle_at @c readlinkat @c renameat diff --git a/manual/llio.texi b/manual/llio.texi index 6db4a70836..afbeca881e 100644 --- a/manual/llio.texi +++ b/manual/llio.texi @@ -180,6 +180,34 @@ new, extended API using 64 bit file sizes and offsets = transparently replaces the old API. @end deftypefun =20 +@deftypefun int openat (int @var{filedes}, const char *@var{filename}, int= @var{flags}[, mode_t @var{mode}]) +@standards{POSIX.1, fcntl.h} +@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{@acsfd{}}} +This function is the descriptor-relative variant of the @code{open} +function. @xref{Descriptor-Relative Access}. + +Note that the @var{flags} argument of @code{openat} does not accept +@code{AT_@dots{}} flags, only the flags described for the @code{open} +function above. + +The @code{openat} function can fail for additional reasons: + +@table @code +@item EBADF +The @var{filedes} argument is not a valid file descriptor. + +@item ENOTDIR +The descriptor @var{filedes} is not associated with a directory, and +@var{filename} is a relative file name. +@end table +@end deftypefun + +@deftypefun int openat (int @var{filedes}, const char *@var{filename}, int= @var{flags}[, mode_t @var{mode}]) +@standards{GNU, fcntl.h} +The large-file variant of the @code{openat}, similar to how +@code{open64} is the large-file variant of @code{open}. +@end deftypefun + @deftypefn {Obsolete function} int creat (const char *@var{filename}, mode= _t @var{mode}) @standards{POSIX.1, fcntl.h} @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{@acsfd{}}} diff --git a/manual/startup.texi b/manual/startup.texi index 21c48cd037..52126ef59e 100644 --- a/manual/startup.texi +++ b/manual/startup.texi @@ -664,9 +664,10 @@ basis there may be information that is not available a= ny other way. @c Reads from hwcap or iterates over constant auxv. This function is used to inquire about the entries in the auxiliary vector. The @var{type} argument should be one of the @samp{AT_} symbols -defined in @file{elf.h}. If a matching entry is found, the value is -returned; if the entry is not found, zero is returned and @code{errno} is -set to @code{ENOENT}. +defined in @file{elf.h}. (There is no relationship between these types +and the file name lookup flags in @file{fcntl.h}.) If a matching entry +is found, the value is returned; if the entry is not found, zero is +returned and @code{errno} is set to @code{ENOENT}. @end deftypefun =20 For some platforms, the key @code{AT_HWCAP} is the easiest way to inquire