From patchwork Fri Dec 4 13:25:54 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: Florian Weimer X-Patchwork-Id: 41305 Return-Path: X-Original-To: patchwork@sourceware.org Delivered-To: patchwork@sourceware.org Received: from server2.sourceware.org (localhost [IPv6:::1]) by sourceware.org (Postfix) with ESMTP id 9100D386180E; Fri, 4 Dec 2020 13:26:13 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 9100D386180E DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=sourceware.org; s=default; t=1607088373; bh=hyuYO22gA/gcsAtKOVE9Qd7R3RcyMzuB115jiH1vD8w=; h=To:Subject:Date:List-Id:List-Unsubscribe:List-Archive:List-Post: List-Help:List-Subscribe:From:Reply-To:Cc:From; b=YmnLQI4TKUAA3oJmrCzJFeiC+06q23kjDXGi0+glTP6Qj6ux785CZMcTl31qfhQBw V0HccH0Gw9h93N9nJhEesPefuwQb4IUSNj42PnSZXIHd0ExzFz98SGK69wEjXwpL2M wxGZhac3B3HO7WZexzx5YE3o/qSC8lv15rxLKAoU= X-Original-To: libc-alpha@sourceware.org Delivered-To: libc-alpha@sourceware.org Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [216.205.24.124]) by sourceware.org (Postfix) with ESMTP id 63FCF386180E for ; Fri, 4 Dec 2020 13:26:09 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 63FCF386180E Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-465-nqs1RfAvNGq0D0LG3w96Yg-1; Fri, 04 Dec 2020 08:26:06 -0500 X-MC-Unique: nqs1RfAvNGq0D0LG3w96Yg-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 3ECF4107ACFA; Fri, 4 Dec 2020 13:26:05 +0000 (UTC) Received: from oldenburg2.str.redhat.com (ovpn-112-44.ams2.redhat.com [10.36.112.44]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 3784527C59; Fri, 4 Dec 2020 13:25:56 +0000 (UTC) To: libc-alpha@sourceware.org Subject: [PATCH v4] manual: Add Descriptor-Relative Access section Date: Fri, 04 Dec 2020 14:25:54 +0100 Message-ID: <87a6utpusd.fsf@oldenburg2.str.redhat.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com X-Spam-Status: No, score=-12.8 required=5.0 tests=BAYES_00, DKIMWL_WL_HIGH, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, GIT_PATCH_0, RCVD_IN_DNSWL_NONE, RCVD_IN_MSPIKE_H3, RCVD_IN_MSPIKE_WL, 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-Patchwork-Original-From: Florian Weimer via Libc-alpha From: Florian Weimer Reply-To: Florian Weimer Cc: Michael Kerrisk Errors-To: libc-alpha-bounces@sourceware.org Sender: "Libc-alpha" From: Florian Weimer 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.) --- v4: Use “file name” instead of “pathname”. Some small fixes. manual/filesys.texi | 154 ++++++++++++++++++++++++++++++++++++++++++++++++++-- manual/llio.texi | 28 ++++++++++ manual/startup.texi | 7 ++- 3 files changed, 182 insertions(+), 7 deletions(-) diff --git a/manual/filesys.texi b/manual/filesys.texi index 73e630842e..9ee0e17abd 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 +207,118 @@ An I/O error occurred. @end table @end deftypefun +@node Descriptor-Relative Access +@section Descriptor-Relative Access +@cindex file name 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 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 of such functions also +accept an additional flags argument which changes the behavior of the +file name lookup based on the passed @code{AT_@dots{}} flags. + +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 and the +@code{O_RDONLY} file access mode, with or without the @code{O_DIRECTORY} +flag. @xref{Opening and Closing Files}. Or it can be created implicity +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 relatively to that +directory, just as if the 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. + +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 descriptor's parent directory using a file name +argument @code{"../example"}, or in the root directory using +@code{"/example"}. + +If the file descriptor refers to a directory, the empty string @code{""} +is not a valid file name argument. + +@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. + +If @code{AT_FDCWD} is used, the empty string @code{""} is not a valid +file name argument. + +@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 (opened +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, and some (such as @code{openat}) do not accept a +flags argument at all. + +In the flag descriptions below, the @dfn{effective final path component} +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 final path component 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 final path component, the operation +applies to the file or directory in 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 final path component 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 final path component 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} argument to the @code{open} function. +Even with the @code{AT_SYMLINK_NOFOLLOW} flag present, symbolic links in +a non-final position of the path 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}). @node Accessing Directories @section Accessing Directories @@ -1250,10 +1363,11 @@ A hardware error occurred while trying to read or write the to filesystem. 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 file descriptor @code{AT_FDCWD} +denotes the current directory. The @var{flags} argument is a combination of the following flags: @@ -2095,6 +2209,38 @@ replaces the interface for small files on 32-bit machines. @c available. @c @safety{@mtsafe{}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}} + +@deftypefun int fstatat (int @var{filedes}, const char *@var{filename}, struct 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 for this function. + +@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 + @deftypefun int lstat (const char *@var{filename}, struct stat *@var{buf}) @standards{BSD, sys/stat.h} @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} diff --git a/manual/llio.texi b/manual/llio.texi index c0a53e1a6e..aae0340755 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 +@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 openat64 (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 9bf24123f5..daedd4ff79 100644 --- a/manual/startup.texi +++ b/manual/startup.texi @@ -664,9 +664,10 @@ basis there may be information that is not available any 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 For some platforms, the key @code{AT_HWCAP} is the easiest way to inquire