From 3de73f974fab55430177c811c9c9ba3f251d5747 Mon Sep 17 00:00:00 2001 From: Florian Weimer Date: Wed, 7 Aug 2024 14:57:41 +0200 Subject: [PATCH] manual: Add Descriptor-Relative Access section Reference this new section from the O_PATH documentation. 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.) Reviewed-by: Adhemerval Zanella --- manual/filesys.texi | 196 ++++++++++++++++++++++++++++++++++++++++++-- manual/llio.texi | 41 ++++++++- manual/startup.texi | 8 +- 3 files changed, 235 insertions(+), 10 deletions(-) diff --git a/manual/filesys.texi b/manual/filesys.texi index 47d929744e..aabb68385b 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 +207,151 @@ 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 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 passed @code{AT_@dots{}} +flags. + +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 program wishes to access a directory tree which is being modified +concurrently, perhaps even by a different user on the system, the +program must avoid looking up file names with multiple components, in +order to detect symbolic links, using the @code{O_NOFOLLOW} flag +(@pxref{Open-time Flags}) or the @code{AT_SYMLINK_FOLLOW} flag +(described below). Without directory-relative access, it is necessary +to use the @code{fchdir} function to change the working directory +(@pxref{Working Directory}), which is not thread-safe. + +@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 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 +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 +directory referred to by the file 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. + +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. It is possible to use @code{"."} to +refer to the directory itself. Also see @code{AT_EMPTY_PATH} below. + +@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 component of the file name are still followed. +@end vtable + +@strong{Note:} 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}). @xref{Auxiliary Vector}. @node Accessing Directories @section Accessing Directories @@ -1250,10 +1396,11 @@ 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 value @code{AT_FDCWD} denotes the +current directory. The @var{flags} argument is a combination of the following flags: @@ -2091,9 +2238,44 @@ function is available under the name @code{fstat} and so transparently replaces the interface for small files on 32-bit machines. @end deftypefun -@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}, 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 + +When the sources are compiled with @code{_FILE_OFFSET_BITS == 64} this +function is in fact @code{fstatat64} since the LFS interface transparently +replaces the normal implementation. +@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}. + +When the sources are compiled with @code{_FILE_OFFSET_BITS == 64} this +function is available under the name @code{fstatat} and so transparently +replaces the interface for small files on 32-bit machines. +@end deftypefun @deftypefun int lstat (const char *@var{filename}, struct stat *@var{buf}) @standards{BSD, sys/stat.h} diff --git a/manual/llio.texi b/manual/llio.texi index ea84196abd..a035c3e20f 100644 --- a/manual/llio.texi +++ b/manual/llio.texi @@ -181,6 +181,43 @@ 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 + +When the sources are compiled with @code{_FILE_OFFSET_BITS == 64} this +function is in fact @code{openat64} since the LFS interface transparently +replaces the normal implementation. +@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}. + +When the sources are translated with @code{_FILE_OFFSET_BITS == 64} this +function is actually available under the name @code{openat}. I.e., the +new, extended API using 64 bit file sizes and offsets transparently +replaces the old API. +@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{}}} @@ -3807,7 +3844,9 @@ contains it is still needed), and permissions are checked when the descriptor is used later on. For example, such descriptors can be used with the @code{fexecve} -function (@pxref{Executing a File}). +function (@pxref{Executing a File}). Other applications involve the +@samp{*at} function variants, along with the @code{AT_EMPTY_PATH} flag. +@xref{Descriptor-Relative Access}. This access mode is specific to Linux. On @gnuhurdsystems{}, it is possible to use @code{O_EXEC} explicitly, or specify no access modes diff --git a/manual/startup.texi b/manual/startup.texi index 747beed4d9..8ac3b97eed 100644 --- a/manual/startup.texi +++ b/manual/startup.texi @@ -664,8 +664,12 @@ basis there may be information that is not available any other way. 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}. +returned; if the entry is not found, zero is returned and @code{errno} +is set to @code{ENOENT}. + +@strong{Note:} There is no relationship between the @samp{AT_} contants +defined in @file{elf.h} and the file name lookup flags in +@file{fcntl.h}. @xref{Descriptor-Relative Access}. @end deftypefun For some platforms, the key @code{AT_HWCAP} is the easiest way to inquire