Re: [PATCH] manual: Add Descriptor-Relative Access section

[Carlos O'Donell <carlos@redhat.com>]

On 5/4/20 7:16 AM, Florian Weimer wrote:
> * 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:
> 
> […]
> 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.
> 

This documentation is a great improvement. Thanks for this.

Please post a [PATCH v2] for this and we'll see if patchwork catches it.

Please also fixup your scripts to use '---' 3-dash for the diffstat header.

> 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.

OK.

>  * 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
>  
> +@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.

OK.

> +
> +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.

OK.

> +
> +@item
> +If a progrem wishes to access a directory tree which is being modified

s/progrem/program/g

> +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.

OK.

> +
> +@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.

OK.

> +
> +@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}.

OK.

> +
> +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.

OK.

> +
> +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"}.

OK. Good point.

> +
> +@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.

OK.

> +
> +@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}).

OK.

> +
> +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.

OK.

> +
> +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.

OK.

> +
> +@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.

OK.

> +
> +@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.

OK.

> +
> +@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.)

OK.

> +
> +@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.

OK.

> +@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}).

OK. I didn't think of this, but yes, they are both AT_* constants.

>  
>  @node Accessing Directories
>  @section Accessing Directories
> @@ -1250,10 +1393,11 @@ 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.

OK.

>  
>  The @var{flags} argument is a combination of the following flags:
>  
> @@ -2091,9 +2235,36 @@ 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.
> +
> +@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

OK.

>  
>  @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)

OK.

>  @c statx
>  @c mkdirat
>  @c mkfifoat
>  @c name_to_handle_at
> -@c openat

OK.

>  @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
>  
> +@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}])

s/openat/openat64/g

> +@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

OK.

> +
>  @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 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}.

OK.

>  @end deftypefun
>  
>  For some platforms, the key @code{AT_HWCAP} is the easiest way to inquire
> 


-- 
Cheers,
Carlos.