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manual: Clarify File Access Modes section and add O_PATH

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Kees Cook reported that the current text is misleading:

  <https://lore.kernel.org/lkml/202005150847.2B1ED8F81@keescook/>
This commit is contained in:
Florian Weimer 2020-12-03 10:59:50 +01:00
parent f9ba73d056
commit e960d8313d
1 changed files with 44 additions and 28 deletions
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manual/llio.texi
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@ -3563,10 +3563,9 @@ The symbols in this section are defined in the header file
@node Access Modes
@subsection File Access Modes
The file access modes allow a file descriptor to be used for reading,
writing, or both. (On @gnuhurdsystems{}, they can also allow none of these,
and allow execution of the file as a program.) The access modes are chosen
when the file is opened, and never change.
The file access mode allows a file descriptor to be used for reading,
writing, both, or neither. The access mode is determined when the file
is opened, and never change.
@deftypevr Macro int O_RDONLY
@standards{POSIX.1, fcntl.h}
@ -3583,7 +3582,43 @@ Open the file for write access.
Open the file for both reading and writing.
@end deftypevr
On @gnuhurdsystems{} (and not on other systems), @code{O_RDONLY} and
@deftypevr Macro int O_PATH
@standards{Linux, fcntl.h}
Obtain a file descriptor for the file, but do not open the file for
reading or writing. Permission checks for the file itself are skipped
when the file is opened (but permission to access the directory that
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}).
This access mode is specific to Linux. On @gnuhurdsystems{}, it is
possible to use @code{O_EXEC} explicitly, or specify no access modes
at all (see below).
@end deftypevr
The portable file access modes @code{O_RDONLY}, @code{O_WRONLY}, and
@code{O_RDWR} may not correspond to individual bits. To determine the
file access mode with @code{fcntl}, you must extract the access mode
bits from the retrieved file status flags, using the @code{O_ACCMODE}
mask.
@deftypevr Macro int O_ACCMODE
@standards{POSIX.1, fcntl.h}
This macro is a mask that can be bitwise-ANDed with the file status flag
value to recover the file access mode, assuming that a standard file
access mode is in use.
@end deftypevr
If a non-standard file access mode is used (such as @code{O_PATH} or
@code{O_EXEC}), masking with @code{O_ACCMODE} may give incorrect
results. These non-standard access modes are identified by individual
bits and have to be checked directly (without masking with
@code{O_ACCMODE} first).
On @gnuhurdsystems{} (but not on other systems), @code{O_RDONLY} and
@code{O_WRONLY} are independent bits that can be bitwise-ORed together,
and it is valid for either bit to be set or clear. This means that
@code{O_RDWR} is the same as @code{O_RDONLY|O_WRONLY}. A file access
@ -3591,40 +3626,21 @@ mode of zero is permissible; it allows no operations that do input or
output to the file, but does allow other operations such as
@code{fchmod}. On @gnuhurdsystems{}, since ``read-only'' or ``write-only''
is a misnomer, @file{fcntl.h} defines additional names for the file
access modes. These names are preferred when writing GNU-specific code.
But most programs will want to be portable to other POSIX.1 systems and
should use the POSIX.1 names above instead.
access modes.
@deftypevr Macro int O_READ
@standards{GNU, fcntl.h (optional)}
Open the file for reading. Same as @code{O_RDONLY}; only defined on GNU.
Open the file for reading. Same as @code{O_RDONLY}; only defined on GNU/Hurd.
@end deftypevr
@deftypevr Macro int O_WRITE
@standards{GNU, fcntl.h (optional)}
Open the file for writing. Same as @code{O_WRONLY}; only defined on GNU.
Open the file for writing. Same as @code{O_WRONLY}; only defined on GNU/Hurd.
@end deftypevr
@deftypevr Macro int O_EXEC
@standards{GNU, fcntl.h (optional)}
Open the file for executing. Only defined on GNU.
@end deftypevr
To determine the file access mode with @code{fcntl}, you must extract
the access mode bits from the retrieved file status flags. On
@gnuhurdsystems{},
you can just test the @code{O_READ} and @code{O_WRITE} bits in
the flags word. But in other POSIX.1 systems, reading and writing
access modes are not stored as distinct bit flags. The portable way to
extract the file access mode bits is with @code{O_ACCMODE}.
@deftypevr Macro int O_ACCMODE
@standards{POSIX.1, fcntl.h}
This macro stands for a mask that can be bitwise-ANDed with the file
status flag value to produce a value representing the file access mode.
The mode will be @code{O_RDONLY}, @code{O_WRONLY}, or @code{O_RDWR}.
(On @gnuhurdsystems{} it could also be zero, and it never includes the
@code{O_EXEC} bit.)
Open the file for executing. Only defined on GNU/Hurd.
@end deftypevr
@node Open-time Flags