Re: [patch] chmod.2, lchmod.3: Document lchmod()

Michael Kerrisk (man-pages)
2021-01-18T16:12:58Z

Hi Bruno,

Ping!

Thanks,

Michael

On 1/11/21 9:02 AM, Michael Kerrisk (man-pages) wrote:
> On 1/10/21 11:05 AM, Bruno Haible wrote:
>> Dmitry V. Levin wrote:
>>>> Here is a patch to update the man pages accordingly.
>>>
>>> Please note the important limitation of that implementation:
>>> it doesn't work when /proc is not mounted, see
>>> https://sourceware.org/bugzilla/show_bug.cgi?id=26401
>>
>> Thanks for the pointer. Revised patch attached.
>
> Hi Bruno,
>
> Thank you for the patch.
>
> But, I have a question: is lchown() actually usable? That is,
> are there any kinds of links whose mode can be changed? I
> can't seem to use it with normal symlinks, or for that matter,
> magic links:
>
> [[
> $ cat t_lchmod.c
> /*#* t_lchmod.c
>
> Copyright Michael Kerrisk, Jan 2021
> */
> #define _DEFAULT_SOURCE
> #include <sys/stat.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <unistd.h>
> #include <string.h>
>
> typedef enum { FALSE, TRUE } Boolean;
> int
> main(int argc, char *argv[])
> {
> if (argc < 2) {
> fprintf(stderr, "Usage: %s <file> <mode>\n", argv[0]);
> exit(EXIT_FAILURE);
> }
>
> int mode = strtoul(argv[2], NULL, 0);
> if (lchmod(argv[1], mode) == -1) {
> perror("lchmod");
> exit(EXIT_FAILURE);
> }
>
> exit(EXIT_SUCCESS);
> }
>
> $ touch a
> $ ln -s a b
> $ ./t_lchmod b 700
> lchmod: Operation not supported
> $ ./t_lchmod /proc/self/fd/0 700
> lchmod: Operation not supported
> ]]
>
>
> Thanks,
>
> Michael
>
> =====
>
>
>>From 56ce64325fa9a9184b820eac908ecc5d53a5154b Mon Sep 17 00:00:00 2001
> From: Bruno Haible <bruno@clisp.org>
> Date: Sun, 10 Jan 2021 05:20:30 +0100
> Subject: [PATCH] chmod.2, lchmod.3: Document lchmod().
>
> ---
> man2/chmod.2 | 70 ++++++++++++++++++++++++++++++++++++++++++++++++++++++-----
> man3/lchmod.3 | 1 +
> 2 files changed, 65 insertions(+), 6 deletions(-)
> create mode 100644 man3/lchmod.3
>
> diff --git a/man2/chmod.2 b/man2/chmod.2
> index a54aec7..f1709ef 100644
> --- a/man2/chmod.2
> +++ b/man2/chmod.2
> @@ -29,15 +29,16 @@
> .\" <michael@cantor.informatik.rwth-aachen.de>: NFS details
> .\" Modified 2004-06-23 by Michael Kerrisk <mtk.manpages@gmail.com>
> .\"
> -.TH CHMOD 2 2017-09-15 "Linux" "Linux Programmer's Manual"
> +.TH CHMOD 2 2021-01-10 "Linux" "Linux Programmer's Manual"
> .SH NAME
> -chmod, fchmod, fchmodat \- change permissions of a file
> +chmod, fchmod, lchmod, fchmodat \- change permissions of a file
> .SH SYNOPSIS
> .nf
> .B #include <sys/stat.h>
> .PP
> .BI "int chmod(const char *" pathname ", mode_t " mode );
> .BI "int fchmod(int " fd ", mode_t " mode );
> +.BI "int lchmod(const char *" pathname ", mode_t " mode );
> .PP
> .BR "#include <fcntl.h>" " /* Definition of AT_* constants */"
> .B #include <sys/stat.h>
> @@ -68,6 +69,12 @@ Feature Test Macro Requirements for glibc (see
> .\" || (_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED)
> .fi
> .PP
> +.BR lchmod ():
> +.nf
> + Since glibc 2.32:
> + _DEFAULT_SOURCE
> +.fi
> +.PP
> .BR fchmodat ():
> .nf
> Since glibc 2.10:
> @@ -80,10 +87,12 @@ The
> .BR chmod ()
> and
> .BR fchmod ()
> -system calls change a files mode bits.
> +system calls and the
> +.BR lchmod ()
> +function change a file's mode bits.
> (The file mode consists of the file permission bits plus the set-user-ID,
> set-group-ID, and sticky bits.)
> -These system calls differ only in how the file is specified:
> +These functions differ only in how the file is specified:
> .IP * 2
> .BR chmod ()
> changes the mode of the file specified whose pathname is given in
> @@ -93,6 +102,11 @@ which is dereferenced if it is a symbolic link.
> .BR fchmod ()
> changes the mode of the file referred to by the open file descriptor
> .IR fd .
> +.IP *
> +.BR lchmod ()
> +is like
> +.BR chmod (),
> +but does not dereference symbolic links.
> .PP
> The new file mode is specified in
> .IR mode ,
> @@ -220,8 +234,13 @@ can either be 0, or include the following flag:
> If
> .I pathname
> is a symbolic link, do not dereference it:
> -instead operate on the link itself.
> -This flag is not currently implemented.
> +instead operate on the link itself, like
> +.BR lchmod ().
> +(By default,
> +.BR fchmodat ()
> +dereferences symbolic links, like
> +.BR chmod ().)
> +This flag is implemented since glibc 2.32.
> .PP
> See
> .BR openat (2)
> @@ -304,6 +323,17 @@ See above.
> The same errors that occur for
> .BR chmod ()
> can also occur for
> +.BR lchmod ().
> +The following additional errors can occur for
> +.BR lchmod ():
> +.TP
> +.B EOPNOTSUPP
> +.B /proc
> +is not mounted.
> +.PP
> +The same errors that occur for
> +.BR chmod ()
> +can also occur for
> .BR fchmodat ().
> The following additional errors can occur for
> .BR fchmodat ():
> @@ -323,14 +353,31 @@ is relative and
> is a file descriptor referring to a file other than a directory.
> .TP
> .B ENOTSUP
> +(Before glibc 2.32.)
> .I flags
> specified
> .BR AT_SYMLINK_NOFOLLOW ,
> which is not supported.
> +.TP
> +.B EOPNOTSUPP
> +(Since glibc 2.32.)
> +.I flags
> +specified
> +.BR AT_SYMLINK_NOFOLLOW ,
> +and
> +.B /proc
> +is not mounted.
> .SH VERSIONS
> .BR fchmodat ()
> was added to Linux in kernel 2.6.16;
> library support was added to glibc in version 2.4.
> +.PP
> +.BR lchmod ()
> +and the handling of
> +.B AT_SYMLINK_NOFOLLOW
> +in
> +.BR fchmodat ()
> +were added in glibc version 2.32.
> .SH CONFORMING TO
> .BR chmod (),
> .BR fchmod ():
> @@ -362,6 +409,17 @@ glibc constructs a pathname based on the symbolic link in
> that corresponds to the
> .IR dirfd
> argument.
> +.SH BUGS
> +.BR lchmod ()
> +and
> +.BR fchmodat ()
> +with
> +.B AT_SYMLINK_NOFOLLOW
> +flag fail with error
> +.B EOPNOTSUPP
> +when the
> +.B /proc
> +file system is not mounted.
> .SH SEE ALSO
> .BR chmod (1),
> .BR chown (2),
> diff --git a/man3/lchmod.3 b/man3/lchmod.3
> new file mode 100644
> index 0000000..92647d2
> --- /dev/null
> +++ b/man3/lchmod.3
> @@ -0,0 +1 @@
> +.so man2/chmod.2
>

--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/