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

Michael Kerrisk (man-pages)
2021-01-11T08:03:30Z

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
--
2.7.4

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