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@xxxxxxxxx> > 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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>: NFS details > .\" Modified 2004-06-23 by Michael Kerrisk <mtk.manpages@xxxxxxxxx> > .\" > -.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/
