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 -- 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
