Commit Graph

539 Commits

Author SHA1 Message Date
Samanta Navarro
b2d202cb5d libmisc: fix grammar
Signed-off-by: Samanta Navarro <ferivoz@riseup.net>
2023-01-26 22:44:39 -06:00
Christian Göttsche
c99d8d0a08 Avoid comparisons of different signs
Comparisons if different signedness can result in unexpected results.
Add casts to ensure operants are of the same type.

    gettime.c: In function 'gettime':
    gettime.c:58:26: warning: comparison of integer expressions of different signedness: 'long long unsigned int' and 'time_t' {aka 'long int'} [-Wsign-compare]
       58 |         } else if (epoch > fallback) {
          |                          ^

Cast to time_t, since epoch is less than ULONG_MAX at this point.

    idmapping.c: In function 'write_mapping':
    idmapping.c:202:48: warning: comparison of integer expressions of different signedness: 'int' and 'long unsigned int' [-Wsign-compare]
      202 |                 if ((written <= 0) || (written >= (bufsize - (pos - buf)))) {
          |                                                ^~

    newgidmap.c: In function ‘main’:
    newgidmap.c:178:40: warning: comparison of integer expressions of different signedness: ‘int’ and ‘long unsigned int’ [-Wsign-compare]
      178 |         if ((written <= 0) || (written >= sizeof(proc_dir_name))) {
          |                                        ^~
    newuidmap.c: In function ‘main’:
    newuidmap.c:107:40: warning: comparison of integer expressions of different signedness: ‘int’ and ‘long unsigned int’ [-Wsign-compare]
      107 |         if ((written <= 0) || (written >= sizeof(proc_dir_name))) {
          |                                        ^~
2023-01-25 12:31:17 +01:00
Christian Göttsche
43508ac476 Drop redundant declaration
environ is exported in <unistd.h>.

    env.c:29:15: warning: redundant redeclaration of 'environ' [-Wredundant-decls]
       29 | extern char **environ;
          |               ^~~~~~~
    login.c:92:15: warning: redundant redeclaration of ‘environ’ [-Wredundant-decls]
       92 | extern char **environ;
          |               ^~~~~~~
    sulogin.c:40:15: warning: redundant redeclaration of ‘environ’ [-Wredundant-decls]
       40 | extern char **environ;
          |               ^~~~~~~
    newgrp.c:32:15: warning: redundant redeclaration of ‘environ’ [-Wredundant-decls]
       32 | extern char **environ;
          |               ^~~~~~~
2023-01-25 12:31:17 +01:00
Christian Göttsche
46d3058341 copydir: fix impl usage
copydir.c: In function 'copy_dir':
    copydir.c:517:32: warning: passing argument 1 of 'copy_tree' from incompatible pointer type [-Wincompatible-pointer-types]
      517 |             return (copy_tree (src, dst, false, reset_selinux,
          |                                ^~~
          |                                |
          |                                const struct path_info *
    In file included from copydir.c:20:
    ../lib/prototypes.h:108:35: note: expected 'const char *' but argument is of type 'const struct path_info *'
      108 | extern int copy_tree (const char *src_root, const char *dst_root,
          |                       ~~~~~~~~~~~~^~~~~~~~
    copydir.c:517:37: warning: passing argument 2 of 'copy_tree' from incompatible pointer type [-Wincompatible-pointer-types]
      517 |             return (copy_tree (src, dst, false, reset_selinux,
          |                                     ^~~
          |                                     |
          |                                     const struct path_info *
    ../lib/prototypes.h:108:57: note: expected 'const char *' but argument is of type 'const struct path_info *'
      108 | extern int copy_tree (const char *src_root, const char *dst_root,
          |                                             ~~~~~~~~~~~~^~~~~~~~

Fixes: 74c17c71 ("Add support for skeleton files from /usr/etc/skel")
2023-01-25 12:31:17 +01:00
Alejandro Colomar
b2bed465e8 Use getnameinfo(3) instead of our own equivalent
I didn't know getnameinfo(3) existed, so I implemented it, or something
similar to it called inet_sockaddr2str().  Let's use the standard API.

Link: <https://inbox.sourceware.org/libc-alpha/0f25d60f-f183-b518-b6c1-6d46aa63ee57@gmail.com/T/>
Link: <https://stackoverflow.com/a/42190913/6872717>
Link: <https://github.com/shadow-maint/shadow/pull/617>
Link: <https://software.codidact.com/posts/287748>
Cc: Zack Weinberg <zack@owlfolio.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
2023-01-20 10:23:03 -06:00
Alejandro Colomar
ac8b81c2b7 Prefer getrandom(3)/getentropy(3) over arc4random(3bsd)
arc4random(3) without kernel support is unsafe, as it can't know when to
drop the buffer.  Since we depend on libbsd since recently, we have
arc4random(3) functions always available, and thus, this code would have
always called arc4random_buf(3bsd), which is unsafe.  Put it after some
better alternatives, at least until in a decade or so all systems have a
recent enough glibc.

glibc implements arc4random(3) safely, since it's just a wrapper around
getrandom(2).

Link: <https://inbox.sourceware.org/libc-alpha/20220722122137.3270666-1-adhemerval.zanella@linaro.org/>
Link: <https://inbox.sourceware.org/libc-alpha/5c29df04-6283-9eee-6648-215b52cfa26b@cs.ucla.edu/T/>
Cc: Cristian Rodríguez <crrodriguez@opensuse.org>
Cc: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Cc: Guillem Jover <guillem@hadrons.org>
Cc: Björn Esser <besser82@fedoraproject.org>
Reviewed-by: "Jason A. Donenfeld" <Jason@zx2c4.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
2023-01-16 10:12:31 +01:00
Alejandro Colomar
bb3a89577c Add inet_sockaddr2str() to wrap inet_ntop(3)
Signed-off-by: Alejandro Colomar <alx@kernel.org>
2023-01-02 08:20:43 +01:00
Alejandro Colomar
220b352b70 Use strlcpy(3) instead of its pattern
-  Since strncpy(3) is not designed to write strings, but rather
   (null-padded) character sequences (a.k.a. unterminated strings), we
   had to manually append a '\0'.  strlcpy(3) creates strings, so they
   are always terminated.  This removes dependencies between lines, and
   also removes chances of accidents.

-  Repurposing strncpy(3) to create strings requires calculating the
   location of the terminating null byte, which involves a '-1'
   calculation.  This is a source of off-by-one bugs.  The new code has
   no '-1' calculations, so there's almost-zero chance of these bugs.

-  strlcpy(3) doesn't padd with null bytes.  Padding is relevant when
   writing fixed-width buffers to binary files, when interfacing certain
   APIs (I believe utmpx requires null padding at lease in some
   systems), or when sending them to other processes or through the
   network.  This is not the case, so padding is effectively ignored.

-  strlcpy(3) requires that the input string is really a string;
   otherwise it crashes (SIGSEGV).  Let's check if the input strings are
   really strings:

   -  lib/fields.c:
      -  'cp' was assigned from 'newft', and 'newft' comes from fgets(3).

   -  lib/gshadow.c:
      -  strlen(string) is calculated a few lines above.

   -  libmisc/console.c:
      -  'cons' comes from getdef_str, which is a bit cryptic, but seems
         to generate strings, I guess.1

   -  libmisc/date_to_str.c:
      -  It receives a string literal.  :)

   -  libmisc/utmp.c:
      -  'tname' comes from ttyname(3), which returns a string.

   -  src/su.c:
      -  'tmp_name' has been passed to strcmp(3) a few lines above.

Signed-off-by: Alejandro Colomar <alx@kernel.org>
2022-12-22 18:03:39 -06:00
Iker Pedrosa
a48d77bdef strtoday.c: remove unused defines.h inclusion
Signed-off-by: Iker Pedrosa <ipedrosa@redhat.com>
2022-12-22 10:39:45 -06:00
Iker Pedrosa
bb0c89d944 strtoday.c: remove USE_GETDATE as it was always used
Signed-off-by: Iker Pedrosa <ipedrosa@redhat.com>
2022-12-22 10:39:45 -06:00
Iker Pedrosa
e4441489bc strtoday.c: remove POSIX 1995 conditional dependency
Since the project is supposed to be POSIX.1-2001 compliant it doesn't
make sense to have that added conditionally.

Signed-off-by: Iker Pedrosa <ipedrosa@redhat.com>
2022-12-22 10:39:45 -06:00
Alejandro Colomar
d96bb2868d Assume struct stat has st_atim and st_mtim fields
That's required by POSIX.1-2008.

Link: <https://github.com/shadow-maint/shadow/pull/600>
Cc: Christian Göttsche <cgzones@googlemail.com>
Cc: Iker Pedrosa <ipedrosa@redhat.com>
Cc: Mike Frysinger <vapier@gentoo.org>
Cc: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
2022-12-22 09:49:02 -06:00
Alejandro Colomar
e2df287aad Don't redefine errno(3)
It is Undefined Behavior to declare errno (see NOTES in its manual page).
Instead of using the errno dummy declaration, use one that doesn't need
a comment.

Signed-off-by: Alejandro Colomar <alx@kernel.org>
2022-12-22 11:43:29 +01:00
Alejandro Colomar
b990b167d4 Cosmetic fixes
Previous commits, to keep readability of the diffs, left the code that
was previously wrapped by preprocessor coditionals untouched.  Apply
some minor cosmetic changes to merge it in the surrounding code.

Signed-off-by: Alejandro Colomar <alx@kernel.org>
2022-12-22 10:31:43 +01:00
Alejandro Colomar
170b76cdd1 Disable utmpx permanently
On Linux, utmpx and utmp are identical.  However, documentation (manual
pages) covers utmp, and just says about utmpx that it's identical to
utmp.  It seems that it's preferred to use utmp, at least by reading the
manual pages.

Moreover, we were defaulting to utmp (utmpx had to be explicitly enabled
at configuration time).  So, it seems safer to just make it permanent,
which should not affect default builds.

Signed-off-by: Alejandro Colomar <alx@kernel.org>
2022-12-22 10:31:43 +01:00
Michael Vetter
74c17c7167 Add support for skeleton files from /usr/etc/skel
This patch is used by openSUSE to make useradd look for
skeleton files in /usr/etc/skel additionally to /etc/skel
in accordance with
https://uapi-group.org/specifications/specs/base_directory_specification/
2022-12-19 09:43:03 -06:00
Alejandro Colomar
6b6e005ce1 Remove comments that survived the Helicoprion
The OSes that are referred to by these comments, are extinct, but
their comments survived, fossilized in amber.

Reported-by: Iker Pedrosa <ipedrosa@redhat.com>
Cc: Christian Göttsche <cgzones@googlemail.com>
Cc: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
2022-12-15 16:22:05 -06:00
Alejandro Colomar
61299d69ad Assume B[0-9]* macros are defined
All of the macros we're using are required by POSIX.1-2001.

Cc: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
2022-12-15 16:22:05 -06:00
Alejandro Colomar
f51c6838ac Assume SIGTTOU is defined
It is required by POSIX.1-2001.

Cc: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
2022-12-15 16:22:05 -06:00
Alejandro Colomar
307502d8b5 Assume SIGTSTP is defined
It is required by POSIX.1-2001.

Cc: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
2022-12-15 16:22:05 -06:00
Alejandro Colomar
74c8015730 Assume RLIMIT_STACK is defined
It is required by POSIX.1-2001.

Cc: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
2022-12-15 16:22:05 -06:00
Alejandro Colomar
c916715a6c Assume RLIMIT_NOFILE is defined
It is required by POSIX.1-2001.

Cc: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
2022-12-15 16:22:05 -06:00
Alejandro Colomar
5ebf28c999 Assume RLIMIT_FSIZE is defined
It is required by POSIX.1-2001.

Cc: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
2022-12-15 16:22:05 -06:00
Alejandro Colomar
91adf3b8bb Assume RLIMIT_DATA is defined
It is required by POSIX.1-2001.

Cc: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
2022-12-15 16:22:05 -06:00
Alejandro Colomar
891d8dbedd Assume RLIMIT_CPU is defined
It is required by POSIX.1-2001.

Cc: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
2022-12-15 16:22:05 -06:00
Alejandro Colomar
7a4906fc75 Assume RLIMIT_AS is defined
It is required by POSIX.1-2001.

Cc: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
2022-12-15 16:22:05 -06:00
Alejandro Colomar
e1a39e1dfc Assume RLIMIT_CORE is defined
It is required by POSIX.1-2001.

Cc: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
2022-12-15 16:22:05 -06:00
Alejandro Colomar
cbc363f671 Assume getgrgid_r(3) exists
It is required by POSIX.1-2001.

Cc: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
2022-12-15 16:22:05 -06:00
Alejandro Colomar
88eb38f4ab Assume getgrnam_r(3) exists
It is required by POSIX.1-2001.

Cc: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
2022-12-15 16:22:05 -06:00
Alejandro Colomar
e5e5df1966 Assume getpwuid_r(3) exists
It is required by POSIX.1-2001.

Cc: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
2022-12-15 16:22:05 -06:00
Alejandro Colomar
e788001977 Assume getpwnam_r(3) exists
It is required by POSIX.1-2001.

Cc: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
2022-12-15 16:22:05 -06:00
Alejandro Colomar
55c62b663f Assume l64a(3) exists
It is required by POSIX.1-2001.

Cc: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
2022-12-15 16:22:05 -06:00
Alejandro Colomar
b76d9b540a Remove preprocessor conditionals that are always true
Since the last commit, LIMITS is always defined.  Remove the dummy
macro, and all conditionals on it.

Signed-off-by: Alejandro Colomar <alx@kernel.org>
2022-12-15 16:22:05 -06:00
Alejandro Colomar
9d695340b4 Assume <sys/resource.h> exists
It is required by POSIX.1-2001.

Cc: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
2022-12-15 16:22:05 -06:00
Alejandro Colomar
cdaa04e460 Remove uses of ulimit(3)
The function is obsolete.  It is recommended to use getrlimit(2) instead
(see the manual page for ulimit(3) or the POSIX manual for it).  Since
getrlimit(2) is required by POSIX.1-2001, we can rely on it.

Cc: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
2022-12-15 16:22:05 -06:00
Alejandro Colomar
0527fa677b Add indentation to heavy use of preprocessor conditionals
This clarifies which code is under which conditions,
for further clenaup.

Signed-off-by: Alejandro Colomar <alx@kernel.org>
2022-12-15 16:22:05 -06:00
Guillem Jover
2a5b8810bb agetpass: Hook into build-system
Signed-off-by: Guillem Jover <guillem@hadrons.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
2022-12-05 10:47:19 +01:00
Alex Colomar
155c9421b9 libmisc: agetpass(), erase_pass(): Add functions for getting passwords safely
There are several issues with getpass(3).

Many implementations of it share the same issues that the infamous
gets(3).  In glibc it's not so terrible, since it's a wrapper
around getline(3).  But it still has an important bug:

If the password is long enough, getline(3) will realloc(3) memory,
and prefixes of the password will be laying around in some
deallocated memory.

See the getpass(3) manual page for more details, and especially
the commit that marked it as deprecated, which links to a long
discussion in the linux-man@ mailing list.

So, readpassphrase(3bsd) is preferrable, which is provided by
libbsd on GNU systems.  However, using readpassphrase(3) directly
is a bit verbose, so we can write our own wrapper with a simpler
interface similar to that of getpass(3).

One of the benefits of writing our own interface around
readpassphrase(3) is that we can hide there any checks that should
be done always and which would be error-prone to repeat every
time.  For example, check that there was no truncation in the
password.

Also, use malloc(3) to get the buffer, instead of using a global
buffer.  We're not using a multithreaded program (and it wouldn't
make sense to do so), but it's nice to know that the visibility of
our passwords is as limited as possible.

erase_pass() is a clean-up function that handles all clean-up
correctly, including zeroing the entire buffer, and then
free(3)ing the memory.  By using [[gnu::malloc(erase_pass)]], we
make sure that we don't leak the buffers in any case, since the
compiler will be able to enforce clean up.

Link: <https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit?id=7ca189099d73bde954eed2d7fc21732bcc8ddc6b>
Reported-by: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
2022-12-05 10:47:19 +01:00
Iker Pedrosa
d324c6776b libmisc: minimum id check for system accounts
The minimum id allocation for system accounts shouldn't be 0 as this is
reserved for root.

Signed-off-by: Tomáš Mráz <tm@t8m.info>
Signed-off-by: Iker Pedrosa <ipedrosa@redhat.com>
2022-10-06 20:09:35 -05:00
Alejandro Colomar
3dc1754e50 Use libc MAX() and MIN()
glibc, musl, FreeBSD, and OpenBSD define the MAX() and MIN()
macros in <sys/param.h> with the same definition that we use.
Let's not redefine it here and use the system one, as it's
effectively the same as we define (modulo whitespace).

See:

shadow (previously):

alx@asus5775:~/src/shadow/shadow$ grepc -ktm MAX
./lib/defines.h:318:#define MAX(x,y) (((x) > (y)) ? (x) : (y))

glibc:

alx@asus5775:~/src/gnu/glibc$ grepc -ktm -x 'sys/param.h$' MAX
./misc/sys/param.h:103:#define MAX(a,b) (((a)>(b))?(a):(b))

musl:

alx@asus5775:~/src/musl/musl$ grepc -ktm -x 'sys/param.h$' MAX
./include/sys/param.h:19:#define MAX(a,b) (((a)>(b))?(a):(b))

OpenBSD:

alx@asus5775:~/src/bsd/openbsd/src$ grepc -ktm -x 'sys/param.h$' MAX
./sys/sys/param.h:193:#define	MAX(a,b) (((a)>(b))?(a):(b))

FreeBSD:

alx@asus5775:~/src/bsd/freebsd/freebsd-src$ grepc -ktm -x 'sys/param.h$' MAX
./sys/sys/param.h:333:#define	MAX(a,b) (((a)>(b))?(a):(b))

Signed-off-by: Alejandro Colomar <alx@kernel.org>
2022-09-30 16:13:36 -05:00
Alex Colomar
0d9799de04 Don't test for NULL before calling free(3)
free(3) accepts NULL, since the oldest ISO C.  I guess the
paranoid code was taking care of prehistoric implementations of
free(3).  I've never known of an implementation that doesn't
conform to this, so let's simplify this.

Remove xfree(3), which was effectively an equivalent of free(3).

Signed-off-by: Alejandro Colomar <alx@kernel.org>
2022-09-29 16:03:53 +02:00
Samanta Navarro
cde221b858 copy_tree: carefully treat permissions
The setuid, setgid, and sticky bits are not copied during copy_tree.

Also start with very restrictive permissions before setting ownerships.

This prevents situations in which users in a group with less permissions
than others could win a race in opening the file before permissions are
removed again.

Proof of concept:

$ echo $HOME
/home/uwu
$ install -o uwu -g fandom -m 604 /dev/null /home/uwu/owo
$ ls -l /home/uwu/owo
-rw----r-- 1 uwu fandom 0 Sep  4 00:00 /home/uwu/owo

If /tmp is on another filesystem, then "usermod -md /tmp/uwu uwu" leads
to this temporary situation:

$ ls -l /tmp/uwu/owo
-rw----r-- 1 root root  0 Sep  4 00:00 /tmp/uwu/owo

This means that between openat and chownat_if_needed a user of group
fandom could open /tmp/uwu/owo and read the content when it is finally
written into the file.
2022-09-14 10:11:32 +02:00
Samanta Navarro
10cd68e0f0 copy_tree: do not block on fifos
Fixes regression introduced in faeab50e71.

If a directory contains fifos, then openat blocks until the other side
of the fifo is connected as well.

This means that users can prevent "usermod -m" from completing if their
home directories contain at least one fifo.
2022-09-09 15:19:12 +02:00
Samanta Navarro
f3bdb28e57 copy_tree: use fchmodat instead of chmod
Fixes regression introduced in faeab50e71
for setups configured without acl support.
2022-09-09 15:19:12 +02:00
Alexander Kanavin
cfc981df2a shadow: use relaxed usernames
The groupadd from shadow does not allow upper case group names, the
same is true for the upstream shadow. But distributions like
Debian/Ubuntu/CentOS has their own way to cope with this problem,
this patch is picked up from Fedora [1] to relax the usernames
restrictions to allow the upper case group names, and the relaxation is
POSIX compliant because POSIX indicate that usernames are composed of
characters from the portable filename character set [A-Za-z0-9._-].

[1] https://src.fedoraproject.org/rpms/shadow-utils/blob/rawhide/f/shadow-4.8-goodname.patch

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
2022-09-02 20:27:14 -05:00
Christian Göttsche
faeab50e71 Avoid races in copy_tree()
Use *at() functions to pin the directory operating in to avoid being
redirected by unprivileged users replacing parts of paths by symlinks to
privileged files.

Introduce a path_info struct with the full path and dirfd and name
information for *at() functions, since the full path is needed for link
resolution, SELinux label lookup and ACL attributes.
2022-08-17 12:34:01 -05:00
Christian Göttsche
6cbec2d0aa Address minor compiler warnings
copydir.c:666:44: warning: unsigned conversion from 'int' to '__mode_t' {aka 'unsigned int'} changes value from '-4096' to '4294963200' [-Wsign-conversion]
      666 |         if (   (mknod (dst, statp->st_mode & ~07777, statp->st_rdev) != 0)
          |                                            ^

    copydir.c:116:1: warning: missing initializer for field 'quote' of 'struct error_context' [-Wmissing-field-initializers]
      116 | };
          | ^
    In file included from copydir.c:27:
    /usr/include/attr/error_context.h:30:23: note: 'quote' declared here
       30 |         const char *(*quote) (struct error_context *, const char *);
          |                       ^~~~~
2022-08-17 12:34:01 -05:00
Christian Göttsche
f606314f0c More robust file content copy in copy_tree()
Bail out on read(2) failure, continue on EINTR, support short writes and
increase chunk size.
2022-08-17 12:34:01 -05:00
Christian Göttsche
1d281273b1 Fail if regular file pre-exists in copy_tree()
Similar to the default behavior of mkdir(2), symlink(2), link(2) and
mknod(2).
2022-08-17 12:34:01 -05:00
Christian Göttsche
dab764d019 Require symlink support
Require lstat(2), lchown(2), S_IFLNK and S_ISLNK from POSIX.1-2001.

Already unconditionally used in lib/tcbfuncs.c and lib/run_part.c.
2022-08-17 12:34:01 -05:00