[PATCH v3 1/2] man/man3/getcwd.3: say more clear that syscall can return "(unreachable)", but modern glibc wrapper cannot

[Askar Safin <safinaskar@xxxxxxxxxxxx>]

I verified using expirement that modern glibc wrapper getcwd actually never returns "(unreachable)".
Also I have read modern glibc sources for all 3 functions documented here.
All they don't return "(unreachable)".
Also I changed "pathname" to "pathnames".

Now let me describe my expirement:

	d-user@comp:/tmp$ cat getcwd.c
	#include <unistd.h>
	#include <stdio.h>
	#include <sys/syscall.h>

	int
	main(void)
	{
		char  buf[1000];

		if (syscall(SYS_getcwd, buf, sizeof(buf)) == -1)
			perror("SYS_getcwd");
		else
			printf("SYS_getcwd: %s\n", buf);

		if (getcwd(buf, sizeof(buf)) == NULL)
			perror("getcwd");
		else
			printf("getcwd: %s\n", buf);

		return 0;
	}
	d-user@comp:/tmp$ gcc -Wall -Wextra -o getcwd getcwd.c
	d-user@comp:/tmp$ sudo unshare --mount bash
	d-root@comp:/tmp# mkdir /tmp/dir
	d-root@comp:/tmp# mount -t tmpfs tmpfs /tmp/dir
	d-root@comp:/tmp# cd /tmp/dir
	d-root@comp:/tmp/dir# umount -l .
	d-root@comp:/tmp/dir# /tmp/getcwd
	SYS_getcwd: (unreachable)/
	getcwd: No such file or directory
	d-root@comp:/tmp/dir# exit
	exit

Cc: Carlos O'Donell <carlos@xxxxxxxxxx>
Link: <https://sourceware.org/bugzilla/show_bug.cgi?id=18203>
Link: <https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=52a713fdd0a30e1bd79818e2e3c4ab44ddca1a94>
Signed-off-by: Askar Safin <safinaskar@xxxxxxxxxxxx>
---
 man/man3/getcwd.3 | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/man/man3/getcwd.3 b/man/man3/getcwd.3
index 685585a60..919ffb08f 100644
--- a/man/man3/getcwd.3
+++ b/man/man3/getcwd.3
@@ -246,7 +246,10 @@ without changing its current directory into the new root).
 Such behavior can also be caused by an unprivileged user by changing
 the current directory into another mount namespace.
 When dealing with pathname from untrusted sources, callers of the
-functions described in this page
+functions described in this page (until glibc 2.27)
+or the raw
+.BR getcwd ()
+system call
 should consider checking whether the returned pathname starts
 with '/' or '(' to avoid misinterpreting an unreachable path
 as a relative pathname.
-- 
2.39.5