man/man3/getcwd.3: VERSIONS: The syscall can return "(unreachable)", but modern glibc...

I verified using an expirement (see below) that the modern glibc wrapper
getcwd() actually never returns "(unreachable)".  I have also read the
modern glibc sources for all three functions documented here.  None of
them return "(unreachable)".

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

Link: <https://sourceware.org/bugzilla/show_bug.cgi?id=18203>
Link: <https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=52a713fdd0a30e1bd79818e2e3c4ab44ddca1a94>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Signed-off-by: Askar Safin <safinaskar@zohomail.com>
Message-ID: <20250220091926.3985504-2-safinaskar@zohomail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

diff --git a/man/man3/getcwd.3 b/man/man3/getcwd.3
index 685585a604fa537c8b729de5c9b925a4fd0f7a2f..ba4ef28c6c60628ffcef8d16d9f0be999770be18 100644 (file)
--- 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 (before 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.