getcwd() returns with stale errno value upon error

[Richard Weinberger]

Configuration Information [Automatically generated, do not change][RW: The 
configuration info generated by bashbug is useless, as it is a cross build]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS: -g -O2
uname output: Linux foxxylove 6.4.0-150600.23.47-default #1 SMP PREEMPT_DYNAMIC 
Thu Apr  3 03:44:04 UTC 2025 (2854fd7) x86_64 x86_64 x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 5.2
Patch Level: 37
Release Status: release

Description:
        getcwd() as implemented in lib/sh/getcwd.c can return with NULL and a
        stale errno value if the algorithm fails. It aborts if readdir()
        returns NULL and no matching directory entry was found.
        But bash's getcwd() assumes that NULL from readdir() is always an error,
        which is not the case.

        This can happen on filesystems where d_ino != st_ino such as
        on Linux's OverlayFS.

        I have observed error messages like:
        shell-init: error retrieving current directory: getcwd: cannot access 
parent directories: Inappropriate ioctl for device
        Or:
        shell-init: error retrieving current directory: getcwd: cannot access 
parent directories: Success

Repeat-By:
        The problem happens when the bash getcwd() implementation is used and 
the filesystem
        has odd semantics regarding inode numbers reported by readdir() and 
stat().
        Such semantics can happen with OverayFS on Linux but also with many 
other network or
        userspace filesystems.

Fix:
        I suggest as a fix setting errno to 0 before calling readdir() to 
distinguish
        between the error and end-of-directory case and setting errno to EINVAL 
or such
        if the algorithm is unable to determine the directory name.