[Bug libc/22899] New: Use 64-bit readdir() in generic POSIX getcwd()

["nick.alcock at oracle dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=22899

            Bug ID: 22899
           Summary: Use 64-bit readdir() in generic POSIX getcwd()
           Product: glibc
           Version: 2.28
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: nick.alcock at oracle dot com
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

The Linux-specific getcwd() implementation calls the getcwd() syscall, but can
in theory fall back to the generic POSIX implementation. For a long time it
would always do this except when the path was longer than a page (most
unlikely), so bugs in the generic implementation went almost unnoticed: but
after the fix for CVE-2018-1000001 in #22679 it will also fall back to the
generic implementation when disconnected paths are encountered (perhaps due to
umount -l or directory deletion).

Unfortunately, this breaks getcwd() on such paths in 32-bit binaries when,
during the search of all parent directories for elements of the cwd, any ino_t
which is not representable in 64 bits is encountered. This is clearly shown by
io/test-getcwd-abspath, which returns e.g.

tst-getcwd-abspath.c:42: numeric comparison failure
   left: 75 (0x4b); from: errno
  right: 2 (0x2); from: ENOENT
tst-getcwd-abspath.c:47: numeric comparison failure
   left: 75 (0x4b); from: errno
  right: 2 (0x2); from: ENOENT
error: 2 test failures

in this situation (75 == EOVERFLOW, ultimately returned from getdents()).

The fix is to call readdir64() from this implementation.

Patch in preparation.

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