tree ea5b0dc49aaa16f25e2262cd5560070ae65923b9
parent 21cdd22f035efd50902b7780fc6ea53bd1684357
author Nick Kralevich <nnk@google.com> 1422763066 -0800
committer Nick Kralevich <nnk@google.com> 1422911837 -0800

Add fchmodat(AT_SYMLINK_NOFOLLOW) and fchmod O_PATH support

Many libc functions have an option to not follow symbolic
links. This is useful to avoid security sensitive code
from inadvertantly following attacker supplied symlinks
and taking inappropriate action on files it shouldn't.
For example, open() has O_NOFOLLOW, chown() has
lchown(), stat() has lstat(), etc.

There is no such equivalent function for chmod(), such as lchmod().
To address this, POSIX introduced fchmodat(AT_SYMLINK_NOFOLLOW),
which is intended to provide a way to perform a chmod operation
which doesn't follow symlinks.

Currently, the Linux kernel doesn't implement AT_SYMLINK_NOFOLLOW.
In GLIBC, attempting to use the AT_SYMLINK_NOFOLLOW flag causes
fchmodat to return ENOTSUP. Details are in "man fchmodat".

Bionic currently differs from GLIBC in that AT_SYMLINK_NOFOLLOW
is silently ignored and treated as if the flag wasn't present.

This patch provides a userspace implementation of
AT_SYMLINK_NOFOLLOW for bionic. Using open(O_PATH | O_NOFOLLOW),
we can provide a way to atomically change the permissions on
files without worrying about race conditions.

As part of this change, we add support for fchmod on O_PATH
file descriptors, because it's relatively straight forward
and could be useful in the future.

The basic idea behind this implementation comes from
https://sourceware.org/bugzilla/show_bug.cgi?id=14578 , specifically
comment #10.

Change-Id: I1eba0cdb2c509d9193ceecf28f13118188a3cfa7