fchmodat(AT_SYMLINK_NOFOLLOW)

[Eric Blake]

glibc's current implementation of fchmodat() is non-compliant when
AT_SYMLINK_NOFOLLOW is used.  qemu just recently rediscovered what musl
libc has already known for three years: on kernels new enough to support
O_PATH, you can get a race-free implementation that supports
AT_SYMLINK_NOFOLLOW even without kernel support of the proposed
fchmodat2() syscall, by using fstat() to double-check whether the
open(O_PATH) name is a symlink, before using chmod(/proc/self/fd/nnn) to
change the mode without a race.

Are we interested in improving gnulib's fchmodat() module to implement
this workaround?

musl libc also claims that with Linux' O_PATH, it is possible to
implement POSIX-compliant O_SEARCH/O_EXEC fds.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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