I was looking at POSIX ACL on-disk and in-memory code and it looks like
there is a subtle dependency between the on-disk format and what (IMHO)
would be considered in-memory declarations.

When a POSIX ACL is read from disk, posix_acl_from_mode() copies the file
mode (S_I[RWX][UGO]) into the e_perm fields of the ACL default entries.
Similarly, in posix_acl_equiv_mode() and posix_acl_create_masq() it uses
S_IRWXO to mask the e_perm flags.

However, later on in posix_acl_permission() it directly uses the "want"
flag contains MAY_{READ,WRITE,EXEC} flags and compares those to e_perm of
each ACL entry.

In posix_acl_valid() it compares e_perm with ACL_{READ,WRITE,EXECUTE}.

While the MAY_[RWX] and ACL_[RWX] currently have the same value as
S_I[RWX]OTH, it isn't very clear that these flags MUST all have the same
values or POSIX ACLs will break.

This definitely doesn't seem quite right.  Are the ACL_* constants the
values to be used, with "conversion" in between the flags/modes?  Should
there be a BUILD_BUG_ON() that trips if those values ever differ?

Cheers, Andreas