identity assertion in back-ldap
by Howard Chu
It appears that one of the more basic modes of operation is now missing in
back-ldap: if a direct bind is performed, then subsequent operations using
that identity should not do any proxyAuthz/identity assertion at all. They
should just re-use the already bound connection. I expected that this was the
behavior for idassert mode=legacy but apparently that's not the case.
Trawling thru the old discussions (from May 2004) wasn't too enlightening. I
suspect there's a bug here, but I'm not sure which of the assert modes to fix.
Also, the slapd-ldap(5) manpage now says "Direct binds are always proxied" but
it's not clear what the significance of that statement is. I.e., yes,
back-ldap has always passed direct binds through straight to the remote
server. Since this sentence occurs within the description of mode=legacy, are
we to imply that this is now only true for legacy mode, and in other modes
direct Binds may be munged in some other way?
The function ldap_back_is_proxy_authz() behaves as if the latter were true -
it returns 1 even if the op is a Bind request. But in fact the Bind will
always be sent thru to the remote server. What happens next, though, depends
on these various settings. I.e., after the connection has bound successfully,
it might be marked as bound with that ID, and available for further use by
that ID, or it may get left marked as anonymous, to be re-used for future Bind
requests.
The code and the manpage are both pretty unclear on what settings should be
used to get this combination of behavior:
assert identity on any ID that was not authenticated by this backend
passthru anything that was authenticated by this backend
--
-- Howard Chu
CTO, Symas Corp. http://www.symas.com
Director, Highland Sun http://highlandsun.com/hyc/
Chief Architect, OpenLDAP http://www.openldap.org/project/
10 years, 6 months
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MDB, read/write mmap
by Howard Chu
A suggestion was made to use a read/write mmap (as an option), to allow writes
to be performed with no syscall overhead. I'm thinking that might be ok as a
completely separate version of the library, because a fair bit of the code
would need to change to accommodate that update style, and it would push the
library over the 32K boundary.
Also this isn't as cool a suggestion as it sounds - it completely gives up
MDB's current immunity to corruption, and in fact makes reliability much less
stable. When you write through an mmap, you have absolutely no idea when the
OS is going to get around to flushing the data back to disk. You have no idea
what order the flushes will occur in. You can force the OS's hand, by calling
msync on every page you want to flush, in the order you want them flushed, but
then you'll just get back to having syscall overhead again, and by calling
msync in a particular order, you defeat the underlying filesystem's ability to
schedule the writes for optimum seeks.
Currently, by using writev, we can push a lot of data to the OS, and then when
we call fdatasync() at the end, the OS schedules those writes as it sees fit.
Right now the only ordering dependency MDB has is that all of the data pages
must be flushed successfully before flushing the meta page, so we can afford
to let the OS schedule all of the data page writes, and then do an explicitly
synchronous write of the meta page.
So, with a writable mmap, we're stuck with the choice of either (a) not
knowing at all whether our data has been flushed, or (b) being forced to
explicitly flush every page ourselves, in a predetermined order which we have
no way of knowing whether or not it's optimal for the current disk layout.
It seems to me this can only be a viable mode of operation if you're always
going to run asynch and don't care much about transaction durability or DB
recoverability. Running in this mode offers absolutely zero crash resistance;
the entire DB will almost always be irreparably damaged after a system crash.
Would you run like that, if it offered you the potential of maybe 10x faster
write performance? (It could be useful for slapadd -q, certainly.)
--
-- Howard Chu
CTO, Symas Corp. http://www.symas.com
Director, Highland Sun http://highlandsun.com/hyc/
Chief Architect, OpenLDAP http://www.openldap.org/project/
10 years, 6 months
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