On krb5_db_alloc()

Simo Sorce simo at redhat.com
Mon May 16 09:47:45 EDT 2016

On Thu, 2016-05-12 at 14:27 -0400, Greg Hudson wrote:
> In the krb5 project, we try to operate on the assumption that
> differently compiled code units might use incompatible memory
> allocators.  This situation arises frequently on Windows and rarely on
> Unix-like systems.  For most interfaces, accomodating this assumption
> means providing a free operation corresponding to every allocation
> operation; for example, libkrb5 callers are expected to free the result
> of krb5_cc_full_name() with krb5_free_string() instead of just free().
> For the KDB layer, there are three layers which might be differently
> compiled: the caller, the krb5 libraries (libkdb5/libkrb5), and the KDB
> module.  Accomodating different allocators is less trivial than for most
> interfaces; it is common for callers of libkdb5 to fetch a principal
> entry, modify it (such as by changing the key data or tl-data), put the
> principal entry, and then free it.
> Our current design tries to accomodate KDB modules with different
> allocators by including alloc() and free() methods in the KDB interface,
> typically implemented by just calling realloc() and free() inside the
> module.  When a libkdb5 caller modifies a principal entry, it is
> expected to allocate new memory for any fields or sub-fields with
> krb5_db_alloc() and free any old fields with krb5_db_free().
> Experience has shown that it's really hard to get this right.  If a
> caller or custom KDB module needs to manipulate the principal name, it
> shouldn't use krb5_copy_principal(), krb5_build_principal(), or
> krb5_free_principal(), because those use the libkrb5 allocator.  Similar
> problems apply to manipulating key data and occasionally tl-data.
> When we screw up this discipline (and we have, many times), we don't
> notice except by code inspection.  We are usually testing against
> in-tree KDB modules which are built in the same way as the libraries,
> and we don't support libkdb5 on Windows.
> All of the above concerns also apply to policy entries, although policy
> entries have a simpler structure.
> I would like to change the contract so that principal and policy entry
> memory is all allocated with the allocator visible to the krb5
> libraries, not the one visible to the KDB module.  I can see two ways to
> accomplish this:
> 1. Whenever we fetch a principal or policy entry, immediately copy it
> into library-allocated memory and then ask the module to free its copy.

This is wasteful.

> 2. Document that KDB modules must allocate memory using krb5 library
> functions or a compatible allocator.  Modules could call krb5_db_alloc()
> to allocate an individual memory region, or a function like
> krb5_copy_principal() for a more complicated field.  KDB modules can
> also get away with just using malloc() as long as this stuff isn't used
> on Windows.
> In both of these proposals, krb5_db_alloc() and krb_db_free() would
> change to just call realloc() or free() themselves.  The kdb_vftabl
> entries for those functions would be removed.
> Comments?

I'm for 2.


Simo Sorce * Red Hat, Inc * New York

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