[Bug 200589] Kerberos authentication slow in many processes simultaneously

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=200589

            Bug ID: 200589
           Summary: Kerberos authentication slow in many processes
                    simultaneously
           Product: Base System
           Version: 8.4-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: bin
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: martin.beran at kernun.cz

Our Kernun HTTP proxy performs Kerberos (Negotiate) authentication in Active
Directory by calling gss_acquire_cred, gss_accept_sec_context. When there are
many (several thousand) proxy processes authenticating simultaneously,
authentication operation becomes slow. 

A probable cause is in the Kerberos library, which uses exclusive fcntl lock on
the keytab file. It is slow when many processes are trying to obtain the lock
simultaneously. Moreover, gss_acquire_cred reads the keytab file twice and
gss_accept_sec_context once. Each reading of the keytab file consists of may
read syscalls, each reading a few bytes. Maybe it would be more efficient to
lock the keytab using a shared lock, or, optionally, not to lock it. Also, the
keytab could be read by larger blocks, using fewer read syscalls.

Observed with Heimdal Kerberos from the base system.

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