Commit f2633200 authored by Benjamin Kaduk's avatar Benjamin Kaduk Committed by Ben Kaduk
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Document RAND_DRBG fork-safety locking model 



Add some more exposition on why unlocked access to the global rand_fork_count
is safe, and provide a comment for the struct rand_drbg_st fork_count field.

Reviewed-by: default avatarMatt Caswell <matt@openssl.org>
Reviewed-by: default avatarMatthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/4110)
parent fffb1c54

crypto/rand/rand_lcl.h

+17 −1
Original line number Diff line number Diff line
@@ -116,6 +116,12 @@ struct rand_drbg_st { 
    RAND_DRBG *parent;

    int secure; /* 1: allocated on the secure heap, 0: otherwise */

    int type; /* the nid of the underlying algorithm */

    /*

     * Stores the value of the rand_fork_count global as of when we last

     * reseeded.  The DRG reseeds automatically whenever drbg->fork_count !=

     * rand_fork_count.  Used to provide fork-safety and reseed this DRBG in

     * the child process.

     */

    int fork_count;

    unsigned short flags; /* various external flags */
@@ -202,7 +208,17 @@ struct rand_drbg_st { 
/* The global RAND method, and the global buffer and DRBG instance. */

extern RAND_METHOD rand_meth;



/* How often we've forked (only incremented in child). */

/*

 * A "generation count" of forks.  Incremented in the child process after a

 * fork.  Since rand_fork_count is increment-only, and only ever written to in

 * the child process of the fork, which is guaranteed to be single-threaded, no

 * locking is needed for normal (read) accesses; the rest of pthread fork

 * processing is assumed to introduce the necessary memory barriers.  Sibling

 * children of a given parent will produce duplicate values, but this is not

 * problematic because the reseeding process pulls input from the system CSPRNG

 * and/or other global sources, so the siblings will end up generating

 * different output streams.

 */

extern int rand_fork_count;



/* DRBG helpers */