rados_connect not thread-safe when using nss (documentation)

I'm not sure whether rados_connect is expected to be threadsafe or not,
so this is just a documentation patch rather than a fix; I'd appreciate
your opinion on whether this is expected behaviour or not.

The race condition is in the call to ceph::crypto::init when called by
common_init_finish, the issue being that it calls NSS_NoDB_Init (not
threadsafe) without locking. It can be reproduced (probabilistically) by
calling rados_connect on different rados_t objects simultaneously, due
to NSS_NoDB_Init's use of PR_CallOnce in nspr (which keeps global state,
and while PR_CallOnce is intended as a locking function, the locking
itself isn't thread-safe, and can pass PR_Lock a null pointer).

The observed behaviour is a segfault on calling rados_connect.
Backtrace, for reference:

    secmodName=secmodName@entry=0x7ffff4f64817 "", updateDir=updateDir@entry=0x7ffff4f64817 "", updCertPrefix=updCertPrefix@entry=0x7ffff4f64817 "",
    updKeyPrefix=updKeyPrefix@entry=0x7ffff4f64817 "", updateID=updateID@entry=0x7ffff4f64817 "", updateName=updateName@entry=0x7ffff4f64817 "",
    initContextPtr=initContextPtr@entry=0x0, initParams=initParams@entry=0x0, readOnly=readOnly@entry=1, noCertDB=noCertDB@entry=1, noModDB=noModDB@entry=1,
    forceOpen=forceOpen@entry=1, noRootInit=noRootInit@entry=1, optimizeSpace=optimizeSpace@entry=1, noSingleThreadedModules=noSingleThreadedModules@entry=0,
    allowAlreadyInitializedModules=allowAlreadyInitializedModules@entry=0, dontFinalizeModules=dontFinalizeModules@entry=0) at nssinit.c:551

Signed-off-by: Sharif Olorin <sio@tesser.org>

diff --git a/src/include/rados/librados.h b/src/include/rados/librados.h
index f0d45af34b86a1921826e37ddc4ddcdac49bed78..7254e3ad0ba4ed260f6f06ee1e8b59b4884751b7 100644 (file)
--- a/src/include/rados/librados.h
+++ b/src/include/rados/librados.h
@@ -355,6 +355,9 @@ int rados_ping_monitor(rados_t cluster, const char *mon_id,
  *
  * @post If this succeeds, any function in librados may be used
  *
+ * @warning This function is not thread-safe if using libnss (built with
+ * --with-nss); callers must implement their own locking mechanisms.
+ *
  * @param cluster The cluster to connect to.
  * @returns 0 on sucess, negative error code on failure
  */