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On Unix (lib/freebl/unix_rand.c), RNG_SystemInfoForRNG calls RNG_FileForRNG on the directories /tmp, /var/tmp, and /usr/tmp. RNG_FileForRNG first calls fopen on the directory, and then calls fread on the FILE * pointer. It seems that the goal is to do a directory listing (to get a list of the files in a directory). What actually happens is that fopen succeeds but fread fails. So the RNG_FileForRNG calls on the directories don't seem to do what they are intended for.
Once upon a time, the (binary) contents of a directory could be read, just as any other file's contents could be read, by opening them and reading them. Besides file names, they also contains inode numbers (IIRC), which were rather unpredictable (especially in /tmp) and therefore were thought to be a pretty good source of entropy. This code clearly reflects that era, days gone by now, apparently, except on HP where the results still look interesting, but may not have much entropy.