unittest_crypto: Don't exceed limit for getentropy

[badone]

CryptoRandom::get_bytes calls getentropy directly if available and it
enforces a limit of 256 bytes.

Signed-off-by: Brad Hubbard bhubbard@redhat.com

[badone]

Thread 1 "unittest_crypto" hit Breakpoint 1, getentropy (
buffer=buffer@entry=0x5555558c7ea0, length=length@entry=16)
at ../sysdeps/unix/sysv/linux/getentropy.c:32
32 if (length > 256)
(gdb) l
27 int
28 getentropy (void buffer, size_t length)
29 {
30 / The interface is documented to return EIO for buffer lengths
31 longer than 256 bytes. */
32 if (length > 256)
33 {
34 __set_errno (EIO);
35 return -1;
36 }

[cbodley]

thanks @badone. do you or @tchaikov have suggestions for how to better document/enforce this limitation? currently it's just this comment in CryptoRandom:

  /// copy up to 256 random bytes into the given buffer. throws on failure
  void get_bytes(char *buf, int len);

edit: i suppose this value should be exposed by the interface, so callers can loop over 256-byte chunks if they need to

[badone]

@cbodley I'll look into how we can enforce this at compile time in a different PR, thanks.