Am Donnerstag, 28. Juli 2016, 18:07:32 CEST schrieb Alex Xu:
Hi Alex,
> Linux 4.6, also tried 4.7, qemu 2.6, using this C program:
I am not sure what problem you are referring to, but that is an expected
behavior.
You get partial reads when reading from /dev/random with a minimum of 64
bits. On the other hand getrandom(2) is woken up after the input_pool
received 128 bits of entropy.
In you strace you see that after reading 16 bytes from /dev/random, the
getrandom unblocks and starts delivering.
Note, in virtualized environments the current Linux /dev/random
implementation collects massively less entropy compared to a bare-metal
system. Hence the long wait time of your 90 to 100 secs until getrandom
unblocks.
Besides, even without reading from /dev/random, your getrandom will wait that
long.
And finally, you have a coding error that is very very common but fatal when
reading from /dev/random: you do not account for short reads which implies
that your loop continues even in the case of short reads.
Fix your code with something like the following:
int read_random(char *buf, size_t buflen)
{
int fd = 0;
ssize_t ret = 0;
size_t len = 0;
fd = open("/dev/random", O_RDONLY|O_CLOEXEC);
if(0 > fd)
return fd;
do {
ret = read(fd, (buf + len), (buflen - len));
if (0 < ret)
len += ret;
} while ((0 < ret || EINTR == errno || ERESTART == errno)
&& buflen > len);
...
Ciao
Stephan
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