Re: getrandom syscall

["Daniel Cegiełka" <daniel.cegielka@gmail.com>]

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2015-01-28 17:21 GMT+01:00 Rich Felker <dalias@libc.org>:
> On Wed, Jan 28, 2015 at 05:12:31PM +0100, Daniel Cegiełka wrote:
>>
>> int getentropy(void *buf, size_t buflen)
>> {
>>       int ret;
>>
>>       if (buflen > 256)
>>               goto failure;
>>       ret = getrandom(buf, buflen, 0);
>>       if (ret < 0)
>>               return ret;
>>       if (ret == buflen)
>>               return 0;
>> failure:
>>       errno = EIO;
>>       return -1;
>> }
>
> Is it intentional to fall through to failure when ret is positive but
> less than buflen? Can this happen?

This is a Theodore Tso version...


>
>> #include "syscall.h"
>>
>> int getrandom(void *buf, size_t buflen, unsigned int flags)
>> {
>>       return syscall(SYS_getrandom, buf, buflen, flags);
>> }
>
> If I read the documentation correctly, the removed EINTR handling is
> irrelevant since the kernel guarantees not to EINTR for <=256 bytes
> with the default flags, right?

yes, and if it is above 256, it can be blocked and there is no guarantee.



> Rich

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int getentropy(void *buf, size_t len)
{
	int ret, pre_errno = errno;

	if (len > 256) {
		errno = EIO;
		return -1;
	}
	while ((ret = __syscall(SYS_getrandom, buf, len, 0)) == -EINTR);
	if (ret != len)
		return -1;
	errno = pre_errno;
	return 0;
}