[gnutls-devel] GnuTLS | Dummy getrandom() definition can cause have_getrandom() = 1, causing TLS failure (#892)

[Development of GNU's TLS library]

Edward Stangler created an issue: https://gitlab.com/gnutls/gnutls/issues/892



## Description of problem:

When compiling on Linux that doesn't have getrandom() at all:

```
/* if defined(__linux__) && !defined(HAVE_GETRANDOM) && !defined(SYS_getrandom) */

#   define getrandom(dst,s,flags) -1
```

Then if this executes (on a Linux with kernel < 3.17, for example) during GNU TLS init:

```
static unsigned have_getrandom(void)
{
        char c;
        int ret;
        ret = getrandom(&c, 1, 1/*GRND_NONBLOCK*/);
        if (ret == 1 || (ret == -1 && errno == EAGAIN))
                return 1;
        return 0;
}
```

Then have_getrandom() = 1 when GNU TLS init is executed right after an API call that sets errno = EAGAIN.  This causes TLS failure.

This actually happened.  At a very bad time.

Our patch was to change this line:

```
#   define getrandom(dst,s,flags) -1
```

to this:

```
static ssize_t _getrandom0(void *buf, size_t buflen, unsigned int flags)
{
        errno = ENOSYS;
        return -1;
}
#   define getrandom(dst,s,flags) _getrandom0(dst,s,flags)
```

## Version of gnutls used:
3.6.10

## Distributor of gnutls (e.g., Ubuntu, Fedora, RHEL)
None

## How reproducible:
Always

Steps to Reproduce:

 * one
 * two
 * three

## Actual results:


## Expected results:

-- 
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