getrandom: Fail with ENOSYS when the system has no randomness source

getrandom: Fail with ENOSYS when the system has no randomness source

[Bruno Haible]

On IRIX 6.5, 'test-getrandom' fails:

$ ./test-getrandom 
../../tests/test-getrandom.c:43: assertion 'errno == ENOSYS' failed

The glibc documentation
<https://www.gnu.org/software/libc/manual/html_node/Unpredictable-Bytes.html>
says that when "The operating system does not implement a randomness source"
the function should return with errno set to ENOSYS. This is what our unit
test is verifying. So, it's our getrandom() implementation that needs a fix.


2021-05-09  Bruno Haible  <bruno@clisp.org>

	getrandom: Fail with ENOSYS when the system has no randomness source.
	* lib/getrandom.c (getrandom): When open() fails, set errno to ENOSYS.

diff --git a/lib/getrandom.c b/lib/getrandom.c
index 41212fb..6160118 100644
--- a/lib/getrandom.c
+++ b/lib/getrandom.c
@@ -178,7 +178,10 @@ getrandom (void *buffer, size_t length, unsigned int flags)
                     + (flags & GRND_NONBLOCK ? O_NONBLOCK : 0));
       fd = open (randdevice[devrandom], oflags);
       if (fd < 0)
-        return fd;
+        {
+          errno = ENOSYS;
+          return -1;
+        }
       randfd[devrandom] = fd;
     }
 

Re: getrandom: Fail with ENOSYS when the system has no randomness source

[Paul Eggert]

On 5/9/21 7:51 AM, Bruno Haible wrote:
>         if (fd < 0)
> -        return fd;
> +        {
> +          errno = ENOSYS;
> +          return -1;
> +        }

If 'open' fails with errno equal to (say) EINTR or EAGAIN or EMFILE, 
this doesn't mean the operating system lacks a randomness source; it 
merely means the 'open' failed. And the libc manual allows getrandom to 
fail with EMFILE or with any other valid error number. (FWIW, the Hurd 
implementation of getrandom simply passes the errno of 'open' through.)

How about if we instead change that code to something like this:

  if (fd < 0)
    {
       if (errno == ENOENT || errno == ENOTDIR)
         errno = ENOSYS;
       return fd;
    }

That is, if /dev/random (or whatever) doesn't exist, we assume the OS is 
like IRIX and lacks randomness support, so we fail with ENOSYS; 
otherwise we pass errno through as that's more useful to the caller.

Re: getrandom: Fail with ENOSYS when the system has no randomness source

[Bruno Haible]

Paul Eggert wrote:
> On 5/9/21 7:51 AM, Bruno Haible wrote:
> >         if (fd < 0)
> > -        return fd;
> > +        {
> > +          errno = ENOSYS;
> > +          return -1;
> > +        }
> 
> If 'open' fails with errno equal to (say) EINTR or EAGAIN or EMFILE, 
> this doesn't mean the operating system lacks a randomness source; it 
> merely means the 'open' failed. And the libc manual allows getrandom to 
> fail with EMFILE or with any other valid error number. (FWIW, the Hurd 
> implementation of getrandom simply passes the errno of 'open' through.)
> 
> How about if we instead change that code to something like this:
> 
>   if (fd < 0)
>     {
>        if (errno == ENOENT || errno == ENOTDIR)
>          errno = ENOSYS;
>        return fd;
>     }
> 
> That is, if /dev/random (or whatever) doesn't exist, we assume the OS is 
> like IRIX and lacks randomness support, so we fail with ENOSYS; 
> otherwise we pass errno through as that's more useful to the caller.

Good point. Thanks for the correction. Done:


2021-05-13  Bruno Haible  <bruno@clisp.org>

	getrandom: Produce a better error code.
	Reported by Paul Eggert in
	<https://lists.gnu.org/archive/html/bug-gnulib/2021-05/msg00025.html>.
	* lib/getrandom.c (getrandom): When open() fails with an error that does
	not indicate the absence of the file, fail with that error code, not
	with ENOSYS.

diff --git a/lib/getrandom.c b/lib/getrandom.c
index 6160118..9e90e64 100644
--- a/lib/getrandom.c
+++ b/lib/getrandom.c
@@ -179,7 +179,8 @@ getrandom (void *buffer, size_t length, unsigned int flags)
       fd = open (randdevice[devrandom], oflags);
       if (fd < 0)
         {
-          errno = ENOSYS;
+          if (errno == ENOENT || errno == ENOTDIR)
+            errno = ENOSYS;
           return -1;
         }
       randfd[devrandom] = fd;