MNT_NOEXEC on root filesystem with diskless PXE boot?

[Colin Percival]

Tom Alsberg wrote:
> Perhaps this should go to -STABLE, I just couldn't be sure.

It will get more attention on freebsd-stable@, so I'm CCing that list.

> We are trying out FreeBSD 5.4-PRERELEASE on diskless clients.  I
> noticed one problem, being that when setting the LD_LIBRARY_PATH
> (or for that matter, LD_PRELOAD, and LD_LIBMAP_DISABLE) environment
> variables, nothing will run, as /libexec/ld-elf.so.1 complains:
> 
> Cannot execute objects on /
> 
> According to the sources, this was added in 5.4, and will happen
> if / is mounted noexec.

Yes, that's quite correct -- although I can't imagine how a bug which
caused / to be labelled as "noexec" managed to avoid causing major
problems until now.

I don't know anything about NFS, but hopefully someone on -stable
will be able to work out what's going on from the rest of your
email (quoted below).

Colin Percival

> In this case, / is mounted by the BTX PXE loader over NFS (from a
> FreeBSD 5.3 server, right now).  "mount" does not show the noexec
> flag.  However, with the attached little C program I verified that
> statfs really returns this flag (0x00000006).
> 
> Now, I see that on FreeBSD 5.3 diskless clients this flag is also
> returned on / - just it happened that nobody looked at it until
> the change in rtld.c of FreeBSD 5.4:
> 
> if (fs.f_flags & MNT_NOEXEC) {
> 	_rtld_error("Cannot execute objects on %s\n", fs.f_mntonname);
> 	close(fd);
> 	return NULL;
> }
> 
> I didn't yet understand (didn't check much) - why does statfs report
> the MNT_NOEXEC flag on the / filesystem (and only the / filesystem,
> when it's mounted from NFS by the bootloader - not any other
> NFS filesystems)?  BTW, this happens also with NetApp as the NFS 
> server - just to rule out any possibility of relation here.
> 
>   Ideas appreciated,
>   -- Tom
> 
> 
> 
> ------------------------------------------------------------------------
> 
> #include <stdio.h>
> #include <fcntl.h>
> #include <sys/param.h>
> #include <sys/mount.h>
> 
> 
> int main(int argc, char *argv[])
> {
>     if (argc != 2) {
> 	fprintf(stderr, "invalid number of arguments");
> 	return -1;
>     }
> 
>     struct statfs stbuf;
> 
>     if (statfs(argv[1], &stbuf) != 0) {
> 	perror("fstatfs");
> 	return -1;
>     }
> 
>     printf("FLAGS: 0x%08X\n", stbuf.f_flags);
>     if (stbuf.f_flags & MNT_NOEXEC)
> 	printf("MNT_NOEXEC\n");
> 
>     return 0;
> }