Libguestfs - Jul 2019 - Re: [nbdkit] [filter/nozero] large binary size with GCC 9

Hi all,

It seems GCC 9 does not put read-only zero-initialized variables into the BSS
section anymore; instead it is put into RODATA.
(See the thread at [0], especially [1])

In filter/nozero a 64M large, static, zeroed, read-only array is declared.
The new behavior of GCC puts this array as-is into the binary inflating the
size by a factor of around 10000.

(Clang 8 and older GCCs work "correctly")

It would be possible to remove the "const" qualifier or use some
compiler
attribute trickery to get the previous behavior back.

Thanks,
Thomas

[0] https://gcc.gnu.org/ml/gcc/2019-04/msg00034.html
[1] https://gcc.gnu.org/ml/gcc/2019-04/msg00049.html

On Thu, Jul 04, 2019 at 08:38:07PM +0200, Thomas Weißschuh
wrote:
> Hi all,
> 
> It seems GCC 9 does not put read-only zero-initialized variables into the
BSS
> section anymore; instead it is put into RODATA.
> (See the thread at [0], especially [1])
> 
> In filter/nozero a 64M large, static, zeroed, read-only array is declared.
> The new behavior of GCC puts this array as-is into the binary inflating the
> size by a factor of around 10000.
> 
> (Clang 8 and older GCCs work "correctly")
> 
> It would be possible to remove the "const" qualifier or use some
compiler
> attribute trickery to get the previous behavior back.
> 
> Thanks,
> Thomas
> 
> [0] https://gcc.gnu.org/ml/gcc/2019-04/msg00034.html
> [1] https://gcc.gnu.org/ml/gcc/2019-04/msg00049.html
Thanks, I can confirm this is indeed a problem (with GCC 9.1.1):

-rwxrwxr-x. 1 rjones rjones 67141400 Jun 10 09:36 nbdkit-nozero-filter.so

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-df lists disk usage of guests without needing to install any
software inside the virtual machine.  Supports Linux and Windows.
http://people.redhat.com/~rjones/virt-df/

On 7/4/19 1:38 PM, Thomas Weißschuh wrote:
> Hi all,
> 
> It seems GCC 9 does not put read-only zero-initialized variables into the
BSS
> section anymore; instead it is put into RODATA.
> (See the thread at [0], especially [1])
> 
Thanks for the pointers.
> In filter/nozero a 64M large, static, zeroed, read-only array is declared.
> The new behavior of GCC puts this array as-is into the binary inflating the
> size by a factor of around 10000.
Odd that you are only seeing this for the nozero filter, since we also
have the same sort of buffer in server/plugins.c.  Oh, I see a
difference between the two - one is function-local, the other is
file-local.  Does this patch make a difference? (I need to fire up a
rawhide VM to test it myself...)  If it doesn't, then removing the
'const' seems like the easiest trick to work around this compiler
pessimization (yes, I can see why the compiler writers argued that
sticking things in .rodata adds a bit more security against errant code
accidentally trying to corrupt the buffer; but as the buffer shouldn't
be escaping, it's already undefined behavior for such code to happen).

diff --git i/filters/nozero/nozero.c w/filters/nozero/nozero.c
index 16ec710b..21863707 100644
--- i/filters/nozero/nozero.c
+++ w/filters/nozero/nozero.c
@@ -45,7 +45,6 @@

 #define MAX_WRITE (64 * 1024 * 1024)

-static const char buffer[MAX_WRITE];
 static enum ZeroMode {
   NONE,
   EMULATE,
@@ -111,6 +110,8 @@ nozero_zero (struct nbdkit_next_ops *next_ops, void
*nxdata,

   while (count) {
     uint32_t size = MIN (count, MAX_WRITE);
+    static const char buffer[MAX_WRITE]; /* Always contains zeroes */
+
     if (next_ops->pwrite (nxdata, buffer, size, offs, flags, err) == -1)
       return -1;
     offs += size;


-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

On Fri, 2019-07-05T08:04-0500, Eric Blake wrote:
> On 7/4/19 1:38 PM, Thomas Weißschuh wrote:
> > In filter/nozero a 64M large, static, zeroed, read-only array is
declared.
> > The new behavior of GCC puts this array as-is into the binary
inflating the
> > size by a factor of around 10000.
> 
> Odd that you are only seeing this for the nozero filter, since we also
> have the same sort of buffer in server/plugins.c.  Oh, I see a
> difference between the two - one is function-local, the other is
> file-local.  Does this patch make a difference? (I need to fire up a
> rawhide VM to test it myself...)  If it doesn't, then removing the
> 'const' seems like the easiest trick to work around this compiler
> pessimization (yes, I can see why the compiler writers argued that
> sticking things in .rodata adds a bit more security against errant code
> accidentally trying to corrupt the buffer; but as the buffer shouldn't
> be escaping, it's already undefined behavior for such code to happen).
It actually also happens for server/plugins.c.

I assumed that filter/nozero got somehow statically linked into the server
(which does not really make sense), so it would have been the same instance of
the problem. Then I forgot to confirm this afterwards.
> diff --git i/filters/nozero/nozero.c w/filters/nozero/nozero.c
> index 16ec710b..21863707 100644
> --- i/filters/nozero/nozero.c
> +++ w/filters/nozero/nozero.c
> ...
Nope, same thing.


Thomas