Re: Problem Installing Linux Kernel Module compiled with gcc-3.2.x

Re: Problem Installing Linux Kernel Module compiled with gcc-3.2.x

[Bernd Jendrissek]

Not *exactly* on-topic for gcc@gcc.gnu.org I suppose, but here goes.

[Cc'ed to linux-kernel@vger.kernel.org]

On Fri, May 30, 2003 at 09:26:51AM -0600, Kendrick Hamilton wrote:
> 	I have a module for a custom developped PCI card. The device 
> driver is written for the Linux 2.4 series kernels. When I build the 
> module and the Linux kernel with gcc-2.95.3, the module installs 
> correctly. When I build the module and the Linux kernel with gcc-3.2.3 
> (also other gcc-3.2.x), the module installs but the Linux kernel crashes 
> in random places outside of the module. Do you have any suggestions of 
> what to look for? I can email you the complete module source code. I have 
> not tried gcc-3.3 because I cannot compile the current Linux kernel with 
> it (there is a known bug that is being fixed and should be out in 
> Linux-2.4.21).

Been there, done that, got the T-shirt.  I was lucky: while my module
installed, it broke in a fairly harmless way.  (It just didn't work; it
didn't screw with my system.)

If you look at linux/include/linux/spinlock.h, you'll see:

/*
 * Your basic spinlocks, allowing only a single CPU anywhere
 *
 * Most gcc versions have a nasty bug with empty initializers.
 */
#if (__GNUC__ > 2)
  typedef struct { } spinlock_t;
  #define SPIN_LOCK_UNLOCKED (spinlock_t) { }
#else
  typedef struct { int gcc_is_buggy; } spinlock_t;
  #define SPIN_LOCK_UNLOCKED (spinlock_t) { 0 }
#endif

There are a couple of spinlock_t's (directly or through other structs) in
the task_struct.  So when your module accesses parts of the "current"
task_struct beyond the first spinlock_t, you better hope it's reading and
not writing (which was the case with my module).

I bet your module modifies "current".

Hmm, actually I thought the kernel had a mechanism to prevent a GCC 3.x
module from being loaded into a GCC 2.x kernel and vice versa?

Re: Problem Installing Linux Kernel Module compiled with gcc-3.2.x

[Kendrick Hamilton]

I have been manually recompillng the module and kernel to ensure they are 
both compiled with the same version of gcc. When I do switch gcc versions, 
I cp .config to config, make mrproper, cp config .config, make dep, make 
all modules modules_install install; reboot; make clean on my driver the 
make it.

 On Fri, 30 May 2003, Bernd Jendrissek wrote:

> Not *exactly* on-topic for gcc@gcc.gnu.org I suppose, but here goes.
> 
> [Cc'ed to linux-kernel@vger.kernel.org]
> 
> On Fri, May 30, 2003 at 09:26:51AM -0600, Kendrick Hamilton wrote:
> > 	I have a module for a custom developped PCI card. The device 
> > driver is written for the Linux 2.4 series kernels. When I build the 
> > module and the Linux kernel with gcc-2.95.3, the module installs 
> > correctly. When I build the module and the Linux kernel with gcc-3.2.3 
> > (also other gcc-3.2.x), the module installs but the Linux kernel crashes 
> > in random places outside of the module. Do you have any suggestions of 
> > what to look for? I can email you the complete module source code. I have 
> > not tried gcc-3.3 because I cannot compile the current Linux kernel with 
> > it (there is a known bug that is being fixed and should be out in 
> > Linux-2.4.21).
> 
> Been there, done that, got the T-shirt.  I was lucky: while my module
> installed, it broke in a fairly harmless way.  (It just didn't work; it
> didn't screw with my system.)
> 
> If you look at linux/include/linux/spinlock.h, you'll see:
> 
> /*
>  * Your basic spinlocks, allowing only a single CPU anywhere
>  *
>  * Most gcc versions have a nasty bug with empty initializers.
>  */
> #if (__GNUC__ > 2)
>   typedef struct { } spinlock_t;
>   #define SPIN_LOCK_UNLOCKED (spinlock_t) { }
> #else
>   typedef struct { int gcc_is_buggy; } spinlock_t;
>   #define SPIN_LOCK_UNLOCKED (spinlock_t) { 0 }
> #endif
> 
> There are a couple of spinlock_t's (directly or through other structs) in
> the task_struct.  So when your module accesses parts of the "current"
> task_struct beyond the first spinlock_t, you better hope it's reading and
> not writing (which was the case with my module).
> 
> I bet your module modifies "current".
> 
> Hmm, actually I thought the kernel had a mechanism to prevent a GCC 3.x
> module from being loaded into a GCC 2.x kernel and vice versa?
> 

-- 
Kendrick Hamilton E.I.T.
SED Systems, a division of Calian Ltd.
18 Innovation Blvd.
PO Box 1464
Saskatoon, Saskatchewan
Canada
S7N 3R1

Hamilton@sedsystems.ca
Tel: (306) 933-1453
Fax: (306) 933-1486

Re: Problem Installing Linux Kernel Module compiled with gcc-3.2.x

[Bernd Jendrissek]

On Fri, May 30, 2003 at 11:31:57AM -0600, Kendrick Hamilton wrote:
> I have been manually recompillng the module and kernel to ensure they are 
> both compiled with the same version of gcc. When I do switch gcc versions, 
> I cp .config to config, make mrproper, cp config .config, make dep, make 
> all modules modules_install install; reboot; make clean on my driver the 
> make it.

Aargh.  Now if I had actually *read* your message I'd have picked that up.

Well, it's not maybe some *other* module that gets left behind in
/lib/modules/$VERSION?  No, that doesn't make too much sense.  That
doesn't gel with the crashes happening from the time you load *your*
module.

Uh, could it maybe be (gasp!) a *bug* in your module?  Maybe some
assumption your code is making is being invalidated by a new! improved!
optimization in GCC 3.x?  I know my module ha[ds] bugs...

Although... I must say that ever since I recompiled 2.4.18 with 3.2.x
(now 3.2.3), my machine seems somewhat less stable.  (I think) I *had* to
reboot yesterday after just 16 days' uptime after X or something else
with the keyboard went berserk.  But I'm not quite ready yet to "blame"
GCC for that.

>  On Fri, 30 May 2003, Bernd Jendrissek wrote:
> 
> > Not *exactly* on-topic for gcc@gcc.gnu.org I suppose, but here goes.
> > 
> > [Cc'ed to linux-kernel@vger.kernel.org]
> > 
> > On Fri, May 30, 2003 at 09:26:51AM -0600, Kendrick Hamilton wrote:
> > > 	I have a module for a custom developped PCI card. The device 
> > > driver is written for the Linux 2.4 series kernels. When I build the 
> > > module and the Linux kernel with gcc-2.95.3, the module installs 
> > > correctly. When I build the module and the Linux kernel with gcc-3.2.3 
> > > (also other gcc-3.2.x), the module installs but the Linux kernel crashes 
> > > in random places outside of the module. Do you have any suggestions of 
> > > what to look for? I can email you the complete module source code. I have 
> > > not tried gcc-3.3 because I cannot compile the current Linux kernel with 
> > > it (there is a known bug that is being fixed and should be out in 
> > > Linux-2.4.21).
> > 
> > Been there, done that, got the T-shirt.  I was lucky: while my module
> > installed, it broke in a fairly harmless way.  (It just didn't work; it
> > didn't screw with my system.)
> > 
> > If you look at linux/include/linux/spinlock.h, you'll see:
> > 
> > /*
> >  * Your basic spinlocks, allowing only a single CPU anywhere
> >  *
> >  * Most gcc versions have a nasty bug with empty initializers.
> >  */
> > #if (__GNUC__ > 2)
> >   typedef struct { } spinlock_t;
> >   #define SPIN_LOCK_UNLOCKED (spinlock_t) { }
> > #else
> >   typedef struct { int gcc_is_buggy; } spinlock_t;
> >   #define SPIN_LOCK_UNLOCKED (spinlock_t) { 0 }
> > #endif
> > 
> > There are a couple of spinlock_t's (directly or through other structs) in
> > the task_struct.  So when your module accesses parts of the "current"
> > task_struct beyond the first spinlock_t, you better hope it's reading and
> > not writing (which was the case with my module).
> > 
> > I bet your module modifies "current".
> > 
> > Hmm, actually I thought the kernel had a mechanism to prevent a GCC 3.x
> > module from being loaded into a GCC 2.x kernel and vice versa?

Re: Problem Installing Linux Kernel Module compiled with gcc-3.2.x

[Joe Buck]

On Fri, May 30, 2003 at 07:22:40PM +0200, Bernd Jendrissek wrote:

> If you look at linux/include/linux/spinlock.h, you'll see:
> 
> /*
>  * Your basic spinlocks, allowing only a single CPU anywhere
>  *
>  * Most gcc versions have a nasty bug with empty initializers.
>  */
> #if (__GNUC__ > 2)
>   typedef struct { } spinlock_t;
>   #define SPIN_LOCK_UNLOCKED (spinlock_t) { }
> #else
>   typedef struct { int gcc_is_buggy; } spinlock_t;
>   #define SPIN_LOCK_UNLOCKED (spinlock_t) { 0 }
> #endif

Yuk!  What is the benefit of introducing this incompatibility?  #ifdefs
are harmful to maintainance, and it's only one word, so why not always
put in the dummy struct member?

> Hmm, actually I thought the kernel had a mechanism to prevent a GCC 3.x
> module from being loaded into a GCC 2.x kernel and vice versa?

Is there any reason, other than the above-described bit of evil, for doing
this (forbidding mixing)?  It prevents the bug-finding approach I
described earlier (a binary search for finding miscompiled code) from
working.

Re: Problem Installing Linux Kernel Module compiled with gcc-3.2.x

[Bernd Jendrissek]

On Fri, May 30, 2003 at 10:33:29AM -0700, Joe Buck wrote:
> On Fri, May 30, 2003 at 07:22:40PM +0200, Bernd Jendrissek wrote:
> 
> > If you look at linux/include/linux/spinlock.h, you'll see:
> > 
> > /*
> >  * Your basic spinlocks, allowing only a single CPU anywhere
> >  *
> >  * Most gcc versions have a nasty bug with empty initializers.
> >  */
> > #if (__GNUC__ > 2)
> >   typedef struct { } spinlock_t;
> >   #define SPIN_LOCK_UNLOCKED (spinlock_t) { }
> > #else
> >   typedef struct { int gcc_is_buggy; } spinlock_t;
> >   #define SPIN_LOCK_UNLOCKED (spinlock_t) { 0 }
> > #endif
> 
> Yuk!  What is the benefit of introducing this incompatibility?  #ifdefs
> are harmful to maintainance, and it's only one word, so why not always
> put in the dummy struct member?

I dont speak for the kernel people, but...

I suppose some people just insist on squeezing every last cycle out of
their machines.  For my home PC (a 486 with 5MB RAM running linux 2.0.30),
I am quite grateful for such cycle and bit saving.  Believe me, I notice
whether I have apache running or not. :)

Hmm, yes, it does seem to be just one word.  grep -r spinlock_t . |wc -l
says 1013 here, that's across *all* architectures.  IOW 4052 bytes - that's
*one page* - on i386!

Never mind what definition tcc will give to __GNUC__

So there I thought I was going to justify the kernel.  Instead I mostly
agree with Joe!  I'm also sure there have been flamewars about this...

> > Hmm, actually I thought the kernel had a mechanism to prevent a GCC 3.x
> > module from being loaded into a GCC 2.x kernel and vice versa?
> 
> Is there any reason, other than the above-described bit of evil, for doing
> this (forbidding mixing)?  It prevents the bug-finding approach I
> described earlier (a binary search for finding miscompiled code) from
> working.

Between GCC 2.x and 3.x the *major* version changed (duh).  I would
imagine that people are/were (justifiably?) concerned that ABI's might
have changed.  From your response, I assume there are no ABI changes
for C at least?  I suppose a gratuitous ABI change would constitute a
bug, though...

BTW I said "I thought" - it appears there is in fact no such mechanism.

Okay, so here's a PR (Public Relations, not Problem Report) patch just
for you, Joe:     <with a fistful of smileys :)>

(It also gets rid of some of that crazy 2-space indentation.)

diff -u linux/include/linux/spinlock.h.borig linux/include/linux/spinlock.h
--- linux/include/linux/spinlock.h.borig        Tue May 13 17:05:57 2003
+++ linux/include/linux/spinlock.h      Fri May 30 20:29:42 2003
@@ -53,13 +53,8 @@
  *
  * Most gcc versions have a nasty bug with empty initializers.
  */
-#if (__GNUC__ > 2)
-  typedef struct { } spinlock_t;
-  #define SPIN_LOCK_UNLOCKED (spinlock_t) { }
-#else
-  typedef struct { int gcc_is_buggy; } spinlock_t;
-  #define SPIN_LOCK_UNLOCKED (spinlock_t) { 0 }
-#endif
+typedef struct { int gcc_was_buggy; } spinlock_t;
+#define SPIN_LOCK_UNLOCKED (spinlock_t) { 0 }
 
 #define spin_lock_init(lock)   do { } while(0)
 #define spin_lock(lock)                (void)(lock) /* Not "unused variable". */

Re: Problem Installing Linux Kernel Module compiled with gcc-3.2.x

[Joe Buck]

On Fri, May 30, 2003 at 08:43:32PM +0200, Bernd Jendrissek wrote:
> > Is there any reason, other than the above-described bit of evil, for doing
> > this (forbidding mixing)?  It prevents the bug-finding approach I
> > described earlier (a binary search for finding miscompiled code) from
> > working.
> 
> Between GCC 2.x and 3.x the *major* version changed (duh).  I would
> imagine that people are/were (justifiably?) concerned that ABI's might
> have changed.  From your response, I assume there are no ABI changes
> for C at least?  I suppose a gratuitous ABI change would constitute a
> bug, though...

There are no ABI changes for C.

Re: Problem Installing Linux Kernel Module compiled with gcc-3.2.x

[Alan Cox]

On Gwe, 2003-05-30 at 20:02, Joe Buck wrote:
> > Between GCC 2.x and 3.x the *major* version changed (duh).  I would
> > imagine that people are/were (justifiably?) concerned that ABI's might
> > have changed.  From your response, I assume there are no ABI changes
> > for C at least?  I suppose a gratuitous ABI change would constitute a
> > bug, though...
> 
> There are no ABI changes for C.

Not quite true but close. The padding in gcc 2.96 however is actually to
work around a compiler bug