div64: Rework 64-bit type safety checks in do_div().

div64: Rework 64-bit type safety checks in do_div().

[Paul Mundt]

The current do_div() implementation has a bogus pointer compare to
generate build warnings on mismatch on 32-bit, unfortunately this not
only triggers for size mismatch, but also _any_ type mismatch, even on
reasonable 64-bit values:

In file included from kernel/sched.c:869:
kernel/sched_debug.c: In function 'nsec_high':
kernel/sched_debug.c:38: warning: comparison of distinct pointer types lacks a cast
kernel/sched_debug.c:41: warning: comparison of distinct pointer types lacks a cast
kernel/sched_debug.c: In function 'nsec_low':
kernel/sched_debug.c:51: warning: comparison of distinct pointer types lacks a cast
...

The options are to either 'fix' all callers of do_div() to make sure
they're using a uint64_t explicitly, or to update do_div() to make sure
that the value is 64-bits, regardless of specific type. Currently
everything that uses the generic do_div() causes a warning when using one
of 'u64', 'long long', etc. instead of 'uint64_t'.

Half-assed empirical testing indicates that the number of false positives
far outweighs any benefits of this type of checking:

$ git grep uint64_t | wc -l
947
$ git grep u64 | wc -l
13942

In short, screw uint64_t and its fan club.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>

---

 include/asm-generic/div64.h |    7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/include/asm-generic/div64.h b/include/asm-generic/div64.h
index a4a4937..63e7768 100644
--- a/include/asm-generic/div64.h
+++ b/include/asm-generic/div64.h
@@ -19,6 +19,7 @@
 
 #include <linux/types.h>
 #include <linux/compiler.h>
+#include <linux/kernel.h>
 
 #if BITS_PER_LONG == 64
 
@@ -39,13 +40,11 @@ static inline uint64_t div64_64(uint64_t dividend, uint64_t divisor)
 
 extern uint32_t __div64_32(uint64_t *dividend, uint32_t divisor);
 
-/* The unnecessary pointer compare is there
- * to check for type safety (n must be 64bit)
- */
+/* The BUILD_BUG_ON() is there to check for type safety (n must be 64bit) */
 # define do_div(n,base) ({				\
 	uint32_t __base = (base);			\
 	uint32_t __rem;					\
-	(void)(((typeof((n)) *)0) == ((uint64_t *)0));	\
+	BUILD_BUG_ON(sizeof(n) != sizeof(uint64_t));	\
 	if (likely(((n) >> 32) == 0)) {			\
 		__rem = (uint32_t)(n) % __base;		\
 		(n) = (uint32_t)(n) / __base;		\

Re: div64: Rework 64-bit type safety checks in do_div().

[Andrew Morton]

On Mon, 17 Dec 2007 10:48:05 +0900 Paul Mundt <lethal@linux-sh.org> wrote:

> The current do_div() implementation has a bogus pointer compare to
> generate build warnings on mismatch on 32-bit, unfortunately this not
> only triggers for size mismatch, but also _any_ type mismatch, even on
> reasonable 64-bit values:
> 
> In file included from kernel/sched.c:869:
> kernel/sched_debug.c: In function 'nsec_high':
> kernel/sched_debug.c:38: warning: comparison of distinct pointer types lacks a cast
> kernel/sched_debug.c:41: warning: comparison of distinct pointer types lacks a cast
> kernel/sched_debug.c: In function 'nsec_low':
> kernel/sched_debug.c:51: warning: comparison of distinct pointer types lacks a cast
> ...
> 
> The options are to either 'fix' all callers of do_div() to make sure
> they're using a uint64_t explicitly, or to update do_div() to make sure
> that the value is 64-bits, regardless of specific type. Currently
> everything that uses the generic do_div() causes a warning when using one
> of 'u64', 'long long', etc. instead of 'uint64_t'.

u64 and uint64_t should be identical?

> Half-assed empirical testing indicates that the number of false positives
> far outweighs any benefits of this type of checking:
> 
> $ git grep uint64_t | wc -l
> 947
> $ git grep u64 | wc -l
> 13942
> 
> In short, screw uint64_t and its fan club.

I don't get it.  Are u64 and uint64_t different on any arch?

> diff --git a/include/asm-generic/div64.h b/include/asm-generic/div64.h
> index a4a4937..63e7768 100644
> --- a/include/asm-generic/div64.h
> +++ b/include/asm-generic/div64.h
> @@ -19,6 +19,7 @@
>  
>  #include <linux/types.h>
>  #include <linux/compiler.h>
> +#include <linux/kernel.h>
>  
>  #if BITS_PER_LONG == 64
>  
> @@ -39,13 +40,11 @@ static inline uint64_t div64_64(uint64_t dividend, uint64_t divisor)
>  
>  extern uint32_t __div64_32(uint64_t *dividend, uint32_t divisor);
>  
> -/* The unnecessary pointer compare is there
> - * to check for type safety (n must be 64bit)
> - */
> +/* The BUILD_BUG_ON() is there to check for type safety (n must be 64bit) */
>  # define do_div(n,base) ({				\
>  	uint32_t __base = (base);			\
>  	uint32_t __rem;					\
> -	(void)(((typeof((n)) *)0) == ((uint64_t *)0));	\
> +	BUILD_BUG_ON(sizeof(n) != sizeof(uint64_t));	\
>  	if (likely(((n) >> 32) == 0)) {			\
>  		__rem = (uint32_t)(n) % __base;		\
>  		(n) = (uint32_t)(n) / __base;		\

The mismatch which I've seen triggering a lot is doing do_div() on an s64
when it expects a u64.

And I think that _is_ a bug, isn't it?  do_div(-10, 10) should return -1,
but as the implementation will convert -10 to <monstrously large +ve
number>, the return value will be wildly wrong?

I'm thinking that the problem here is that x86's do_div(s64, ...) doesn't
warn.  So people write wrong code and then the problems only crop up on
architectures which use asm-generic/div64.h, which does warn?

Re: div64: Rework 64-bit type safety checks in do_div().

[Paul Mundt]

(Adding Ingo to CC regarding kernel/lockdep_proc.c..)

On Sun, Dec 16, 2007 at 07:04:18PM -0800, Andrew Morton wrote:
> On Mon, 17 Dec 2007 10:48:05 +0900 Paul Mundt <lethal@linux-sh.org> wrote:
> > The options are to either 'fix' all callers of do_div() to make sure
> > they're using a uint64_t explicitly, or to update do_div() to make sure
> > that the value is 64-bits, regardless of specific type. Currently
> > everything that uses the generic do_div() causes a warning when using one
> > of 'u64', 'long long', etc. instead of 'uint64_t'.
> 
> u64 and uint64_t should be identical?
> 
Er, yes, that was supposed to be an 's64'. It only applies to sign
mismatch.

> > -/* The unnecessary pointer compare is there
> > - * to check for type safety (n must be 64bit)
> > - */
> > +/* The BUILD_BUG_ON() is there to check for type safety (n must be 64bit) */
> >  # define do_div(n,base) ({				\
> >  	uint32_t __base = (base);			\
> >  	uint32_t __rem;					\
> > -	(void)(((typeof((n)) *)0) == ((uint64_t *)0));	\
> > +	BUILD_BUG_ON(sizeof(n) != sizeof(uint64_t));	\
> >  	if (likely(((n) >> 32) == 0)) {			\
> >  		__rem = (uint32_t)(n) % __base;		\
> >  		(n) = (uint32_t)(n) / __base;		\
> 
> The mismatch which I've seen triggering a lot is doing do_div() on an s64
> when it expects a u64.
> 
> And I think that _is_ a bug, isn't it?  do_div(-10, 10) should return -1,
> but as the implementation will convert -10 to <monstrously large +ve
> number>, the return value will be wildly wrong?
> 
If it's supposed to be u64 only, then yes, the existing check should be
ok. There are a lot of places (time keeping code, lockdep, etc.) that
operate on signed values though, and from the comments in some places
this seems to be intentional (ie, kernel/lockdep_proc.c has this gem):

static void snprint_time(char *buf, size_t bufsiz, s64 nr)
{
        unsigned long rem;

        rem = do_div(nr, 1000); /* XXX: do_div_signed */
        snprintf(buf, bufsiz, "%lld.%02d", (long long)nr, ((int)rem+5)/10);
}

> I'm thinking that the problem here is that x86's do_div(s64, ...) doesn't
> warn.  So people write wrong code and then the problems only crop up on
> architectures which use asm-generic/div64.h, which does warn?

That seems to be an accurate asessment, yes. If do_div(s64, ...) is buggy
behaviour, then the current check is fine, and the callsites should be
corrected. Though if there's code in-tree that relies on s64 do_div, that seems
to be a more problematic issue.

Re: div64: Rework 64-bit type safety checks in do_div().

[Al Viro]

On Mon, Dec 17, 2007 at 12:20:19PM +0900, Paul Mundt wrote:
> (Adding Ingo to CC regarding kernel/lockdep_proc.c..)
 
> That seems to be an accurate asessment, yes. If do_div(s64, ...) is buggy
> behaviour, then the current check is fine, and the callsites should be
> corrected. Though if there's code in-tree that relies on s64 do_div, that seems
> to be a more problematic issue.

It is a bug and the only existing callers that manage to work are those that
make sure that signed value is positive.  Still asking for trouble...