Re: [PATCH] mtd: nand: Fix return type of __DIVIDE() when called with 32-bit

[Boris Brezillon]

On Mon, 14 May 2018 14:13:39 +0200
Boris Brezillon <boris.brezillon@xxxxxxxxxxx> wrote:

> On Mon, 14 May 2018 14:00:19 +0200
> Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
> 
> > Hi Boris,
> > 
> > On Mon, May 14, 2018 at 1:46 PM, Boris Brezillon
> > <boris.brezillon@xxxxxxxxxxx> wrote:  
> > > On Mon, 14 May 2018 13:32:30 +0200
> > > Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:    
> > >> On Mon, May 14, 2018 at 1:23 PM, Boris Brezillon
> > >> <boris.brezillon@xxxxxxxxxxx> wrote:    
> > >> > On Mon, 14 May 2018 12:49:37 +0200
> > >> > Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:    
> > >> >> The __DIVIDE() macro checks whether it is called with a 32-bit or 64-bit
> > >> >> dividend, to select the appropriate divide-and-round-up routine.
> > >> >> As the check uses the ternary operator, the result will always be
> > >> >> promoted to a type that can hold both results, i.e. unsigned long long.
> > >> >>
> > >> >> When using this result in a division on a 32-bit system, this may lead
> > >> >> to link errors like:
> > >> >>
> > >> >>     ERROR: "__udivdi3" [drivers/mtd/nand/raw/nand.ko] undefined!
> > >> >>
> > >> >> Fix this by casting the result of the 64-bit division to the type of the
> > >> >> dividend.
> > >> >>
> > >> >> Fixes: 8878b126df769831 ("mtd: nand: add ->exec_op() implementation")
> > >> >> Signed-off-by: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
> > >> >> ---
> > >> >> This fixes the root cause of the link failure seen with
> > >> >> m68k/allmodconfig since commit 3057fcef385348fe ("mtd: rawnand: Make
> > >> >> sure we wait tWB before polling the STATUS reg").
> > >> >>
> > >> >> An alternative mitigation was posted as "[PATCH] m68k: Implement
> > >> >> ndelay() as an inline function to force type checking/casting"
> > >> >> (https://lkml.org/lkml/2018/5/13/102).
> > >> >> ---
> > >> >>  include/linux/mtd/rawnand.h | 2 +-
> > >> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> > >> >>
> > >> >> diff --git a/include/linux/mtd/rawnand.h b/include/linux/mtd/rawnand.h
> > >> >> index 5dad59b312440a9c..d06dc428ea0102ae 100644
> > >> >> --- a/include/linux/mtd/rawnand.h
> > >> >> +++ b/include/linux/mtd/rawnand.h
> > >> >> @@ -871,7 +871,7 @@ struct nand_op_instr {
> > >> >>  #define __DIVIDE(dividend, divisor) ({                                       \
> > >> >>       sizeof(dividend) == sizeof(u32) ?                               \
> > >> >>               DIV_ROUND_UP(dividend, divisor) :                       \
> > >> >> -             DIV_ROUND_UP_ULL(dividend, divisor);                    \
> > >> >> +             (__typeof__(dividend))DIV_ROUND_UP_ULL(dividend, divisor); \    
> > >> >
> > >> > Hm, it's a bit hard to follow when you place the cast here. One could
> > >> > wonder why a cast to (__typeof__(dividend)) is needed since
> > >> > DIV_ROUND_UP_ULL() already returns a (__typeof__(dividend)) type.    
> > >>
> > >> DIV_ROUND_UP_ULL() does not return __typeof__(dividend), but
> > >> unsigned long long.    
> > >
> > > Except if you entered this branch, that means you passed an unsigned
> > > long long dividend (AKA u64), otherwise you would go in DIV_ROUND_UP().
> > > Am I missing something?    
> > 
> > Sure, inside that branch, it does.
> > But the compiler considers the whole ternary operator construction, i.e.
> > both branches.  
> 
> Yes, and that's my point. The (__typeof__(dividend)) when placed like
> you did is ambiguous. It looks like you're doing a useless cast, while
> what you're actually fixing is the case where dividend is an u32 (AKA
> unsigned long), and from the reader PoV, the code you're fixing
> shouldn't even be reached. Hence my suggestion to move the case one
> level up and add a comment ;-).
> 
> >   
> > >> > How about:
> > >> >
> > >> >         /*
> > >> >          * Cast to type of dividend is needed here to guarantee that the
> > >> >          * result won't be an unsigned long long when the dividend is an
> > >> >          * unsigned long, which is what the compiler does when it sees a    
> > >>
> > >> s/an unsigned long/32-bit/
> > >>    
> > >> >          * ternary operator with 2 different return types.
> > >> >          */
> > >> >         (__typeof__(dividend))(sizeof(dividend) == sizeof(u32) ?        \    
> > >
> > > To be completely safe and handle cases where dividend is an unsigned
> > > short or an unsigned, we should probably have:
> > >
> > >         (__typeof__(dividend))(sizeof(dividend) == sizeof(unsigned long long) ? \    
> >   
> > "> sizeof(u32)"?    
> > 
> > /me starts to think about uint128_t...  
> 
> 	sizeof(dividend) <= sizeof(unsigned long) ?
> 	DIV_ROUND_UP(dividend, divisor) :
> 	DIV_ROUND_UP_ULL(dividend, divisor);
> 
> Problem solved :-).
> 
> > 
> >   
> > >                                DIV_ROUND_UP_ULL(dividend, divisor) :
> > >                                DIV_ROUND_UP(dividend, divisor));
> > >    
> > >> >                                DIV_ROUND_UP(dividend, divisor) :        \
> > >> >                                DIV_ROUND_UP_ULL(dividend, divisor));    
> > >>
> > >> Looks fine to me, too.
> > >>    
> > >> > Actually, I'm not even sure we care about the truncation that could
> > >> > happen on an unsigned long long -> unsigned long cast because the
> > >> > delays we express here will anyway be hundreds of nanosecs/millisecs,
> > >> > so nothing close to the billions of nanosecs/millisecs you can express
> > >> > with an unsigned long.
> > >> >
> > >> > So, maybe we should just do:
> > >> >
> > >> >         (unsigned long)(sizeof(dividend) == sizeof(u32) ?               \
> > >> >                         DIV_ROUND_UP(dividend, divisor) :               \
> > >> >                         DIV_ROUND_UP_ULL(dividend, divisor));
> > >> >
> > >> > to make things more readable.    
> > >>
> > >> That would break callers who pass a 64-bit dividend, and expect to receive
> > >> a 64-bit quotient back (on 32-bit systems).
> > >> Calling e.g. PSEC_TO_NSEC(1000000000000ULL) is valid, passing the
> > >> result to ndelay() isn't ;-)    
> > >
> > > Well, theoretically, yes it's possible, in practice, we only ever pass
> > > u32 types to PSEC_TO_NSEC() and u64 types to PSEC_TO_MSEC(), so why
> > > bother.    
> > 
> > True. But __DIVIDE() sounds too generic to add such unobvious limitations.
> >   
> 
> Is the following version okay?
> 
> --->8---  
> From 59160e26383e1aca1eafc7078d858b91dd0074ce Mon Sep 17 00:00:00 2001
> From: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
> Date: Mon, 14 May 2018 12:49:37 +0200
> Subject: [PATCH] mtd: nand: Fix return type of __DIVIDE() when called with
>  32-bit
> 
> The __DIVIDE() macro checks whether it is called with a 32-bit or 64-bit
> dividend, to select the appropriate divide-and-round-up routine.
> As the check uses the ternary operator, the result will always be
> promoted to a type that can hold both results, i.e. unsigned long long.
> 
> When using this result in a division on a 32-bit system, this may lead
> to link errors like:
> 
>     ERROR: "__udivdi3" [drivers/mtd/nand/raw/nand.ko] undefined!
> 
> Fix this by casting the result of the division to the type of the
> dividend.
> 
> Fixes: 8878b126df769831 ("mtd: nand: add ->exec_op() implementation")
> Signed-off-by: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
> Signed-off-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxx>
> ---
>  include/linux/mtd/rawnand.h | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/mtd/rawnand.h b/include/linux/mtd/rawnand.h
> index 5dad59b31244..17c919436f48 100644
> --- a/include/linux/mtd/rawnand.h
> +++ b/include/linux/mtd/rawnand.h
> @@ -867,12 +867,18 @@ struct nand_op_instr {
>   * tBERS (during an erase) which all of them are u64 values that cannot be
>   * divided by usual kernel macros and must be handled with the special
>   * DIV_ROUND_UP_ULL() macro.
> + *
> + * Cast to type of dividend is needed here to guarantee that the result won't
> + * be an unsigned long long when the dividend is an unsigned long (or smaller),
> + * which is what the compiler does when it sees ternary operator with 2
> + * different return types (picks the largest type to make sure there's no
> + * loss).
>   */
> -#define __DIVIDE(dividend, divisor) ({					\
> -	sizeof(dividend) == sizeof(u32) ?				\
> -		DIV_ROUND_UP(dividend, divisor) :			\
> -		DIV_ROUND_UP_ULL(dividend, divisor);			\
> -		})
> +#define __DIVIDE(dividend, divisor) ({						\
> +	(__typeof__(dividend))(sizeof(dividend) <= sizeof(unsigned long) ?	\
> +			       DIV_ROUND_UP(dividend, divisor) :		\
> +			       DIV_ROUND_UP_ULL(dividend, divisor)); 		\
> +	})
>  #define PSEC_TO_NSEC(x) __DIVIDE(x, 1000)
>  #define PSEC_TO_MSEC(x) __DIVIDE(x, 1000000000)
>  

Applied this version.

Thanks,

Boris