Re: [PATCH] xfs: Fix 64-bit division on 32-bit in xlog_state_switch_iclogs()

[Geert Uytterhoeven]

Hi Dave,

On Sat, Jun 12, 2021 at 12:46 AM Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> On Fri, Jun 11, 2021 at 08:55:24AM +0200, Geert Uytterhoeven wrote:
> > On Fri, Jun 11, 2021 at 12:02 AM Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> > > On Thu, Jun 10, 2021 at 01:00:01PM +0200, Geert Uytterhoeven wrote:
> > > > On 32-bit (e.g. m68k):
> > > >
> > > >     ERROR: modpost: "__udivdi3" [fs/xfs/xfs.ko] undefined!
> > > >
> > > > Fix this by using a uint32_t intermediate, like before.
> > > >
> > > > Reported-by: noreply@xxxxxxxxxxxxxx
> > > > Fixes: 7660a5b48fbef958 ("xfs: log stripe roundoff is a property of the log")
> > > > Signed-off-by: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
> > > > ---
> > > > Compile-tested only.
> > > > ---
> > > >  fs/xfs/xfs_log.c | 4 ++--
> > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > <sigh>
> > >
> > > 64 bit division on 32 bit platforms is still a problem in this day
> > > and age?
> >
> > They're not a problem.  But you should use the right operations from
> > <linux/math64.h>, iff you really need these expensive operations.
>
> See, that's the whole problem. This *isn't* obviously a 64 bit
> division - BBTOB is shifting the variable down by 9 (bytes to blocks)
> and then using that as the divisor.

BTOBB, not BBTOB ;-)

> The problem is that BBTOB has an internal cast to a 64 bit size,
> and roundup() just blindly takes it and hence we get non-obvious
> compile errors only on 32 bit platforms.

Indeed. Perhaps the macros should be fixed?

    #define BBSHIFT         9
    #define BBSIZE          (1<<BBSHIFT)
    #define BBMASK          (BBSIZE-1)
    #define BTOBB(bytes)    (((__u64)(bytes) + BBSIZE - 1) >> BBSHIFT)
    #define BTOBBT(bytes)   ((__u64)(bytes) >> BBSHIFT)

Why are these two casting bytes to u64? The result will be smaller
due to the shift.
if the type holding bytes was too small, you're screwed anyway.

    #define BBTOB(bbs)      ((bbs) << BBSHIFT)

Why does this one lack the cast? If the passed bbs is ever 32-bit,
it may overflow due to the shift.

> We have type checking macros for all sorts of generic functionality
> - why haven't these generic macros that do division also have type
> checking to catch this? i.e. so that when people build kernels on
> 64 bit machines find out that they've unwittingly broken 32 bit
> builds the moment they use roundup() and/or friends incorrectly?
>
> That would save a lot of extra work having fix crap like this up
> after the fact...

While adding checks would work for e.g. roundup(), it wouldn't work
for plain divisions not involving rounding, as we don't have a way to
catch this for "a / b", except for the link error on 32-bit platforms.

Perhaps the build bots are not monitoring linux-xfs?

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds