On Fri, 4 Jul 2008, Andrew Morton wrote:
>
> probe_kernel_address() should be usable here.
Right you are.
> > +static char *string(char *buf, char *end, char *s, int field_width, int precision, int flags)
> > +{
> > + int len, i;
> > +
> > + if ((unsigned long)s < PAGE_SIZE)
> > + s = "<NULL>";
>
> hm, is that needed for other reasons than "it will fault"?
It's similar to what glibc does - showing a NULL ptr gracefully. They are
pretty common, and it's very rude to SIGSEGV or oops just because you
wanted to print a string out for debugging.
The code is also just copied from the old code - just moved it to a
function of its own.
> otherwise we could walk the whole string with probe_kernel_address()
> before we do anything with it.
That would be pretty slow, wed' be better off then unrolling it (ie doing
all the ugly setup around the whole string).
> If this takes off we might want a register-your-printk-handler
> interface. Maybe.
Yeah.
> We also jump through hoops to print things like sector_t and
> resource_size_t. They always need to be cast to `unsiged long long',
> which generates additional stack space and text in some setups.
Indeed. Though doing it with a pointer is often not a _whole_ lot cleaner,
but yes, it's often nicer to add a '&' than adding a cast.
> And then there's the perennial "need to cast u64 to unsigned long long
> to print it". If we were to do
>
> printk("%SL", (void *)some_u64);
>
> then that's still bloody ugly, but it'll save a little text-n-stack.
No can do. (void *) isn't big enough to hold a u64. So it would have to be
something like this:
printk("%p64i", &some_u64);
instead. Avoiding the cast, and often being more efficient calling
convention on 32-bit.
But it can often generate worse code too - now we're taking an address of
a variable, and that will disable many optimizations because now i's not a
purely local variable that the compiler knows all uses for any more.
So I'm not entirely conviced it's a good idea to do this for just "long
long" cases. Dunno.
Linus
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