I read the gcc docs and you are right, so it seems to be a gcc bug. I am
using gcc 2.95.2. Which version of gcc is OK to compile a linux mips
kernel ?
BTW, the kernel would be smaller by moving all the asm around __copy_user
in __copy_user itself. I am currently doing that. The cost is an added
'jr' to jump to __memcpy. Do you think it is worthwhile to do that ?
Fabrice.
On Tue, 27 Feb 2001, Maciej W. Rozycki wrote:
> On Tue, 27 Feb 2001, Fabrice Bellard wrote:
>
> > I found a serious bug in the assembler macros in asm-mips/uaccess.h. They
> > all do something like that:
> >
> > __asm__ __volatile__( \
> > "move\t$4, %1\n\t" \
> > "move\t$5, %2\n\t" \
> > "move\t$6, %3\n\t" \
> > ".set\tnoreorder\n\t" \
> > __MODULE_JAL(__copy_user) \
> > ...
> >
> > The problem is that you cannot assume that gcc will not put %1, %2 or %3
> > in registers different from $4, $5 or $6. For example, if %2 is put in $4,
> > the code is incorrect. (With gcc-2.95.2 I got a bug in
> > generic_file_write!).
>
> Hmm, haven't looked through gcc sources, but docs state: "It is an error
> for a clobber description to overlap an input or output operand (for
> example, an operand describing a register class with one member, mentioned
> in the clobber list)." I guess it implies clobbers are not used for input
> or output. It's reasonable anyway and if gcc acts otherwise, you might
> just have caught a bug in gcc.
>
> > A possible fix would be to use asm registers:
> >
> > #define copy_from_user(to,from,n) ({ \
> > register void *__cu_to asm("$4"); \
> > register const void *__cu_from asm("$5"); \
> > register long __cu_len asm("$6"); \
> > \
> > __cu_to = (to); \
> > __cu_from = (from); \
> > __cu_len = (n); \
> > if (access_ok(VERIFY_READ, __cu_from, __cu_len)) \
> > __asm__ __volatile__( \
> > ".set\tnoreorder\n\t" \
> > __MODULE_JAL(__copy_user) \
> > ...
> >
> > But I am not sure that it is always correct. Any idea ?
>
> This is fine and saves us three instructions. Go on, make a patch (I'd
> suggest using "__asm__" for consistency, though)!
>
> --
> + Maciej W. Rozycki, Technical University of Gdansk, Poland +
> +--------------------------------------------------------------+
> + e-mail: macro@ds2.pg.gda.pl, PGP key available +
>
