[PATCH] mips: lib: uncached: fix uninitialized variable 'sp'

[PATCH] mips: lib: uncached: fix uninitialized variable 'sp'

[Anders Roxell]

When building mips tinyconfig with clang the following warning show up:

/tmp/arch/mips/lib/uncached.c:40:18: note: initialize the variable 'sp' to silence this warning
        register long sp __asm__("$sp");
                        ^
                         = 0

Rework to make an explicit inline move.

Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
---
 arch/mips/lib/uncached.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/mips/lib/uncached.c b/arch/mips/lib/uncached.c
index 09d5deea747f..21a4b94a0558 100644
--- a/arch/mips/lib/uncached.c
+++ b/arch/mips/lib/uncached.c
@@ -37,10 +37,11 @@
  */
 unsigned long run_uncached(void *func)
 {
-	register long sp __asm__("$sp");
 	register long ret __asm__("$2");
 	long lfunc = (long)func, ufunc;
 	long usp;
+	long sp;
+	asm ("move %0, $sp" : "=r" (sp));
 
 	if (sp >= (long)CKSEG0 && sp < (long)CKSEG2)
 		usp = CKSEG1ADDR(sp);
-- 
2.29.2

Re: [PATCH] mips: lib: uncached: fix uninitialized variable 'sp'

[Nick Desaulniers]

On Fri, Nov 27, 2020 at 12:39 AM Anders Roxell <anders.roxell@linaro.org> wrote:
>
> When building mips tinyconfig with clang the following warning show up:
>
> /tmp/arch/mips/lib/uncached.c:40:18: note: initialize the variable 'sp' to silence this warning
>         register long sp __asm__("$sp");
>                         ^
>                          = 0

Hi Anders, thank you for sending the patch. Do you have the full text
of the warning; it looks like only the note was included?

>
> Rework to make an explicit inline move.
>
> Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
> ---
>  arch/mips/lib/uncached.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/mips/lib/uncached.c b/arch/mips/lib/uncached.c
> index 09d5deea747f..21a4b94a0558 100644
> --- a/arch/mips/lib/uncached.c
> +++ b/arch/mips/lib/uncached.c
> @@ -37,10 +37,11 @@
>   */
>  unsigned long run_uncached(void *func)
>  {
> -       register long sp __asm__("$sp");
>         register long ret __asm__("$2");
>         long lfunc = (long)func, ufunc;
>         long usp;
> +       long sp;
> +       asm ("move %0, $sp" : "=r" (sp));
>
>         if (sp >= (long)CKSEG0 && sp < (long)CKSEG2)
>                 usp = CKSEG1ADDR(sp);
> --
> 2.29.2
>


--
Thanks,
~Nick Desaulniers

Re: [PATCH] mips: lib: uncached: fix uninitialized variable 'sp'

[Anders Roxell]

On Mon, 30 Nov 2020 at 21:22, Nick Desaulniers <ndesaulniers@google.com> wrote:
>
> On Fri, Nov 27, 2020 at 12:39 AM Anders Roxell <anders.roxell@linaro.org> wrote:
> >
> > When building mips tinyconfig with clang the following warning show up:
> >
> > /tmp/arch/mips/lib/uncached.c:40:18: note: initialize the variable 'sp' to silence this warning
> >         register long sp __asm__("$sp");
> >                         ^
> >                          = 0
>
> Hi Anders

Hi Nick,

>, thank you for sending the patch. Do you have the full text
> of the warning; it looks like only the note was included?

oops, looks like I missed this:

/srv/src/kernel/next/arch/mips/lib/uncached.c:45:6: warning: variable
'sp' is uninitialized when used here [-Wuninitialized]
        if (sp >= (long)CKSEG0 && sp < (long)CKSEG2)
            ^~
/srv/src/kernel/next/arch/mips/lib/uncached.c:40:18: note: initialize
the variable 'sp' to silence this warning
        register long sp __asm__("$sp");
                        ^
                         = 0
1 warning generated.

Cheers,
Anders

>
> >
> > Rework to make an explicit inline move.
> >
> > Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
> > ---
> >  arch/mips/lib/uncached.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/mips/lib/uncached.c b/arch/mips/lib/uncached.c
> > index 09d5deea747f..21a4b94a0558 100644
> > --- a/arch/mips/lib/uncached.c
> > +++ b/arch/mips/lib/uncached.c
> > @@ -37,10 +37,11 @@
> >   */
> >  unsigned long run_uncached(void *func)
> >  {
> > -       register long sp __asm__("$sp");
> >         register long ret __asm__("$2");
> >         long lfunc = (long)func, ufunc;
> >         long usp;
> > +       long sp;
> > +       asm ("move %0, $sp" : "=r" (sp));
> >
> >         if (sp >= (long)CKSEG0 && sp < (long)CKSEG2)
> >                 usp = CKSEG1ADDR(sp);
> > --
> > 2.29.2
> >
>
>
> --
> Thanks,
> ~Nick Desaulniers

Re: [PATCH] mips: lib: uncached: fix uninitialized variable 'sp'

[Maciej W. Rozycki]

On Fri, 27 Nov 2020, Anders Roxell wrote:

> When building mips tinyconfig with clang the following warning show up:
> 
> /tmp/arch/mips/lib/uncached.c:40:18: note: initialize the variable 'sp' to silence this warning
>         register long sp __asm__("$sp");
>                         ^
>                          = 0
> 
> Rework to make an explicit inline move.

 It's not uninitialised, because by definition $sp has been initialised.  
This code wouldn't have worked for the last 15 years otherwise.
 
 It is however non-standard usage (which may not have been clarified at 
the time this piece was made; I recall extensive discussions around this 
feature over the years):

"6.47.5.2 Specifying Registers for Local Variables
.................................................
[...]

"The only supported use for this feature is to specify registers for
input and output operands when calling Extended 'asm' (*note Extended
Asm::).  [...]"

so the change description should refer to that as the rationale rather 
than an implementation-specific misleading warning (although that can be 
quoted for a reference of course).

> diff --git a/arch/mips/lib/uncached.c b/arch/mips/lib/uncached.c
> index 09d5deea747f..21a4b94a0558 100644
> --- a/arch/mips/lib/uncached.c
> +++ b/arch/mips/lib/uncached.c
> @@ -37,10 +37,11 @@
>   */
>  unsigned long run_uncached(void *func)
>  {
> -	register long sp __asm__("$sp");
>  	register long ret __asm__("$2");
>  	long lfunc = (long)func, ufunc;
>  	long usp;
> +	long sp;
> +	asm ("move %0, $sp" : "=r" (sp));

 Missing newline between the variable block and code here; you may well 
swap it with the empty line below you have left.  Also use `__asm__' for 
consistency, and remove the extraneous space ahead of the parenthesis as 
we don't use them in Linux.  Using a tab to separate the mnemonic from 
operands is also preferable, i.e. "move\t%0, $sp" in this case (using 
actual tabs works better with multi-line assembly, but this is not one).

 I think this should be OK once the issues I mentioned here have been 
addressed.

  Maciej

Re: [PATCH] mips: lib: uncached: fix uninitialized variable 'sp'

[Maciej W. Rozycki]

On Sun, 6 Dec 2020, Maciej W. Rozycki wrote:

>  I think this should be OK once the issues I mentioned here have been 
> addressed.

 Or alternatively making the variable global would possibly be a better 
fix as it would let the compiler decide if to waste another register to 
keep a copy of $sp.

  Maciej