Re: [PATCH] exynos4210_gic: Suppress gcc9 format-truncation warnings

[David Gibson]

On Mon, Oct 14, 2019 at 01:51:39PM +0100, Peter Maydell wrote:
> On Fri, 4 Oct 2019 at 04:10, David Gibson <address@hidden> wrote:
> >
> > exynos4210_gic_realize() prints the number of cpus into some temporary
> > buffers, but it only allows 3 bytes space for it.  That's plenty - I'm
> > pretty sure that existing machines will only ever set this value to 2
> > (EXYNOS4210_NCPUS).  But the compiler can't really be expected to figure
> > that out.
> >
> > Some[*] gcc9 versions therefore emit -Wformat-truncation warnings.  Fix
> > that by allowing more space in the temporary buffers - these are on stack
> > very briefly before being essentially strdup()ed inside the memory region
> > code, so there's not much cost to doing so.
> >
> > [*] The bizarre thing here, is that I've long gotten these warnings
> > compiling in a 32-bit x86 container as host - Fedora 30 with
> > gcc-9.2.1-1.fc30.i686 - but it compiles just fine on my normal x86_64 host
> > - Fedora 30 with and gcc-9.2.1-1.fc30.x86_64.
> >
> > Signed-off-by: David Gibson <address@hidden>
> > ---
> >  hw/intc/exynos4210_gic.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/hw/intc/exynos4210_gic.c b/hw/intc/exynos4210_gic.c
> > index a1b699b6ba..2e5e47f9ec 100644
> > --- a/hw/intc/exynos4210_gic.c
> > +++ b/hw/intc/exynos4210_gic.c
> > @@ -290,8 +290,8 @@ static void exynos4210_gic_realize(DeviceState *dev, 
> > Error **errp)
> >      SysBusDevice *sbd = SYS_BUS_DEVICE(obj);
> >      const char cpu_prefix[] = "exynos4210-gic-alias_cpu";
> >      const char dist_prefix[] = "exynos4210-gic-alias_dist";
> > -    char cpu_alias_name[sizeof(cpu_prefix) + 3];
> > -    char dist_alias_name[sizeof(cpu_prefix) + 3];
> > +    char cpu_alias_name[sizeof(cpu_prefix) + 10];
> > +    char dist_alias_name[sizeof(cpu_prefix) + 10];
> >      SysBusDevice *gicbusdev;
> >      uint32_t i;
> 
> If we assert() that num_cpu is always <= EXYNOS4210_NCPUS
> is that sufficient to clue gcc in that the buffer can't overflow?

Interestingly, assert(s->num_cpu <= EXYNOS$210_NCPUS) is *not*
sufficient, but assert(i <= EXYNOS4210_NCPUS) within the loop *is*
enough.  I've updated my patch accordingly.

This isn't 4.2 material, obviously.  Should I just sit on it until 5.0
opens, or does one of you have someplace to stage the patch in the
meanwhile?

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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