Re: [Qemu-devel] Re: [PATCH 07/17] block/vvfat.c: fix warnings with _FORTIFY_SOURCE

["Daniel P. Berrange" <berrange@redhat.com>]

On Wed, Jan 20, 2010 at 08:19:26AM +0200, Kirill A. Shutemov wrote:
> On Wed, Jan 20, 2010 at 1:56 AM, Juan Quintela <quintela@redhat.com> wrote:
> > From: Kirill A. Shutemov <kirill@shutemov.name>
> >
> > CC    block/vvfat.o
> > cc1: warnings being treated as errors
> > block/vvfat.c: In function 'commit_one_file':
> > block/vvfat.c:2259: error: ignoring return value of 'ftruncate', declared with attribute warn_unused_result
> > make: *** [block/vvfat.o] Error 1
> >  CC    block/vvfat.o
> > In file included from /usr/include/stdio.h:912,
> >                 from ./qemu-common.h:19,
> >                 from block/vvfat.c:27:
> > In function 'snprintf',
> >    inlined from 'init_directories' at block/vvfat.c:871,
> >    inlined from 'vvfat_open' at block/vvfat.c:1068:
> > /usr/include/bits/stdio2.h:65: error: call to __builtin___snprintf_chk will always overflow destination buffer
> > make: *** [block/vvfat.o] Error 1
> >
> > Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>
> > Signed-off-by: Juan Quintela <quintela@redhat.com>
> > ---
> >  block/vvfat.c |    9 +++++++--
> >  1 files changed, 7 insertions(+), 2 deletions(-)
> >
> > diff --git a/block/vvfat.c b/block/vvfat.c
> > index 063f731..df957e5 100644
> > --- a/block/vvfat.c
> > +++ b/block/vvfat.c
> > @@ -868,7 +868,8 @@ static int init_directories(BDRVVVFATState* s,
> >     {
> >        direntry_t* entry=array_get_next(&(s->directory));
> >        entry->attributes=0x28; /* archive | volume label */
> > -       snprintf((char*)entry->name,11,"QEMU VVFAT");
> > +       memcpy(entry->name,"QEMU VVF",8);
> > +       memcpy(entry->extension,"AT ",3);
> >     }
> 
> Better to use
> 
> memcpy(entry->name, "QEMU VVFAT", 11);
> 
> memcpy() doesn't check bounds.

It doesn't *currently* check bounds. If we want to explicitly
fill 2 fields at once, then we should redeclare this to have a 
union with one part comprising the entire buffer, thus avoiding
the need for delibrate buffer overruns.

Regards,
Daniel
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