Re: [PATCH RFCv4 3/4] lib/test_printf.c: split write-beyond-buffer check in two

[Kent Overstreet]

On 6/17/22 03:15, Rasmus Villemoes wrote:
> On 17/06/2021 16.17, Petr Mladek wrote:
> > On Tue 2021-06-15 23:49:51, Jia He wrote:
> > > From: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
> > >
> > > Before each invocation of vsnprintf(), do_test() memsets the entire
> > > allocated buffer to a sentinel value. That buffer includes leading and
> > > trailing padding which is never included in the buffer area handed to
> > > vsnprintf (spaces merely for clarity):
> > >
> > >    pad  test_buffer      pad
> > >    **** **************** ****
> > >
> > > Then vsnprintf() is invoked with a bufsize argument <=
> > > BUF_SIZE. Suppose bufsize=10, then we'd have e.g.
> > >
> > >   |pad |   test_buffer    |pad |
> > >    **** pizza0 **** ****** ****
> > >   A    B      C    D           E
> > >
> > > where vsnprintf() was given the area from B to D.
> > >
> > > It is obviously a bug for vsnprintf to touch anything between A and B
> > > or between D and E. The former is checked for as one would expect. But
> > > for the latter, we are actually a little stricter in that we check the
> > > area between C and E.
> > >
> > > Split that check in two, providing a clearer error message in case it
> > > was a genuine buffer overrun and not merely a write within the
> > > provided buffer, but after the end of the generated string.
> > >
> > > So far, no part of the vsnprintf() implementation has had any use for
> > > using the whole buffer as scratch space, but it's not unreasonable to
> > > allow that, as long as the result is properly nul-terminated and the
> > > return value is the right one. However, it is somewhat unusual, and
> > > most %<something> won't need this, so keep the [C,D] check, but make
> > > it easy for a later patch to make that part opt-out for certain tests.
> >
> > Excellent commit message.
> >
> > > Signed-off-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
> > > Tested-by: Jia He <justin.he@xxxxxxx>
> > > Signed-off-by: Jia He <justin.he@xxxxxxx>
> >
> > Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>
>
> Hi Petr
>
> It seems Justin's series got stalled, but I still think this patch makes
> sense on its own (especially since another series in flight mucks about
> in this area), so can you please pick it up directly?
>
> The lore link for the above is
> https://lore.kernel.org/lkml/20210615154952.2744-4-justin.he@xxxxxxx/ ,
> while my original submission is at
> https://lore.kernel.org/lkml/20210615085044.1923788-1-linux@xxxxxxxxxxxxxxxxxx/

That patch definitely clarifies things, feel free to add my reviewed-by tag