[U-Boot] [PATCH 03/12] libfdt: Safer access to strings section

[Tom Rini]

On Tue, Apr 10, 2018 at 10:42:45AM -0400, Simon Glass wrote:
> +U-Boot, Tom, Masahiro
> 
> Hi David,
> 
> On 10 April 2018 at 01:22, David Gibson <david at gibson.dropbear.id.au> wrote:
> > On Wed, Apr 04, 2018 at 01:21:10AM +0800, Simon Glass wrote:
> >> Hi David,
> >>
> >> On 3 April 2018 at 23:02, David Gibson <david at gibson.dropbear.id.au> wrote:
> >> >
> >> > On Fri, Mar 30, 2018 at 04:42:21PM +0800, Simon Glass wrote:
> >> > > Hi David,
> >> > >
> >> > > On 26 March 2018 at 07:25, David Gibson <david at gibson.dropbear.id.au> wrote:
> >> > > > fdt_string() is used to retrieve strings from a DT blob's strings section.
> >> > > > It's rarely used directly, but is widely used internally.
> >> > > >
> >> > > > However, it doesn't do any bounds checking, which means in the case of a
> >> > > > corrupted blob it could access bad memory, which libfdt is supposed to
> >> > > > avoid.
> >> > > >
> >> > > > This write a safe alternative to fdt_string, fdt_get_string().  It checks
> >> > > > both that the given offset is within the string section and that the string
> >> > > > it points to is properly \0 terminated within the section.  It also returns
> >> > > > the string's length as a convenience (since it needs to determine to do the
> >> > > > checks anyway).
> >> > > >
> >> > > > fdt_string() is rewritten in terms of fdt_get_string() for compatibility.
> >> > > >
> >> > > > Most of the diff here is actually testing infrastructure.
> >> > > >
> >> > > > Signed-off-by: David Gibson <david at gibson.dropbear.id.au>
> >> > > > ---
> >> > > >  libfdt/fdt_ro.c          | 61 +++++++++++++++++++++++++++++++++++--
> >> > > >  libfdt/libfdt.h          | 18 ++++++++++-
> >> > > >  libfdt/version.lds       |  2 +-
> >> > > >  tests/.gitignore         |  1 +
> >> > > >  tests/Makefile.tests     |  2 +-
> >> > > >  tests/run_tests.sh       |  1 +
> >> > > >  tests/testdata.h         |  1 +
> >> > > >  tests/testutils.c        | 11 +++++--
> >> > > >  tests/trees.S            | 26 ++++++++++++++++
> >> > > >  tests/truncated_string.c | 79 ++++++++++++++++++++++++++++++++++++++++++++++++
> >> > > >  10 files changed, 193 insertions(+), 9 deletions(-)
> >> > > >  create mode 100644 tests/truncated_string.c
> >> > >
> >> > > Similar code-size quesiton here. It looks like a lot of checking code.
> >> > > Can we have an option to remove it?
> >> >
> >> > Again, I'm disinclined without a concrete example of a problem.  Fwiw
> >> > the code size change is +276 bytes on my setup.
> >>
> >> That might not sound like a lot, but the overhead of DT in U-Boot is
> >> about 3KB, so this adds nearly 10%.
> >
> > Hm.  And how much is it compared to the whole U-Boot blob?
> >
> >> The specific problem is that when U-Boot SPL gets too big boards don't
> >> boot. Because we take the upstream libfdt this will affect U-Boot.
> >>
> >> Do you have any thoughts on how we could avoid this size increase?
> >
> > So, again, I'm very disinclined to prioritize size over memory safety
> > without a *concrete* example.  i.e. "We hit this specific problem with
> > size on this specific board that we were really using" rather than
> > just "it might be a problem".

I'm either failing in my Google-fu or is there not an easy way to grab
the patches from patchwork/similar?  But, if you shoot me the series
off-list, I can tell you how much U-Boot targets grow here (we can use
the same script as the kernel to re-sync sources back in, so I can give
you a before/after).  But as Simon notes, we have a number of platforms
that need to use (parts of) libfdt and stick to ~30KiB or less in total,
sometimes including some memory for stack/etc and we've long been using
-ffunction-sections/etc (the latest round of trying to use LTO has me
thinking maybe we can see if that's a valid option finally, but that's
an aside). Thanks!

-- 
Tom
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