Re: [PATCH v4 19/39] unwind_user/sframe: Add support for reading .sframe contents

[Josh Poimboeuf]

On Tue, Jan 28, 2025 at 11:50:25AM +0100, Jens Remus wrote:
> On 28.01.2025 01:39, Andrii Nakryiko wrote:
> > On Fri, Jan 24, 2025 at 1:41 PM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> > > On Fri, Jan 24, 2025 at 10:02:46AM -0800, Andrii Nakryiko wrote:
> > > > On Tue, Jan 21, 2025 at 6:32 PM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> 
> > > > > +       UNSAFE_GET_USER_INC(info, cur, 1, Efault);
> > > > > +       offset_count = SFRAME_FRE_OFFSET_COUNT(info);
> > > > > +       offset_size  = offset_size_enum_to_size(SFRAME_FRE_OFFSET_SIZE(info));
> > > > > +       if (!offset_count || !offset_size)
> > > > > +               return -EFAULT;
> > > > > +
> > > > > +       if (cur + (offset_count * offset_size) > sec->fres_end)
> > > > 
> > > > offset_count * offset_size done in u8 can overflow, no? maybe upcast
> > > > to unsigned long or use check_add_overflow?
> 
> The maximum offset_count * offset_size is 15 * 4 = 60 if I am not wrong:
> 
> > > offset_size is <= 2 as returned by offset_size_enum_to_size().
> 
> SFrame V2 FRE offset sizes are either 1, 2, or 4 bytes.  This is also
> reflected in offset_size_enum_to_size().
> 
> > > offset_count is expected to be <= 3, enforced by the !offset_count check
> > > at the bottom.
> 
> SFrame V2 FRE offset count is 4 bits unsigned, so 0 <= offset_count <= 15.
> > > An overflow here would be harmless as it would be caught by the
> > > !offset_count anyway.  Though I also notice offset_count isn't big
> > > enough to hold the 2-byte SFRAME_FRE_OFFSET_COUNT() value.  Which is
> > > harmless for the same reason, but yeah I'll make offset_count an
> > > unsigned int.
> 
> As mentioned above the FRE offset count is 4 bits, not 2 bytes.  This is
> also reflected in SFRAME_FRE_OFFSET_COUNT().

You are right on both counts, not sure what I was smoking that day.

-- 
Josh