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

On 28.01.2025 01:39, Andrii Nakryiko wrote:
> On Fri, Jan 24, 2025 at 1:41 PM Josh Poimboeuf <[email protected]> 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 <[email protected]> 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().

Regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
[email protected]

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