Re: [PATCH 6/8] x86/fpu: Fix potential underflow in xstate_calculate_size()

[Chang S. Bae]

On 8/16/2026 9:20 PM, Andrei Vagin wrote:
> xstate_calculate_size() calculates the size required for a given set of
> xfeatures. It determines the topmost feature by finding the most
> significant bit in xfeatures using fls64(xfeatures) - 1.
>
> If xfeatures is 0, fls64(0) returns 0, and topmost becomes -1.
> Previously, topmost was unsigned int, so -1 underflowed to UINT_MAX.
> This caused the subsequent check `topmost <= XFEATURE_SSE` to fail, and
> the code proceeded to access xstate arrays using topmost (UINT_MAX) as
> an index, leading to an out-of-bounds access.
>
> Fix this by checking if xfeatures only contains legacy features (FP/SSE)
> or is empty (xfeatures <= XFEATURE_MASK_FPSSE) before calculating
> topmost.
I think this fix makes sense before applying it to something that 
user-space provides via the sigframe:

  Reviewed-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>

Thanks,
Chang