Re: [PATCH 3/3] vdso/gettimeofday: Assert that the clock id fits the dispatch mask

[Thomas Weißschuh]

On Mon, Aug 31, 2026 at 08:55:57PM +0800, Zhan Xusheng wrote:
> The clock id dispatch turns the id into a bit in a u32:
> 
>   if (!vdso_clockid_valid(clock))
>       return false;
>   msk = 1U << clock;
> 
> vdso_clockid_valid() admits everything up to CLOCK_AUX_LAST, which is 23,
> so the shift is in range.  Nothing states the dependency though, and
> raising MAX_AUX_CLOCKS past 16 would take CLOCK_AUX_LAST to 32 or beyond
> and make the shift undefined.
> 
> Assert it at both dispatch sites.  The condition is on a parameter rather
> than a constant, so it relies on the compiler deriving the range from the
> vdso_clockid_valid() bail-out above it.  gcc 13 and clang 18 both do: x86
> vdso64 and vdso32 build clean, and raising MAX_AUX_CLOCKS to 17 fails the
> assert as intended.
> 
> Suggested-by: Thomas Weißschuh <thomas.weissschuh@xxxxxxxxxxxxx>

Reviewed-by: Thomas Weißschuh <thomas.weissschuh@xxxxxxxxxxxxx>

Thanks!

> Signed-off-by: Zhan Xusheng <zhanxusheng@xxxxxxxxxx>
> ---
>  lib/vdso/gettimeofday.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/lib/vdso/gettimeofday.c b/lib/vdso/gettimeofday.c
> index f7a591aba59f..ef4dcc614489 100644
> --- a/lib/vdso/gettimeofday.c
> +++ b/lib/vdso/gettimeofday.c
> @@ -285,6 +285,7 @@ __cvdso_clock_gettime_common(const struct vdso_time_data *vd, clockid_t clock,
>  	 * Convert the clockid to a bitmask and use it to check which
>  	 * clocks are handled in the VDSO directly.
>  	 */
> +	BUILD_BUG_ON(clock >= BITS_PER_TYPE(msk));
>  	msk = 1U << clock;
>  	if (likely(msk & VDSO_HRES))
>  		vc = &vc[CS_HRES_COARSE];
> @@ -438,6 +439,7 @@ bool __cvdso_clock_getres_common(const struct vdso_time_data *vd, clockid_t cloc
>  	 * Convert the clockid to a bitmask and use it to check which
>  	 * clocks are handled in the VDSO directly.
>  	 */
> +	BUILD_BUG_ON(clock >= BITS_PER_TYPE(msk));
>  	msk = 1U << clock;
>  	if (msk & (VDSO_HRES | VDSO_RAW)) {
>  		/*
> -- 
> 2.43.0
>