Re: [PATCH 3/3] compat: make compat_sysinfo() scale far enough to actually fit

[Arnd Bergmann]

On Thu, Nov 5, 2026, at 01:31, Bradley Morgan wrote:
> compat_sysinfo() checks whether totalram or totalswap is too big for the
> compat struct and scales the values down to fit, but the loop stops as
> soon as s.mem_unit reaches PAGE_SIZE.  That caps the compression at one
> page, 12 bits with 4K pages.  On a 64 bit kernel do_sysinfo() always
> hands it values in bytes with mem_unit == 1, its own overflow math runs
> on unsigned long so it does not bail until 16 EiB, so the loop runs its
> 12 turns, the values come back out as a page count, and anything past
> 2^32 pages still does not fit.  Storing that into the u32 field then
> just drops the top bits.
>
> So a 32 bit process on a box past 2^32 pages gets a wrapped number
> instead of a coarse one, with 4K pages that starts at 16 TiB:
>
>   ram      old                  new
>   8 TiB    2147483648 x 4096    unchanged
>   16 TiB   0 x 4096             2147483648 x 8192
>   17 TiB   268435456 x 4096     2281701376 x 8192
>   32 TiB   0 x 4096             2147483648 x 16384
>
> The 17 TiB row is the nasty one, a 32 bit process is told the box has
> 1 TiB of ram.  Keep scaling until the values fit instead of until the
> unit reaches a page, the unit then says how coarse the number is, which
> is what mem_unit is for.
>
> Nothing that works today changes.  I ran both loops side by side over
> ram from 1 MiB to 2^56 with 4K and 64K pages, they disagree on exactly
> the inputs where the old one dropped bits and agree on every one it got
> right.
>
> Signed-off-by: Bradley Morgan <brads@xxxxxxxxxxxxxx>

Makes sense, thanks for the fix!

Reviewed-by: Arnd Bergmann <arnd@xxxxxxxx>