[PATCH] cmd: setexpr: reject division and modulo by zero

[Quentin Schulz]

Hi Naveen,

On 7/8/26 4:33 PM, Naveen Kumar Chaudhary wrote:
> The '/' and '%' arms of the operator switch in do_setexpr() perform
> a / b and a % b directly on the user-supplied ulongs with no check
> that b is non-zero. The consequences are architecture-dependent:
> 
>    - On x86 and sandbox, integer divide-by-zero raises an exception
>      (#DE / SIGFPE) which aborts the shell.
>    - On arm64, arm, and RISC-V, the UDIV/SDIV instructions are defined
>      to return 0 on a zero divisor and do not trap, so 'setexpr x 10
>      / 0' silently succeeds and stores 0 into the environment
>      variable, which is a plausible-looking but garbage result.
> 
> Neither behaviour is acceptable for a scripting primitive. Check b
> before dividing and print a clear error, returning failure so
> scripts can detect it.
> 
> ++++++ Before Fix ++++++++
> => setexpr x 0xa / 0
> => printenv x
> x=0
> 
> ++++++ After Fix +++++++++
> => setexpr x 0xa / 0
> Error: division by zero
> => setexpr x 0xa % 0
> Error: modulo by zero
> =>
> Signed-off-by: Naveen Kumar Chaudhary <naveen.osdev at gmail.com>

Makes sense to me, though we definitely need new tests for those, c.f.
test/cmd/setexpr.c, so we don't regress in the future.

Cheers,
Quentin