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

[Naveen Kumar Chaudhary]

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>
---
 cmd/setexpr.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/cmd/setexpr.c b/cmd/setexpr.c
index c45fa85c887..38a7fbeb86c 100644
--- a/cmd/setexpr.c
+++ b/cmd/setexpr.c
@@ -517,9 +517,17 @@ static int do_setexpr(struct cmd_tbl *cmdtp, int flag, int argc,
 			value = a * b;
 			break;
 		case '/':
+			if (b == 0) {
+				printf("Error: division by zero\n");
+				return 1;
+			}
 			value = a / b;
 			break;
 		case '%':
+			if (b == 0) {
+				printf("Error: modulo by zero\n");
+				return 1;
+			}
 			value = a % b;
 			break;
 		default:
-- 
2.43.0