Re: [PATCH] bpf, arm32: Correct check_imm24

[Russell King - ARM Linux <linux@armlinux.org.uk>]

On Thu, May 10, 2018 at 11:20:13AM +0800, Wang YanQing wrote:
> imm24 is signed, so the right range is:
> [-(2<<(24 - 1)), (2<<(24 - 1)) - 1]

2 << (24 - 1) is the same as 1 << 24.

> -#define check_imm(bits, imm) do {				\
> -	if ((((imm) > 0) && ((imm) >> (bits))) ||		\
> -	    (((imm) < 0) && (~(imm) >> (bits)))) {		\
> -		pr_info("[%2d] imm=%d(0x%x) out of range\n",	\
> -			i, imm, imm);				\
> +#define check_imm_range(min, max, imm) do {			\
> +	if (imm < min || imm > max) {				\
> +		pr_info("[%2d] imm=%d is out of range\n",	\
> +			i, imm);				\
>  		return -EINVAL;					\
>  	}							\
>  } while (0)
> -#define check_imm24(imm) check_imm(24, imm)
> +#define check_imm24(imm) check_imm_range(-16777216, 16777215, imm)

How is this any different?

If imm is 16777216, then "imm > max" in your version is true.
In the original version, "imm > 0" is true, so we then test for
"16777216 >> 24" being non-zero.  That's also true, so the test
condition fires.

If imm is 16777215, then "imm > max" is false in your version.
In the original version, the conditions also evaluate to false.

For the -16777217 case, "imm < min" in your version is true.
In the original version, "imm < 0" is true, so we then test for
"~(-16777217) >> 24" being non-zero.  This is the same as
"16777216 >> 24" being non-zero, which is true so the condition
fires.

With -16777216, the same thing happens, both end up evaluating
to false.

So, the two cases end up producing identical results, and there
is no actual effect from this change.

However, your commit message is correct - there is a bug here.
That's obvious when you mask the "imm" value with 0x00ffffff,
and realise that an imm value of -16777216 ends up having the
same value in the instruction as an imm value of 0.  So, the
range of "imm" is _half_ that.

 #define check_imm(bits, imm) do {				\
-	if ((((imm) > 0) && ((imm) >> (bits))) ||		\
-	    (((imm) < 0) && (~(imm) >> (bits)))) {		\
+	if ((((imm) > 0) && ((imm) >> (bits - 1))) ||		\
+	    (((imm) < 0) && (~(imm) >> (bits - 1)))) {		\
 		pr_info("[%2d] imm=%d(0x%x) out of range\n",	\
 			i, imm, imm);				\

would fix it.  Alternatively:

 #define check_imm(bits, imm) do {				\
-	if ((((imm) > 0) && ((imm) >> (bits))) ||		\
-	    (((imm) < 0) && (~(imm) >> (bits)))) {		\
+	if ((imm) >= (1 << ((bits) - 1)) ||			\
+	    (imm) < -(1 << ((bits) - 1))) {			\
 		pr_info("[%2d] imm=%d(0x%x) out of range\n",	\
 			i, imm, imm);				\

would also fix it.

-- 
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