Date: Sun, 07 Apr 2024 12:50:00 +0200 From: =?utf-8?Q?Dag-Erling_Sm=C3=B8rgrav?= <des@FreeBSD.org> To: Rocky Hotas <rockyhotas@tilde.team> Cc: freebsd-hackers@freebsd.org Subject: Re: Kernel module: return a number from a device Message-ID: <86r0fh5twn.fsf@ltc.des.dev> In-Reply-To: <oghsl7sgcl6p6ym3wwa3w56hn52cdophcr4jxau4ronxevy4ar@ux223whjcthj> (Rocky Hotas's message of "Sat, 6 Apr 2024 22:27:12 %2B0200") References: <oghsl7sgcl6p6ym3wwa3w56hn52cdophcr4jxau4ronxevy4ar@ux223whjcthj>
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Rocky Hotas <rockyhotas@tilde.team> writes:
> static int
> rolld_read(struct cdev *dev __unused, struct uio *uio, int ioflag __unused)
> {
> uint32_t random_out;
> uint32_t random_item;
> int error;
>
> random_item = arc4random();
> random_out = random_item % d_size;
>
> if ((error = uiomove(&random_out, 1, uio)) != 0)
> uprintf("uiomove failed!\n");
>
> return (error);
> }
Using a uint32_t will work on little-endian systems (such as amd64)
because the least-significant byte, which is the only non-zero byte,
comes first. On big-endian systems, it would simply always return 0.
Furthermore, this won't only return one byte; rather, it will return one
byte _at a time_, very inefficiently. This is why cat appears to hang.
To truly only return one byte, you need to look at uio->uio_offset and
return 0 without calling uiomove(), signaling EOF, if it is non-zero.
In summary, you should write rolld_read() as:
uint8_t roll = arc4random() % d_size;
if (uio->uio_offset > 0)
return (0);
return (uiomove(&roll, 1, uio));
You can also use uiomove_frombuf(), which will take care of that check
for you. It's a bit overkill when you're only writing a single byte,
but if you wanted to output text instead of binary, you could use this:
char roll[2];
roll[0] = '0' + arc4random() % d_size;
roll[1] = '\n';
return (uiomove_frombuf(roll, sizeof(roll), uio));
Obviously, this will only work for d_size <= 9. For larger values, you
will want to use snprintf():
char roll[16];
int len = snprintf(roll, sizeof(roll), "%d\n", arc4random() % d_size);
return (uiomove_frombuf(roll, len, uio));
Have fun,
DES
--
Dag-Erling Smørgrav - des@FreeBSD.org
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