RE: [PATCH v4 2/6] treewide: use prandom_u32_max() when possible

From: David Laight <David.Laight@ACULAB.COM>
To: "'Jason A. Donenfeld'" <Jason@zx2c4.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"patches@lists.linux.dev" <patches@lists.linux.dev>
Cc: "Andreas Noever" <andreas.noever@gmail.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Andy Shevchenko" <andriy.shevchenko@linux.intel.com>,
	"Borislav Petkov" <bp@alien8.de>,
	"Catalin Marinas" <catalin.marinas@arm.com>,
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	"Daniel Borkmann" <daniel@iogearbox.net>,
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Subject: RE: [PATCH v4 2/6] treewide: use prandom_u32_max() when possible
Date: Sat, 8 Oct 2022 22:08:03 +0000	[thread overview]
Message-ID: <01fafe0e56554b1c9c934c458b93473a@AcuMS.aculab.com> (raw)
In-Reply-To: <20221007180107.216067-3-Jason@zx2c4.com>

From: Jason A. Donenfeld
> Sent: 07 October 2022 19:01
> 
> Rather than incurring a division or requesting too many random bytes for
> the given range, use the prandom_u32_max() function, which only takes
> the minimum required bytes from the RNG and avoids divisions.
> 
...
> --- a/lib/cmdline_kunit.c
> +++ b/lib/cmdline_kunit.c
> @@ -76,7 +76,7 @@ static void cmdline_test_lead_int(struct kunit *test)
>  		int rc = cmdline_test_values[i];
>  		int offset;
> 
> -		sprintf(in, "%u%s", prandom_u32_max(256), str);
> +		sprintf(in, "%u%s", get_random_int() % 256, str);
>  		/* Only first '-' after the number will advance the pointer */
>  		offset = strlen(in) - strlen(str) + !!(rc == 2);
>  		cmdline_do_one_test(test, in, rc, offset);
> @@ -94,7 +94,7 @@ static void cmdline_test_tail_int(struct kunit *test)
>  		int rc = strcmp(str, "") ? (strcmp(str, "-") ? 0 : 1) : 1;
>  		int offset;
> 
> -		sprintf(in, "%s%u", str, prandom_u32_max(256));
> +		sprintf(in, "%s%u", str, get_random_int() % 256);
>  		/*
>  		 * Only first and leading '-' not followed by integer
>  		 * will advance the pointer.

Something has gone backwards here....
And get_random_u8() looks a better fit.

	David

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