Re: [PATCH v2 1/2] staging: rtl8723bs: replace private CRC-32 routines with in-kernel ones

[Fabio Aiuto]

Hello David,

On Mon, May 10, 2021 at 01:38:49PM +0000, David Laight wrote:
> > replace private CRC-32 routines with in-kernel ones.
> 
> Have you verified that they compute the same CRC?
> 
> There are all sorts of subtle reasons why the outputs can differ.
> 
> 	David
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
> 

I tried this:

#include <linux/module.h>
#include <linux/crc32poly.h>
#include <linux/crc32.h>

/* Copy pasted from rtl8723bs/core/rtw_security.c */
static signed int bcrc32initialized;
static u32 crc32_table[256];

MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("CRC-32 verifier");
MODULE_AUTHOR("Fabio Aiuto");

/* Copy pasted from rtl8723bs/core/rtw_security.c */
static u8 crc32_reverseBit(u8 data)
{
        return ((u8)((data<<7)&0x80) | 
                ((data<<5)&0x40) | 
                ((data<<3)&0x20) | 
                ((data<<1)&0x10) | 
                ((data>>1)&0x08) | 
                ((data>>3)&0x04) | 
                ((data>>5)&0x02) | 
                ((data>>7)&0x01));
}


/* Copy pasted from rtl8723bs/core/rtw_security.c */
static void crc32_init(void)
{
        if (bcrc32initialized == 1)
                return;
        else {
                signed int i, j;
                u32 c;
                u8 *p = (u8 *)&c, *p1;
                u8 k;

                c = 0x12340000;

                for (i = 0; i < 256; ++i) {
                        k = crc32_reverseBit((u8)i);
                        for (c = ((u32)k) << 24, j = 8; j > 0; --j)
                                c = c & 0x80000000 ? (c << 1) ^ CRC32_POLY_BE : (c << 1);
                        p1 = (u8 *)&crc32_table[i];

                        p1[0] = crc32_reverseBit(p[3]);
                        p1[1] = crc32_reverseBit(p[2]);
                        p1[2] = crc32_reverseBit(p[1]);
                        p1[3] = crc32_reverseBit(p[0]);
                }
                bcrc32initialized = 1;
        }
}

/* Copy pasted from rtl8723bs/core/rtw_security.c */
static __le32 getcrc32(u8 *buf, signed int len)
{
        u8 *p;
        u32  crc;

        if (bcrc32initialized == 0)
                crc32_init();

        crc = 0xffffffff;       /* preload shift register, per CRC-32 spec */

        for (p = buf; len > 0; ++p, --len)
                crc = crc32_table[(crc ^ *p) & 0xff] ^ (crc >> 8);
        return cpu_to_le32(~crc);    /* transmit complement, per CRC-32 spec */
}

static int __init crc32_entry(void)
{
        u8 payload;
        unsigned char crc_priv[4], crc_pub[4];

        payload = (u8)1234;
        /* private crc calculation used in rtl8723bs */
        *((__le32 *)crc_priv) = getcrc32(&payload, 2);
        pr_info("private rtl8723bs crc: %x", *crc_priv);
        /* in-kernel public crc calculation */
        *((__le32 *)crc_pub) = ~crc32_le(~0, &payload, 2);
        pr_info("generic crc: %x", *crc_pub);

        return 0;

}

static void __exit crc32_exit(void)
{
}

module_init(crc32_entry);
module_exit(crc32_exit);

And run it on qemu. Don't know why to display the second pr_info()
I have to rmmod it, but both methods give the same result (62)

[   38.149979] crc32m: module is from the staging directory, the quality is unknown, you have been warned.
[   38.169208] private rtl8723bs crc: 62
[   38.169400] generic crc: 62

thank you,

fabio