SHA-1 hash calculate in Kernel.

[lx]

hi all:
      I built a module for calculate the SHA-1. The code is:
##################
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/crypto.h>
#include <linux/err.h>
#include <linux/scatterlist.h>
MODULE_LICENSE("Dual BSD/GPL");

#define SHA1_LENGTH     20

static int hello_init(void)
{
    /*
     * http://lxr.oss.org.cn/source/fs/ecryptfs/crypto.c?v=2.6.30
     */
    struct scatterlist sg;
    struct hash_desc desc;

    //way 1
    /*
    char *plaintext = "c";
    size_t len = strlen(plaintext);
    */

    //way 2
    /*
    char *plaintext = kmalloc(sizeof(char), GFP_KERNEL);
    plaintext = "c";
    size_t len = 1;
    */

    // way 3.
    /*
    char plaintext[1] = {'c'};
    size_t len = 1;
    */

    // way 4.
    /*
    char *plaintext = (char *)__get_free_page(GFP_KERNEL);
    memcpy(plaintext, "c", 1);
    size_t len = 1;
    */

    int rc = 0;
    int i;
    char hashtext[SHA1_LENGTH];

    memset(hashtext, 0x00, SHA1_LENGTH);
    printk(KERN_INFO "sha1: %s\n", __FUNCTION__);

    sg_init_one(&sg, plaintext, len);
    desc.tfm = crypto_alloc_hash("sha1", 0, CRYPTO_ALG_ASYNC);
    desc.flags = 0;

    rc = crypto_hash_init(&desc);
    if (rc) {
        printk(KERN_ERR "%s: Error initializing crypto hash; rc = [%d]\n",
__func__, rc);
        goto out;
    }
    rc = crypto_hash_update(&desc, &sg, len);
    if (rc) {
        printk(KERN_ERR "%s: Error updating crypto hash; rc = [%d]\n",
__func__, rc);
        goto out;
    }
    rc = crypto_hash_final(&desc, hashtext);
    if (rc) {
        printk(KERN_ERR "%s: Error finalizing crypto hash; rc = [%d]\n",
__func__, rc);
        goto out;
    }
    crypto_free_hash(desc.tfm);

    for (i = 0; i < 20; i++) {
        printk(KERN_INFO "%02x-%d\n", hashtext[i]&0xff, i);
    }

out:
    printk(KERN_INFO "end\n");
    return rc;
}

static void hello_exit(void)
{
    printk(KERN_ALERT "Goodbye, cruel world\n");
}

module_init(hello_init);
module_exit(hello_exit);
##################
I can just get the right result by way 3 and 4, In way 1 and 2 , I get the
wrong result,
I check the result by link: http://www.xorbin.com/tools/sha1-hash-calculator
.
Please tell me why? Thank you.
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