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POMemory comparison causes system halt
 

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    <p>I am working on a kernel module and I need to compare two buffers to find out if they are equivalent. I am using the memcmp function defined in the Linux kernel to do so. My first buffer is like this:</p> <pre><code>cache_buffer = (unsigned char *)vmalloc(4097); cache_buffer[4096] = '/0'; </code></pre> <p>The second buffer is from a page using the page_address() function. </p> <pre><code>page = bio_page(bio); kmap(page); write_buffer = (char *)page_address(page); kunmap(page); </code></pre> <p>I have printed the contents of both buffers before hand and not only to they print correctly, but they also have the same content. So next, I do this:</p> <pre><code>result = memcmp(write_buffer, cache_buffer, 2048); // only comparing up to 2048 positions </code></pre> <p>This causes the kernel to freeze up and I cannot figure out why. I checked the implementation of memcmp and saw nothing that would cause the freeze. Can anyone suggest a cause?</p> <p>Here is the memcmp implementation:</p> <pre><code>int memcmp(const void *cs, const void *ct, size_t count) { const unsigned char *su1, *su2; int res = 0; for (su1 = cs, su2 = ct; 0 &lt; count; ++su1, ++su2, count--) if ((res = *su1 - *su2) != 0) break; return res; } </code></pre> <p>EDIT: The function causing the freeze is memcmp. When I commented it out, everything worked. Also, when I did I memcmp as follows</p> <pre><code>memcmp(write_buffer, write_buffer, 2048); //comparing two write_buffers </code></pre> <p>Everything worked as well. Only when I throw the cache_buffer into the mix is when I get the error. Also, above is a simplification of my actual code. Here is the entire function:</p> <pre><code>static int compare_data(sector_t location, struct bio * bio, struct cache_c * dmc) { struct dm_io_region where; unsigned long bits; int segno; struct bio_vec * bvec; struct page * page; unsigned char * cache_data; char * temp_data; char * write_data; int result, length, i; cache_data = (unsigned char *)vmalloc((dmc-&gt;block_size * 512) + 1); where.bdev = dmc-&gt;cache_dev-&gt;bdev; where.count = dmc-&gt;block_size; where.sector = location &lt;&lt; dmc-&gt;block_shift; printk(KERN_DEBUG "place: %llu\n", where.sector); dm_io_sync_vm(1, &amp;where, READ, cache_data, &amp;bits, dmc); length = 0; bio_for_each_segment(bvec, bio, segno) { if(segno == 0) { page = bio_page(bio); kmap(page); write_data = (char *)page_address(page); //kunmap(page); length += bvec-&gt;bv_len; } else { page = bio_page(bio); kmap(page); temp_data = strcat(write_data, (char *)page_address(page)); //kunmap(page); write_data = temp_data; length += bvec-&gt;bv_len; } } printk(KERN_INFO "length: %u\n", length); cache_data[dmc-&gt;block_size * 512] = '\0'; for(i = 0; i &lt; 2048; i++) { printk("%c", write_data[i]); } printk("\n"); for(i = 0; i &lt; 2048; i++) { printk("%c", cache_data[i]); } printk("\n"); result = memcmp(write_data, cache_data, length); return result; } </code></pre> <p>EDIT #2: Sorry guys. The problem was not memcmp. It was the result of memcmp. When ever it returned a positive or negative number, the function that called my function would play with some pointers, one of which was uninitialized. I don't know why I didn't realize it before. Thanks for trying to help though!</p>
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    1. COI don't know if this has anything to do with your problem but you wrote '/0' instead of '\0'. Try fixing this.
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      1. USIsrael Unterman
    2. COHave you identified precisely which function freezes the kernel ? If so, please edit your question to explicit this. Otherwise, just try adding some `printk`s or run your kernel through a debugger (the second solution is much longer to set up, but once done it will help you a lot :) )
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    3. COit is definitely memcmp, but only when the cache_buffer is involved
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