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By Hyde R.

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This allows the new process to peek at the data written by some other process (a minor security gaffe). The current scullc driver ignores this issue because the scullc device is global and persistent. In particular, if the current process wants to read the previous data written by the last process to access the device, it could do so simply by opening the device in read-only mode rather than write-only mode. If the previous process really wanted to prevent any new processes from reading the data once it was done, it could overwrite the device file with zeros.

If( ebx > maxScullcSize_c ) then // Okay, we'd write beyond the end of the device. // Truncate this write operation and bail if it // turns out we'd transfer zero bytes. printk ( "<1>scull_write: copying %d bytes to posn %d\n", count, (type dword [edx]) ) ); mov( [edx], ebx ); // Get the current file position. printk( "<1>scull_write: %d bytes left to copy\n", eax ) ); // If copy_to_user returns non-zero, we've got an error // (the return value is the number of bytes left to copy). O. dword. However, the device never has more than 2^32 bytes, so we know the carry will always be clear.

Len), ebx ); sub( [edx], ebx ); mov( ebx, count ); endif; Once we get past the code above, there’s little left to do except actually transfer the data. Here’s the code that does this: mov( [edx], ebx ); // Get the current file position. copy_to_user function returns the number of bytes left to transfer, scullc_read’s responsibility is to return the number of bytes it actually transferred. The scullc_read procedure must also update the file position (pointed at by the f_pos parameter) so that it points at the next byte to read from the device.

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