On Tue, Oct 20, 2009 at 4:38 PM, Rajat Jain <Rajat.Jain@xxxxxxxxxxxx> wrote: > > Hi, > >> Thank you for your reply. >> it's interesting, my modified kernel image is run ok on my >> hardware(arm926ejs). i test mounting ramfs and nfs, they are all ok. >> are they occasional? >> >> sorry, i don't comprehend your explanation about it >> In my opinion, if it's possible that the content of >> parameters isn't in memory at the time of the call, the >> "sys_mount" can't get them also. >> >> could u explain it in detail? Thanks > > OK. So here is it. Not all memory used by user space actually needs to > be in RAM all the time. It may be swapped out to disk since the actual > memory in use in a system is much more than its RAM size. When a piece > of memory that is currently swapped out on disk needs to be accessed, it > needs to be brought back into RAM memory - this is done by the page > fault handler. But consider that the Disk IO is a very slow process, and > thus it is relatively a very huge time for the kernel. For this reason, > any memory that is accessed by the kernel needs to be locked down in RAM > so that it cannot be swapped out. > > Secondly, the 4GB virtual address space is split up into user space and > kernel space code (3G/1G split generally). User space cannot access > kernel space virtual addresses and vice versa. Thus the user space > pointer cannot be dereferenced in the kernel. > > Thus, any user data that needs to be accessed firstly needs to be copied > into kernel address space. This done generally by copy_from_user() > function or its varians that sys_mount() uses: > > exact_copy_from_user((void *)page, data, size); > strncpy_from_user(page, filename, len); > > Now, how you code works comes as a surprize to me though... > > Thanks, > > Rajat > > -- > To unsubscribe from this list: send an email with > "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx > Please read the FAQ at http://kernelnewbies.org/FAQ > > Probably this code works because as per linux code, __copy_from_user is defined as memcpy for arm architecture. Correct me if I am wrong here, http://lxr.linux.no/#linux+v2.6.31/arch/arm/include/asm/uaccess.h#L393 -Vinit -- To unsubscribe from this list: send an email with "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx Please read the FAQ at http://kernelnewbies.org/FAQ
