[uml-devel] a question about sigsetjmp() in copy_from/to_user()

[Young Koh <young.koh@gmail.com>]

Hi,

i have a question about copy_from/to_user() implementation in skas mode.

as my understanding,
when copy_from/to_user() is  invoked, before the address translation
happens, UML kernel calls sigsetjmp() to come back when there is a
segmentation fault. and if there is, it seems that the system call an
application triggered eventually returns EFAULT. then, it seems to me
that sigsetjmp() is to catch the error when the application gave the
invalid user space address.

my question is, if so, shouldn't the error be caught when UML kernel
translates the user space address to the kernel space address? i mean,
UML kernel must know the valid memory regions and if the address is
out of the valid regions, then it knows the address is invalid before
UML tries to access the address. why should it use sigsetjmp() and let
a segfault occur?

Thank you in advance,

-Young


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