Re: Security question: "Text file busy" overwriting executables but

[Linus Torvalds]

In article <[e-mail]>,
Linus Torvalds  <[e-mail]> wrote:
>
>The explicit flag is probably a good idea also because of usage patterns
>(PAGE_COPY is a slowdown _if_ the file is actually written to or even
>mapped shared).

Actually, I missed the obvious case: quite often when you do a "read()",
the reader itself will end up writing to the area read into.  In which
case doing the PAGE_COPY would also slow down measurably, due to the
extra overhead of the copy-on-write fault (which not just does the copy
that we tried to avoid, but will take a fault and more VM locks). 

So if we want to do this optimization, we _definitely_ want it to be
explicitly controlled by a flag, like O_DIRECT is.  There are just too
many cases where it's a pessimization, and while the user can often tell
before-hand, the kernel simply cannot. 

		Linus