On Sat, Feb 28, 2015 at 1:14 PM, Benjamin Herrenschmidt
<benh@xxxxxxxxxxxxxxxxxxx> wrote:
>
> BTW. I fail to see how x86 checks PF_INSTR vs. VM_NOEXEC ... or it doesn't ?
It doesn't. x86 traditionally doesn't have an execute bit, so
traditionally "read == exec".
So PF_INSTR really wasn't historically very useful, in that it would
only show if the *first* access to a page was an instruction fetch -
if you did a regular read to brign the page in, then subsequent
instruction fetches would just work.
Then NX came along, and what happens now is
- we handle write faults separately (see the first part of access_error()
- so now we know it was a read or an instruction fetch
- if PF_PROT is set, that means that the present bit was set in the
page tables, so it must have been an exec access to a NX page
- otherwise, we just say "PROTNONE means no access, otherwise
populate the page tables"
.. and if it turns out that it was a PF_INSTR to a NX page, we'll end
up taking the page fault *again* after it's been populated, and now
since the page table was populated, the access_error() will catch it
with the PF_PROT case.
Or something like that. I might have screwed up some detail, but it
should all work.
Linus
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