On Wed, May 13, 2020 at 9:01 AM Christoph Hellwig <hch@xxxxxx> wrote:
>
> + arch_kernel_read(dst, src, type, err_label); \
I'm wondering if
(a) we shouldn't expose this as an interface in general
(b) it wouldn't be named differently..
The reason for (a) is that several users of the
"copy_from_kernel_nofault()" interfaces just seem to want a single
access from kernel mode.
The reason for (b) is that if we do expose this as a normal interface,
it shouldn't be called "arch_kernel_read", and it should have the same
semantics as "get_user_unsafe()".
IOW, maybe we should simply do exactly that: have a
"get_kernel_nofault()" thing that looks exactly like
unsafe_get_user().
On x86, it would basically be identical to unsafe_get_user().
And on architectures that only have the copy function, you'd just have
a fallback something like this:
#define get_kernel_nofault(dst, src, err_label) do { \
typeof (*src) __gkn_result; \
if (probe_kernel_read(&__gkn_result, src) < 0) \
goto err_label; \
(dst) = __gkn_result; \
} while (0)
and now the people who want to read a single kernel word can just do
get_kernel_nofault(n, untrusted_pointer, error);
and they're done.
And some day - when we get reliably "asm goto" wiith outputs - that
"get_kernel_fault()" will literally be a single instruction asm with
the proper exception handler marker, the way "put_user_unsafe()"
already works (and the way "put_kernel_nofault()" would already work
if it does the above).
Linus
