Re: [PATCH v3 20/23] fs: Allow copy_mount_options() to access user-space in a single pass

Catalin Marinas
2020-04-29T13:52:38Z

On Wed, Apr 29, 2020 at 11:26:51AM +0100, Dave P Martin wrote:
> On Tue, Apr 21, 2020 at 03:26:00PM +0100, Catalin Marinas wrote:
> > diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
> > index 32fc8061aa76..566da441eba2 100644
> > --- a/arch/arm64/include/asm/uaccess.h
> > +++ b/arch/arm64/include/asm/uaccess.h
> > @@ -416,6 +416,17 @@ extern unsigned long __must_check __arch_copy_in_user(void __user *to, const voi
> > #define INLINE_COPY_TO_USER
> > #define INLINE_COPY_FROM_USER
> >
> > +static inline bool arch_has_exact_copy_from_user(unsigned long n)
> > +{
> > + /*
> > + * copy_from_user() aligns the source pointer if the size is greater
> > + * than 15. Since all the loads are naturally aligned, they can only
> > + * fail on the first byte.
> > + */
> > + return n > 15;
> > +}
> > +#define arch_has_exact_copy_from_user
>
> Did you mean:
>
> #define arch_has_exact_copy_from_user arch_has_exact_copy_from_user

Yes (and I shouldn't write patches late in the day).

> Mind you, if this expands to 1 I'd have expected copy_mount_options()
> not to compile, so I may be missing something.

I think arch_has_exact_copy_from_user() (with the braces) is looked up
in the function namespace, so the macro isn't expanded. So arguably the
patch is correct but pretty dodgy ;).

I scrapped this in my second attempt in reply to Kevin.

> > diff --git a/fs/namespace.c b/fs/namespace.c
> > index a28e4db075ed..8febc50dfc5d 100644
> > --- a/fs/namespace.c
> > +++ b/fs/namespace.c
> > @@ -3025,13 +3025,16 @@ void *copy_mount_options(const void __user * data)
>
> [ Is this applying a band-aid to duct tape?
>
> The fs presumably knows ahead of time whether it's expecting a string or
> a fixed-size blob for data, so I'd hope we could just DTRT rather than
> playing SEGV roulette here.
>
> This might require more refactoring than makes sense for this series
> though. ]

That's possible but it means moving the copy from sys_mount() to the
specific places where it has additional information (the filesystems).
I'm not even sure it's guaranteed to be strings. If it is, we could just
replace all this with a strncpy_from_user().

--
Catalin