On Wed, Apr 29, 2020 at 02:52:25PM +0100, Catalin Marinas wrote:

On Wed, Apr 29, 2020 at 11:26:51AM +0100, Dave P Martin wrote:

quoted

On Tue, Apr 21, 2020 at 03:26:00PM +0100, Catalin Marinas wrote:

quoted

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).

quoted

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.

Problem solved!

quoted

quoted

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().

Fair enough.  I'll add it to my wishlist...

Cheers
---Dave

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