Re: [PATCH 1/2] x86: unify copy_from_user() size checking

[Guenter Roeck]

On 10/26/2013 03:31 AM, Ingo Molnar wrote:
> * Jan Beulich <JBeulich@xxxxxxxx> wrote:
>
> > Commits 4a3127693001c61a21d1ce680db6340623f52e93 ("x86: Turn the
> > copy_from_user check into an (optional) compile time warning") and
> > 63312b6a6faae3f2e5577f2b001e3b504f10a2aa ("x86: Add a Kconfig option to
> > turn the copy_from_user warnings into errors") touched only the 32-bit
> > variant of copy_from_user(), whereas the original commit
> > 9f0cf4adb6aa0bfccf675c938124e68f7f06349d ("x86: Use
> > __builtin_object_size() to validate the buffer size for
> > copy_from_user()") also added the same code to the 64-bit one.
> >
> > Further the earlier conversion from an inline WARN() to the call to
> > copy_from_user_overflow() went a little too far: When the number of
> > bytes to be copied is not a constant (e.g. [looking at 3.11] in
> > drivers/net/tun.c:__tun_chr_ioctl() or
> > drivers/pci/pcie/aer/aer_inject.c:aer_inject_write()), the compiler
> > will always have to keep the funtion call, and hence there will always
> > be a warning. By using __builtin_constant_p() we can avoid this.
> >
> > And then this slightly extends the effect of
> > CONFIG_DEBUG_STRICT_USER_COPY_CHECKS in that apart from converting
> > warnings to errors in the constant size case, it retains the (possibly
> > wrong) warnings in the non-constant size case, such that if someone is
> > prepared to get a few false positives, (s)he'll be able to recover the
> > current behavior (except that these diagnostics now will never be
> > converted to errors).
> >
> > Since the 32-bit variant (intentionally) didn't call might_fault(), the
> > unification results in this being called twice now. Adding a suitable
> > #ifdef would be the alternative if that's a problem.
>
> Addressing that sanely would be nice.
>
> > I'd like to point out though that with __compiletime_object_size()
> > being restricted to gcc before 4.6, the whole construct is going to
> > become more and more pointless going forward. I would question
> > however that commit 2fb0815c9ee6b9ac50e15dd8360ec76d9fa46a2 ("gcc4:
> > disable __compiletime_object_size for GCC 4.6+") was really necessary,
> > and instead this should have been dealt with as is done here from the
> > beginning.
>
> Can we now revert 2fb0815c9ee6?

I sure don't mind if there is a different and better solution.

Guenter

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