On Mo, 2014-09-29 at 12:41 -0700, Kees Cook wrote:

On Mon, Sep 29, 2014 at 4:04 AM, David Laight [off-list ref] wrote:

quoted

From: Kees Cook

quoted

This makes the size argument a const, since it is always populated by
the caller.

There is almost no point making parameters 'const.
('const foo *' makes sense).

quoted

Additionally double-checks to make sure the copy_from_user
can never overflow, keeping CONFIG_DEBUG_STRICT_USER_COPY_CHECKS happy:

   In function 'copy_from_user',
       inlined from '__tun_chr_ioctl' at drivers/net/tun.c:1871:7:
       ... copy_from_user() buffer size is not provably correct

If 'ifreq_len' could be too big then you want to error the ioctl, not panic.
If it can't be too big you don't need the check.

The ifreq_len comes from the callers, and is the output of "sizeof"
which is const. Changing the function parameter to "const" means any
changes made in the future where the incoming value isn't const, the
compiler will throw a warning.

Hmmm, I think you want something like BUILD_BUG_ON(!
__builtin_constant_p(var)). const in function argument only ensures that
the value cannot be modified in the function.

Bye,
Hannes