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SubjectRe: [PATCH v4 4/9] em28xx: fix em28xx_dvb_init for KASAN
On Tue, Sep 26, 2017 at 9:49 AM, Andrey Ryabinin
<> wrote:
> On 09/26/2017 09:47 AM, Arnd Bergmann wrote:
>> On Mon, Sep 25, 2017 at 11:32 PM, Arnd Bergmann <> wrote:

>> + ret = __builtin_strlen(q);
> I think this is not correct. Fortified strlen called here on purpose. If sizeof q is known at compile time
> and 'q' contains not-null fortified strlen() will panic.

Ok, got it.

>> if (size) {
>> size_t len = (ret >= size) ? size - 1 : ret;
>> if (__builtin_constant_p(len) && len >= p_size)
>> The problem is apparently that the fortified strlcpy calls the fortified strlen,
>> which in turn calls strnlen and that ends up calling the extern '__real_strnlen'
>> that gcc cannot reduce to a constant expression for a constant input.
> Per my observation, it's the code like this:
> if ()
> fortify_panic(__func__);
> somehow prevent gcc to merge several "struct i2c_board_info info;" into one stack slot.
> With the hack bellow, stack usage reduced to ~1,6K:

1.6k is also what I see with my patch, or any other approach I tried
that changes
string.h. With the split up em28xx_dvb_init() function (and without
changes to string.h),
I got down to a few hundred bytes for the largest handler.

> ---
> include/linux/string.h | 4 ----
> 1 file changed, 4 deletions(-)
> diff --git a/include/linux/string.h b/include/linux/string.h
> index 54d21783e18d..9a96ff3ebf94 100644
> --- a/include/linux/string.h
> +++ b/include/linux/string.h
> @@ -261,8 +261,6 @@ __FORTIFY_INLINE __kernel_size_t strlen(const char *p)
> if (p_size == (size_t)-1)
> return __builtin_strlen(p);
> ret = strnlen(p, p_size);
> - if (p_size <= ret)
> - fortify_panic(__func__);
> return ret;
> }
> @@ -271,8 +269,6 @@ __FORTIFY_INLINE __kernel_size_t strnlen(const char *p, __kernel_size_t maxlen)
> {
> size_t p_size = __builtin_object_size(p, 0);
> __kernel_size_t ret = __real_strnlen(p, maxlen < p_size ? maxlen : p_size);
> - if (p_size <= ret && maxlen != ret)
> - fortify_panic(__func__);
> return ret;

I've reduced it further to this change:

--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -227,7 +227,7 @@ static inline const char *kbasename(const char *path)
#define __FORTIFY_INLINE extern __always_inline __attribute__((gnu_inline))
#define __RENAME(x) __asm__(#x)

-void fortify_panic(const char *name) __noreturn __cold;
+void fortify_panic(const char *name) __cold;
void __read_overflow(void) __compiletime_error("detected read beyond
size of object passed as 1st parameter");
void __read_overflow2(void) __compiletime_error("detected read beyond
size of object passed as 2nd parameter");
void __read_overflow3(void) __compiletime_error("detected read beyond
size of object passed as 3rd parameter");
I don't immediately see why the __noreturn changes the behavior here, any idea?

>> Not sure if that change is the best fix, but it seems to address the problem in
>> this driver and probably leads to better code in other places as well.
> Probably it would be better to solve this on the strlcpy side, but I haven't found the way to do this right.
> Alternative solutions:
> - use memcpy() instead of strlcpy(). All source strings are smaller than I2C_NAME_SIZE, so we could
> do something like this - memcpy(info.type, "si2168", sizeof("si2168"));
> Also this should be faster.

This would be very similar to the patch I posted at the start of this
thread to use strncpy(), right?
I was hoping that changing strlcpy() here could also improve other
users that might run into
the same situation, but stay below the 2048-byte stack frame limit.

> - Move code under different "case:" in the switch(dev->model) to the separate function should help as well.
> But it might be harder to backport into stables.

Agreed, I posted this in earlier versions of the patch series, see

The new patch was a result of me trying to come up with a less
invasive version to
make it easier to backport, since I would like to backport the last
patch in the series
that depends on all the earlier ones.


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