From: Arnd Bergmann <arnd@arndb.de>
To: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: "David Laight" <David.Laight@aculab.com>,
"Mauro Carvalho Chehab" <mchehab@kernel.org>,
"Jiri Pirko" <jiri@resnulli.us>,
"Arend van Spriel" <arend.vanspriel@broadcom.com>,
"Kalle Valo" <kvalo@codeaurora.org>,
"David S. Miller" <davem@davemloft.net>,
"Alexander Potapenko" <glider@google.com>,
"Dmitry Vyukov" <dvyukov@google.com>,
"Masahiro Yamada" <yamada.masahiro@socionext.com>,
"Michal Marek" <mmarek@suse.com>,
"Andrew Morton" <akpm@linux-foundation.org>,
"Kees Cook" <keescook@chromium.org>,
"Geert Uytterhoeven" <geert@linux-m68k.org>,
"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
"linux-media@vger.kernel.org" <linux-media@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
"linux-wireless@vger.kernel.org" <linux-wireless@vger.kernel.org>,
"brcm80211-dev-list.pdl@broadcom.com"
<brcm80211-dev-list.pdl@broadcom.com>,
"brcm80211-dev-list@cypress.com" <brcm80211-dev-list@cypress.com>,
"kasan-dev@googlegroups.com" <kasan-dev@googlegroups.com>,
"linux-kbuild@vger.kernel.org" <linux-kbuild@vger.kernel.org>,
"Jakub Jelinek" <jakub@gcc.gnu.org>,
"Martin Liška" <marxin@gcc.gnu.org>,
"stable@vger.kernel.org" <stable@vger.kernel.org>
Subject: Re: [PATCH v4 4/9] em28xx: fix em28xx_dvb_init for KASAN
Date: Wed, 27 Sep 2017 06:26:21 -0700 [thread overview]
Message-ID: <CAK8P3a2C7DBTfQZvRi-QQfrfm1GXktFcXQRmXmzpF4SCa+BADA@mail.gmail.com> (raw)
In-Reply-To: <e7e6418e-4340-5057-aa17-800082aca5fb@virtuozzo.com>
On Tue, Sep 26, 2017 at 9:49 AM, Andrey Ryabinin
<aryabinin@virtuozzo.com> wrote:
>
>
> On 09/26/2017 09:47 AM, Arnd Bergmann wrote:
>> On Mon, Sep 25, 2017 at 11:32 PM, Arnd Bergmann <arnd@arndb.de> 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
https://patchwork.kernel.org/patch/9601025/
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.
Arnd
next prev parent reply other threads:[~2017-09-27 13:26 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-09-22 21:29 [PATCH v4 0/9] bring back stack frame warning with KASAN Arnd Bergmann
2017-09-22 21:29 ` [PATCH v4 1/9] brcmsmac: make some local variables 'static const' to reduce stack size Arnd Bergmann
2017-09-25 4:33 ` Kalle Valo
2017-10-02 13:53 ` [v4, " Kalle Valo
2017-09-22 21:29 ` [PATCH v4 2/9] brcmsmac: split up wlc_phy_workarounds_nphy Arnd Bergmann
2017-10-02 13:55 ` [v4,2/9] " Kalle Valo
2017-10-27 7:51 ` Kalle Valo
2017-09-22 21:29 ` [PATCH v4 3/9] brcmsmac: reindent split functions Arnd Bergmann
2017-09-22 21:29 ` [PATCH v4 4/9] em28xx: fix em28xx_dvb_init for KASAN Arnd Bergmann
2017-09-25 14:41 ` David Laight
2017-09-26 6:32 ` Arnd Bergmann
2017-09-26 6:47 ` Arnd Bergmann
2017-09-26 16:49 ` Andrey Ryabinin
2017-09-27 13:26 ` Arnd Bergmann [this message]
2017-09-28 13:09 ` Andrey Ryabinin
2017-09-28 14:30 ` Arnd Bergmann
2017-10-02 8:33 ` Arnd Bergmann
2017-10-02 8:40 ` [PATCH] string.h: work around for increased stack usage Arnd Bergmann
2017-10-02 9:02 ` Arnd Bergmann
2017-10-02 14:07 ` Andrey Ryabinin
2017-10-03 18:10 ` kbuild test robot
2017-09-22 21:29 ` [PATCH v4 5/9] r820t: fix r820t_write_reg for KASAN Arnd Bergmann
2017-09-22 21:29 ` [PATCH v4 6/9] dvb-frontends: fix i2c access helpers " Arnd Bergmann
2017-09-22 21:29 ` [PATCH v4 7/9] rocker: fix rocker_tlv_put_* functions " Arnd Bergmann
2017-09-26 3:19 ` David Miller
2017-09-22 21:29 ` [PATCH v4 8/9] netlink: fix nla_put_{u8,u16,u32} " Arnd Bergmann
2017-09-26 3:19 ` David Miller
2017-09-22 21:29 ` [PATCH v4 9/9] kasan: rework Kconfig settings Arnd Bergmann
2017-09-26 19:36 ` Andrey Ryabinin
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