Re: [PATCH v3 07/11] tty: improve tty_insert_flip_char() fast path
On Mon, Jun 26, 2017 at 3:58 PM, Arnd Bergmann wrote: > * With asan-stack=1, gcc uses at least 64 bytes per such variable > (two times ASAN_RED_ZONE_SIZE), while clang only uses 16 bytes > (2 * (1< use any more space than with kasan completely disabled > (no -fsanitize=kernel-address). I asked around the Linaro toolchain team today, and arrived at this commit in llvm: https://github.com/llvm-mirror/llvm/commit/daa1bf3b74054 Prior to this, the llvm behavior was the same as gcc, using 64 bytes for each small (<= 16 byte) variable instead of just 16 or 32 as it does now. llvm now also uses a larger redzone (up to 256 bytes) for very large stack objects, which also seems like a good idea. While it would be hard to argue that the gcc behavior is a bug, it should be possible to implement the same optimization in gcc, and that would solve a lot of the stack size issues with KASAN. > Can you say which behavior you find 'sane' or 'not sane' here, > specifically? Maybe we can make future gcc releases use a > smaller redzone like clang does. > > If we find a way to improve gcc so it uses less stack here, we still > have a problem with existing compilers still producing dangerously > high stack usage, as well as annoying warnings for an allmodconfig > build as soon as we start warning about this again. This problem obviously still stands. Arnd
Re: [PATCH v3 07/11] tty: improve tty_insert_flip_char() fast path
On Fri, Jun 23, 2017 at 6:07 PM, Greg Kroah-Hartman
wrote:
> On Thu, Jun 22, 2017 at 07:13:51PM +0200, Arnd Bergmann wrote:
>> kernelci.org reports a crazy stack usage for the VT code when CONFIG_KASAN
>> is enabled:
>>
>> drivers/tty/vt/keyboard.c: In function 'kbd_keycode':
>> drivers/tty/vt/keyboard.c:1452:1: error: the frame size of 2240 bytes is
>> larger than 2048 bytes [-Werror=frame-larger-than=]
>>
>> The problem is that tty_insert_flip_char() gets inlined many times into
>> kbd_keycode(), and also into other functions, and each copy requires 128
>> bytes for stack redzone to check for a possible out-of-bounds access on
>> the 'ch' and 'flags' arguments that are passed into
>> tty_insert_flip_string_flags as a variable-length string.
>>
>> This introduces a new __tty_insert_flip_char() function for the slow
>> path, which receives the two arguments by value. This completely avoids
>> the problem and the stack usage goes back down to around 100 bytes.
>>
>> Without KASAN, this is also slightly better, as we don't have to
>> spill the arguments to the stack but can simply pass 'ch' and 'flag'
>> in registers, saving a few bytes in .text for each call site.
>>
>> This should be backported to linux-4.0 or later, which first introduced
>> the stack sanitizer in the kernel.
>>
>> Cc: sta...@vger.kernel.org
>> Fixes: c420f167db8c ("kasan: enable stack instrumentation")
>> Signed-off-by: Arnd Bergmann
>> ---
>> I already submitted this separately to Greg, but he hasn't replied
>> yet. I assume that it's fine if Andrew picks it up along with the
>> other patches and drops it again in case Greg applies it to linux-next.
>
> I've been traveling in China this week, give me a chance to catch up
> please.
Sorry about the rush, I thought the new version was going to be
uncontroversial.
Having sent a broken patch (unused variable unless tty patch 2/2 is
applied, but that wasn't part of this series) certainly didn't make me
look any better :(
> And no, I don't like this patch either, I think kasan needs to be fixed
> here, not work around it in odd ways in code that is completly
> acceptable to "sane" compilers. But give me a week to catch up on my
> pending stuff first...
I have done some more research, and in particular found out more about
what the compiler does, and why it shows up with some compilers
but not others for this particular file:
* when CONFIG_OPTIMIZE_INLINING is set, gcc-5 and higher decide
to inline put_queue() in keyboard.c, regardless of architecture. gcc-4.9
does not do this, and without CONFIG_OPTIMIZE_INLINING,
put_queue() remains out of line for all versions of gcc. clang-3.9 always
inlines put_queue(), regardless of CONFIG_OPTIMIZE_INLINING.
* with -fsanitize=kernel-address enabled (regardless of asan-stack), both
clang and gcc give each local variable in an inline function a separate
stack address when it gets passed by reference. Clang normally tries
to overlap the addresses (without kasan), gcc apparently does not.
* With asan-stack=1, gcc uses at least 64 bytes per such variable
(two times ASAN_RED_ZONE_SIZE), while clang only uses 16 bytes
(2 * (1<
Re: [PATCH v3 07/11] tty: improve tty_insert_flip_char() fast path
Hi Arnd,
[auto build test WARNING on linuxtv-media/master]
[also build test WARNING on v4.12-rc6 next-20170623]
[if your patch is applied to the wrong git tree, please drop us a note to help
improve the system]
url:
https://github.com/0day-ci/linux/commits/Arnd-Bergmann/bring-back-stack-frame-warning-with-KASAN/20170625-071646
base: git://linuxtv.org/media_tree.git master
config: x86_64-randconfig-b0-06250903 (attached as .config)
compiler: gcc-4.4 (Debian 4.4.7-8) 4.4.7
reproduce:
# save the attached .config to linux build tree
make ARCH=x86_64
All warnings (new ones prefixed by >>):
drivers/tty/tty_buffer.c: In function '__tty_insert_flip_char':
>> drivers/tty/tty_buffer.c:376: warning: unused variable 'flags'
vim +/flags +376 drivers/tty/tty_buffer.c
360 return copied;
361 }
362 EXPORT_SYMBOL(tty_insert_flip_string_flags);
363
364 /**
365 * __tty_insert_flip_char - Add one character to the tty
buffer
366 * @port: tty port
367 * @ch: character
368 * @flag: flag byte
369 *
370 * Queue a single byte to the tty buffering, with an optional flag.
371 * This is the slow path of tty_insert_flip_char.
372 */
373 int __tty_insert_flip_char(struct tty_port *port, unsigned char ch,
char flag)
374 {
375 struct tty_buffer *tb = port->buf.tail;
> 376 int flags = (flag == TTY_NORMAL) ? TTYB_NORMAL : 0;
377
378 if (!tty_buffer_request_room(port, 1))
379 return 0;
380
381 *flag_buf_ptr(tb, tb->used) = flag;
382 *char_buf_ptr(tb, tb->used++) = ch;
383
384 return 1;
---
0-DAY kernel test infrastructureOpen Source Technology Center
https://lists.01.org/pipermail/kbuild-all Intel Corporation
.config.gz
Description: application/gzip
Re: [PATCH v3 07/11] tty: improve tty_insert_flip_char() fast path
On Thu, Jun 22, 2017 at 07:13:51PM +0200, Arnd Bergmann wrote:
> kernelci.org reports a crazy stack usage for the VT code when CONFIG_KASAN
> is enabled:
>
> drivers/tty/vt/keyboard.c: In function 'kbd_keycode':
> drivers/tty/vt/keyboard.c:1452:1: error: the frame size of 2240 bytes is
> larger than 2048 bytes [-Werror=frame-larger-than=]
>
> The problem is that tty_insert_flip_char() gets inlined many times into
> kbd_keycode(), and also into other functions, and each copy requires 128
> bytes for stack redzone to check for a possible out-of-bounds access on
> the 'ch' and 'flags' arguments that are passed into
> tty_insert_flip_string_flags as a variable-length string.
>
> This introduces a new __tty_insert_flip_char() function for the slow
> path, which receives the two arguments by value. This completely avoids
> the problem and the stack usage goes back down to around 100 bytes.
>
> Without KASAN, this is also slightly better, as we don't have to
> spill the arguments to the stack but can simply pass 'ch' and 'flag'
> in registers, saving a few bytes in .text for each call site.
>
> This should be backported to linux-4.0 or later, which first introduced
> the stack sanitizer in the kernel.
>
> Cc: sta...@vger.kernel.org
> Fixes: c420f167db8c ("kasan: enable stack instrumentation")
> Signed-off-by: Arnd Bergmann
> ---
> I already submitted this separately to Greg, but he hasn't replied
> yet. I assume that it's fine if Andrew picks it up along with the
> other patches and drops it again in case Greg applies it to linux-next.
I've been traveling in China this week, give me a chance to catch up
please.
And no, I don't like this patch either, I think kasan needs to be fixed
here, not work around it in odd ways in code that is completly
acceptable to "sane" compilers. But give me a week to catch up on my
pending stuff first...
thanks,
greg k-h
