| Message ID | 20170622171355.267192-8-arnd@arndb.de |
|---|---|
| State | Accepted |
| Commit | 979990c6284814617d8f2179d197f72ff62b5d85 |
| Headers | show |
| Series | None | expand |
On Mon, Jun 26, 2017 at 3:58 PM, Arnd Bergmann <arnd@arndb.de> 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<<kDefaultShadowScale)). With asan-stack=0, they do not > 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
diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c index 4e7a4e9dcf4d..2da05fa37aec 100644 --- a/drivers/tty/tty_buffer.c +++ b/drivers/tty/tty_buffer.c @@ -362,6 +362,30 @@ int tty_insert_flip_string_flags(struct tty_port *port, EXPORT_SYMBOL(tty_insert_flip_string_flags); /** + * __tty_insert_flip_char - Add one character to the tty buffer + * @port: tty port + * @ch: character + * @flag: flag byte + * + * Queue a single byte to the tty buffering, with an optional flag. + * This is the slow path of tty_insert_flip_char. + */ +int __tty_insert_flip_char(struct tty_port *port, unsigned char ch, char flag) +{ + struct tty_buffer *tb = port->buf.tail; + int flags = (flag == TTY_NORMAL) ? TTYB_NORMAL : 0; + + if (!tty_buffer_request_room(port, 1)) + return 0; + + *flag_buf_ptr(tb, tb->used) = flag; + *char_buf_ptr(tb, tb->used++) = ch; + + return 1; +} +EXPORT_SYMBOL(__tty_insert_flip_char); + +/** * tty_schedule_flip - push characters to ldisc * @port: tty port to push from * diff --git a/include/linux/tty_flip.h b/include/linux/tty_flip.h index c28dd523f96e..d43837f2ce3a 100644 --- a/include/linux/tty_flip.h +++ b/include/linux/tty_flip.h @@ -12,6 +12,7 @@ extern int tty_prepare_flip_string(struct tty_port *port, unsigned char **chars, size_t size); extern void tty_flip_buffer_push(struct tty_port *port); void tty_schedule_flip(struct tty_port *port); +int __tty_insert_flip_char(struct tty_port *port, unsigned char ch, char flag); static inline int tty_insert_flip_char(struct tty_port *port, unsigned char ch, char flag) @@ -26,7 +27,7 @@ static inline int tty_insert_flip_char(struct tty_port *port, *char_buf_ptr(tb, tb->used++) = ch; return 1; } - return tty_insert_flip_string_flags(port, &ch, &flag, 1); + return __tty_insert_flip_char(port, ch, flag); } static inline int tty_insert_flip_string(struct tty_port *port,
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: stable@vger.kernel.org Fixes: c420f167db8c ("kasan: enable stack instrumentation") Signed-off-by: Arnd Bergmann <arnd@arndb.de> --- 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. drivers/tty/tty_buffer.c | 24 ++++++++++++++++++++++++ include/linux/tty_flip.h | 3 ++- 2 files changed, 26 insertions(+), 1 deletion(-) -- 2.9.0