[PATCH 3.18 29/68] mtd: avoid stack overflow in MTD CFI code

Greg Kroah-Hartman
2017-05-05T18:34:29Z

From: Arnd Bergmann <arnd@arndb.de>

commit fddcca5107051adf9e4481d2a79ae0616577fd2c upstream.

When map_word gets too large, we use a lot of kernel stack, and for
MTD_MAP_BANK_WIDTH_32, this means we use more than the recommended
1024 bytes in a number of functions:

drivers/mtd/chips/cfi_cmdset_0020.c: In function 'cfi_staa_write_buffers':
drivers/mtd/chips/cfi_cmdset_0020.c:651:1: warning: the frame size of 1336 bytes is larger than 1024 bytes [-Wframe-larger-than=]
drivers/mtd/chips/cfi_cmdset_0020.c: In function 'cfi_staa_erase_varsize':
drivers/mtd/chips/cfi_cmdset_0020.c:972:1: warning: the frame size of 1208 bytes is larger than 1024 bytes [-Wframe-larger-than=]
drivers/mtd/chips/cfi_cmdset_0001.c: In function 'do_write_buffer':
drivers/mtd/chips/cfi_cmdset_0001.c:1835:1: warning: the frame size of 1240 bytes is larger than 1024 bytes [-Wframe-larger-than=]

This can be avoided if all operations on the map word are done
indirectly and the stack gets reused between the calls. We can
mostly achieve this by selecting MTD_COMPLEX_MAPPINGS whenever
MTD_MAP_BANK_WIDTH_32 is set, but for the case that no other
bank width is enabled, we also need to use a non-constant
map_bankwidth() to convince the compiler to use less stack.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
[Brian: this patch mostly achieves its goal by forcing
MTD_COMPLEX_MAPPINGS (and the accompanying indirection) for 256-bit
mappings; the rest of the change is mostly a wash, though it helps
reduce stack size slightly. If we really care about supporting
256-bit mappings though, we should consider rewriting some of this
code to avoid keeping and assigning so many 256-bit objects on the
stack.]
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/mtd/chips/Kconfig | 1 +
include/linux/mtd/map.h | 19 +++++++------------
2 files changed, 8 insertions(+), 12 deletions(-)

--- a/drivers/mtd/chips/Kconfig
+++ b/drivers/mtd/chips/Kconfig
@@ -110,6 +110,7 @@ config MTD_MAP_BANK_WIDTH_16

config MTD_MAP_BANK_WIDTH_32
bool "Support 256-bit buswidth" if MTD_CFI_GEOMETRY
+ select MTD_COMPLEX_MAPPINGS if HAS_IOMEM
default n
help
If you wish to support CFI devices on a physical bus which is
--- a/include/linux/mtd/map.h
+++ b/include/linux/mtd/map.h
@@ -122,18 +122,13 @@
#endif

#ifdef CONFIG_MTD_MAP_BANK_WIDTH_32
-# ifdef map_bankwidth
-# undef map_bankwidth
-# define map_bankwidth(map) ((map)->bankwidth)
-# undef map_bankwidth_is_large
-# define map_bankwidth_is_large(map) (map_bankwidth(map) > BITS_PER_LONG/8)
-# undef map_words
-# define map_words(map) map_calc_words(map)
-# else
-# define map_bankwidth(map) 32
-# define map_bankwidth_is_large(map) (1)
-# define map_words(map) map_calc_words(map)
-# endif
+/* always use indirect access for 256-bit to preserve kernel stack */
+# undef map_bankwidth
+# define map_bankwidth(map) ((map)->bankwidth)
+# undef map_bankwidth_is_large
+# define map_bankwidth_is_large(map) (map_bankwidth(map) > BITS_PER_LONG/8)
+# undef map_words
+# define map_words(map) map_calc_words(map)
#define map_bankwidth_is_32(map) (map_bankwidth(map) == 32)
#undef MAX_MAP_BANKWIDTH
#define MAX_MAP_BANKWIDTH 32