Patch "mtd: avoid stack overflow in MTD CFI code" has been added to the 3.18-stable tree

[<gregkh@linuxfoundation.org>]

This is a note to let you know that I've just added the patch titled

    mtd: avoid stack overflow in MTD CFI code

to the 3.18-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     mtd-avoid-stack-overflow-in-mtd-cfi-code.patch
and it can be found in the queue-3.18 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From fddcca5107051adf9e4481d2a79ae0616577fd2c Mon Sep 17 00:00:00 2001
From: Arnd Bergmann <arnd@arndb.de>
Date: Mon, 29 Feb 2016 13:20:28 +0100
Subject: mtd: avoid stack overflow in MTD CFI code

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


Patches currently in stable-queue which might be from arnd@arndb.de are

queue-3.18/ib-iser-fix-sparse-warnings.patch
queue-3.18/cred-userns-define-current_user_ns-as-a-function.patch
queue-3.18/pci-xilinx-fix-harmless-format-string-warning.patch
queue-3.18/alsa-ppc-awacs-shut-up-maybe-uninitialized-warning.patch
queue-3.18/net-tg3-avoid-uninitialized-variable-warning.patch
queue-3.18/mtd-avoid-stack-overflow-in-mtd-cfi-code.patch
queue-3.18/powerpc-ptrace-fix-out-of-bounds-array-access-warning.patch
queue-3.18/mlx5-avoid-build-warnings-on-32-bit.patch
queue-3.18/drbd-avoid-redefinition-of-bits_per_page.patch