toolchain: granular choice for stack protector

Currently, we only support two levels of stach-smashing protection:
  - entirely disabled,
  - protect _all_ functions with -fstack-protector-all.

-fstack-protector-all tends to be far too aggressive and impacts
performance too much to be worth on a real product.

Add a choice that allows us to select between different levels of
stack-smashing protection:
  - none
  - basic   (NEW)
  - strong  (NEW)
  - all

The differences are documented in the GCC online documentation:
    https://gcc.gnu.org/onlinedocs/gcc-4.9.2/gcc/Optimize-Options.html

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
[yann.morin.1998@free.fr:
  - rebase
  - add legacy handling
  - SSP-strong depends on gcc >= 4.9
  - slightly simple ifeq-block in package/Makefile.in
  - keep the comment in the choice; add a comment shen strong is not
    available
  - drop the defaults (only keep the legacy)
  - update commit log
]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
[Thomas:
 - only show the choice if the toolchain has SSP support
 - add details for the BR2_SSP_ALL option that it has a significant
   performance impact.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Config.in

@@ -522,12 +522,13 @@ config BR2_GOOGLE_BREAKPAD_INCLUDE_FILES
 
 endif
 
-config BR2_ENABLE_SSP
+choice
 	bool "build code with Stack Smashing Protection"
+	default BR2_SSP_ALL if BR2_ENABLE_SSP # legacy
 	depends on BR2_TOOLCHAIN_HAS_SSP
 	help
-	  Enable stack smashing protection support using GCCs
-	  -fstack-protector-all option.
+	  Enable stack smashing protection support using GCC's
+	  -fstack-protector option family.
 
 	  See http://www.linuxfromscratch.org/hints/downloads/files/ssp.txt
 	  for details.
@@ -536,7 +537,43 @@ config BR2_ENABLE_SSP
 	  support. This is always the case for glibc and eglibc
 	  toolchain, but is optional in uClibc toolchains.
 
-comment "enabling Stack Smashing Protection requires support in the toolchain"
+config BR2_SSP_NONE
+	bool "None"
+	help
+	  Disable stack-smashing protection.
+
+config BR2_SSP_REGULAR
+	bool "-fstack-protector"
+	help
+	  Emit extra code to check for buffer overflows, such as stack
+	  smashing attacks. This is done by adding a guard variable to
+	  functions with vulnerable objects. This includes functions
+	  that call alloca, and functions with buffers larger than 8
+	  bytes. The guards are initialized when a function is entered
+	  and then checked when the function exits. If a guard check
+	  fails, an error message is printed and the program exits.
+
+config BR2_SSP_STRONG
+	bool "-fstack-protector-strong"
+	depends on BR2_TOOLCHAIN_GCC_AT_LEAST_4_9
+	help
+	  Like -fstack-protector but includes additional functions to be
+	  protected - those that have local array definitions, or have
+	  references to local frame addresses.
+
+comment "Stack Smashing Protection strong needs a toolchain w/ gcc >= 4.9"
+	depends on !BR2_TOOLCHAIN_GCC_AT_LEAST_4_9
+
+config BR2_SSP_ALL
+	bool "-fstack-protector-all"
+	help
+	  Like -fstack-protector except that all functions are
+	  protected. This option might have a significant performance
+	  impact on the compiled binaries.
+
+endchoice
+
+comment "Stack Smashing Protection needs a toolchain w/ SSP"
 	depends on !BR2_TOOLCHAIN_HAS_SSP
 
 choice

Config.in.legacy

@@ -145,6 +145,14 @@ endif
 ###############################################################################
 comment "Legacy options removed in 2016.02"
 
+# BR2_ENABLE_SSP is still referenced in Config.in (default in choice)
+config BR2_ENABLE_SSP
+	bool "Stack Smashing protection now has different levels"
+	help
+	  The protection offered by SSP can now be selected from different
+	  protection levels. Be sure to review the SSP level in the build
+	  options menu.
+
 config BR2_PACKAGE_DIRECTFB_CLE266
 	bool "cle266 driver for directfb removed"
 	select BR2_LEGACY

package/Makefile.in

@@ -159,7 +159,13 @@ TARGET_CFLAGS += -msep-data
 TARGET_CXXFLAGS += -msep-data
 endif
 
-ifeq ($(BR2_ENABLE_SSP),y)
+ifeq ($(BR2_SSP_REGULAR),y)
+TARGET_CFLAGS += -fstack-protector
+TARGET_CXXFLAGS += -fstack-protector
+else ifeq ($(BR2_SSP_STRONG),y)
+TARGET_CFLAGS += -fstack-protector-strong
+TARGET_CXXFLAGS += -fstack-protector-strong
+else ifeq ($(BR2_SSP_ALL),y)
 TARGET_CFLAGS += -fstack-protector-all
 TARGET_CXXFLAGS += -fstack-protector-all
 endif