[U-Boot] [RFC] ARM: prevent misaligned array inits
Albert ARIBAUD
albert.u.boot at aribaud.net
Tue Oct 2 20:46:04 CEST 2012
Under option -munaligned-access, gcc can perform local char
or 16-bit array initializations using misaligned native
accesses which will throw a data abort exception. Fix files
where these array initializations were unneeded, and for
files known to contain such initializations, enforce gcc
option -mno-unaligned-access.
Signed-off-by: Albert ARIBAUD <albert.u.boot at aribaud.net>
---
Please test this patch with gcc 4.7 on boards which do data aborts
or resets due to misaligned accesses and report result to me.
arch/arm/cpu/arm926ejs/orion5x/cpu.c | 4 +-
board/ti/omap2420h4/sys_info.c | 24 ++++-----
common/Makefile | 3 ++
common/cmd_dfu.c | 2 +-
doc/README.arm-unaligned-accesses | 95 ++++++++++++++++++++++++++++++++++
fs/fat/Makefile | 2 +
fs/ubifs/Makefile | 3 ++
lib/Makefile | 3 ++
8 files changed, 121 insertions(+), 15 deletions(-)
create mode 100644 doc/README.arm-unaligned-accesses
diff --git a/arch/arm/cpu/arm926ejs/orion5x/cpu.c b/arch/arm/cpu/arm926ejs/orion5x/cpu.c
index c3948d3..5a4775a 100644
--- a/arch/arm/cpu/arm926ejs/orion5x/cpu.c
+++ b/arch/arm/cpu/arm926ejs/orion5x/cpu.c
@@ -194,8 +194,8 @@ u32 orion5x_device_rev(void)
*/
int print_cpuinfo(void)
{
- char dev_str[] = "0x0000";
- char rev_str[] = "0x00";
+ char dev_str[7]; /* room enough for 0x0000 plus null byte */
+ char rev_str[5]; /* room enough for 0x00 plus null byte */
char *dev_name = NULL;
char *rev_name = NULL;
diff --git a/board/ti/omap2420h4/sys_info.c b/board/ti/omap2420h4/sys_info.c
index a9f7241..b462aa5 100644
--- a/board/ti/omap2420h4/sys_info.c
+++ b/board/ti/omap2420h4/sys_info.c
@@ -237,18 +237,18 @@ u32 wait_on_value(u32 read_bit_mask, u32 match_value, u32 read_addr, u32 bound)
*********************************************************************/
void display_board_info(u32 btype)
{
- char cpu_2420[] = "2420"; /* cpu type */
- char cpu_2422[] = "2422";
- char cpu_2423[] = "2423";
- char db_men[] = "Menelaus"; /* board type */
- char db_ip[] = "IP";
- char mem_sdr[] = "mSDR"; /* memory type */
- char mem_ddr[] = "mDDR";
- char t_tst[] = "TST"; /* security level */
- char t_emu[] = "EMU";
- char t_hs[] = "HS";
- char t_gp[] = "GP";
- char unk[] = "?";
+ char *cpu_2420 = "2420"; /* cpu type */
+ char *cpu_2422 = "2422";
+ char *cpu_2423 = "2423";
+ char *db_men = "Menelaus"; /* board type */
+ char *db_ip = "IP";
+ char *mem_sdr = "mSDR"; /* memory type */
+ char *mem_ddr = "mDDR";
+ char *t_tst = "TST"; /* security level */
+ char *t_emu = "EMU";
+ char *t_hs = "HS";
+ char *t_gp = "GP";
+ char *unk = "?";
char *cpu_s, *db_s, *mem_s, *sec_s;
u32 cpu, rev, sec;
diff --git a/common/Makefile b/common/Makefile
index 125b2be..19b2130 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -227,6 +227,9 @@ $(obj)env_embedded.o: $(src)env_embedded.c $(obj)../tools/envcrc
$(obj)../tools/envcrc:
$(MAKE) -C ../tools
+$(obj)hush.o: CFLAGS += -mno-unaligned-access
+$(obj)fdt_support.o: CFLAGS += -mno-unaligned-access
+
#########################################################################
# defines $(obj).depend target
diff --git a/common/cmd_dfu.c b/common/cmd_dfu.c
index 62fb890..01d6b3a 100644
--- a/common/cmd_dfu.c
+++ b/common/cmd_dfu.c
@@ -30,7 +30,7 @@
static int do_dfu(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
{
const char *str_env;
- char s[] = "dfu";
+ char *s = "dfu";
char *env_bkp;
int ret;
diff --git a/doc/README.arm-unaligned-accesses b/doc/README.arm-unaligned-accesses
new file mode 100644
index 0000000..00fb1c0
--- /dev/null
+++ b/doc/README.arm-unaligned-accesses
@@ -0,0 +1,95 @@
+Since U-Boot runs on a variety of hardware, some only able to perform
+unaligned accesses with a strong penalty, some unable to perform them
+at all, the policy regarding unaligned accesses is to not perform any,
+unless absolutely necessary because of hardware or standards.
+
+Also, on hardware which permits it, the core is configured to throw
+data abort exceptions on unaligned accesses in order to catch these
+unallowed accesses as early as possible.
+
+Until version 4.7, the gcc default for performing unaligned accesses
+(-mno-unaligned-access) is to emulate unaligned accesses using aligned
+loads and stores plus shifts and masks. Emulated unaligned accesses
+will not be caught by hardware. These accesses may be costly and may
+be actually unnecessary. In order to catch these accesses and remove
+or optimize them, option -munaligned-access is explicitly set for all
+versions of gcc which support it.
+
+From gcc 4.7 onward, the default for performing unaligned accesses
+is to use unaligned native loads and stores (-munaligned-access),
+because the cost of unaligned accesses has dropped. This should not
+affect U-Boot's policy of controlling unaligned accesses, however the
+compiler may generate uncontrolled unaligned on its own in at least
+one known case: when declaring a local initialized char array, e.g.
+
+function foo()
+{
+ char buffer[] = "initial value";
+/* or */
+ char buffer[] = { 'i', 'n', 'i', 't', 0 };
+ ...
+}
+
+Under -munaligned-accesses with optimizations on, this declaration
+causes the compiler to generate native loads from the literal string
+and native stores to the buffer, and the literal string alignment
+cannot be controlled. If it is misaligned, then the core will throw
+a data abort exception.
+
+Quite probably the same might happen for 16-bit array initializations
+where the constant is aligned on a boundary which is a multiple of 2
+but not of 4:
+
+function foo()
+{
+ u16 buffer[] = { 1, 2, 3 };
+ ...
+}
+
+The long term solution to this issue is to add an option to gcc to
+allow controlling the general alignment of data, including constant
+initialization values.
+
+However this will only apply to the version of gcc which will have such
+an option. For other versions, there are four workarounds:
+
+a) Enforce as a rule that array initializations as described above
+ are forbidden. This is generally not acceptable as they are valid,
+ and usual, C constructs. The only case where they could be rejected
+ is when they actually equate to a const char* declaration, i.e. the
+ array is initialized and never modified in the function's scope.
+
+b) Drop the requirement on unaligned accesses at least for ARMv7,
+ i.e. do not throw a data abort exception upon unaligned accesses.
+ But that will allow adding badly aligned code to U-Boot, only for
+ it to fail when re-used with another, more strict, target, possibly
+ once the bad code is already in mainline.
+
+c) Relax the -munified-access rule globally. This will prevent native
+ unaligned accesses of course, but that will also hide any bug caused
+ by a bad unaligned access, making it much harder to diagnose it. It
+ is actually what already happens when building ARM targets with a
+ pre-4.7 gcc, and it may actually already hide some bugs yet unseen
+ until the target gets compiled with m-unaligned-access.
+
+d) Relax the -munified-access rule only for for files susceptible to
+ the local initialized array issue. This minimizes the quantity of
+ code which can hide unwanted misaligned accesses.
+
+Considering the rarity of actual occurrences (as of this writing, 5
+files out of 7840 in U-Boot, or .3%, contain an initialized local char
+array which cannot actually be replaced with a const char*), detection
+if the issue in patches should not be asked from contributors.
+
+Luckily, detecting files susceptible to the issue can be automated
+through a filter/regexp/script which would recognize local char array
+initializations. Automated should err on the false positive side, for
+instance flagging non-local arrays as if they were local if they cannot
+be told apart.
+
+In any case, detection shall be informative only and shall not prevent
+applying the patch.
+
+Upon a positive detection, either option -mno-unaligned-access is
+applied (if not already) to the affected file(s), or if the array is a
+hidden const char*, it should be replaced by one.
diff --git a/fs/fat/Makefile b/fs/fat/Makefile
index 9635d36..5c4a2aa 100644
--- a/fs/fat/Makefile
+++ b/fs/fat/Makefile
@@ -39,6 +39,8 @@ all: $(LIB) $(AOBJS)
$(LIB): $(obj).depend $(OBJS)
$(call cmd_link_o_target, $(OBJS))
+# SEE README.arm-unaligned-accesses
+$(obj)file.o: CFLAGS += -mno-unaligned-access
#########################################################################
diff --git a/fs/ubifs/Makefile b/fs/ubifs/Makefile
index ccffe85..71c40f2 100644
--- a/fs/ubifs/Makefile
+++ b/fs/ubifs/Makefile
@@ -42,6 +42,9 @@ all: $(LIB) $(AOBJS)
$(LIB): $(obj).depend $(OBJS)
$(call cmd_link_o_target, $(OBJS))
+# SEE README.arm-unaligned-accesses
+$(obj)super.o: CFLAGS += -mno-unaligned-access
+
#########################################################################
# defines $(obj).depend target
diff --git a/lib/Makefile b/lib/Makefile
index 45798de..44393ed 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -78,6 +78,9 @@ OBJS := $(addprefix $(obj),$(COBJS))
$(LIB): $(obj).depend $(OBJS)
$(call cmd_link_o_target, $(OBJS))
+# SEE README.arm-unaligned-accesses
+$(obj)bzlib.o: CFLAGS += -mno-unaligned-access
+
#########################################################################
# defines $(obj).depend target
--
1.7.9.5
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