[U-Boot] [PATCH V4] ARM: prevent misaligned array inits

Albert ARIBAUD
2012-10-08T19:50:18Z

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@aribaud.net>
---
V4: added information on how to find relocation offset for pc
common/Makefile missed a comment re README.arm-unaligned-accesses
V3: *really* fix incorrect doc file name in dabort handler message
clarifications and typo fixes in README.arm-unaligned-accesses
V2: fix incorrect doc file name in dabort handler message

arch/arm/cpu/arm926ejs/orion5x/cpu.c | 4 +-
arch/arm/lib/interrupts.c | 2 +-
board/ti/omap2420h4/sys_info.c | 28 ++++----
common/Makefile | 4 ++
common/cmd_dfu.c | 2 +-
doc/README.arm-unaligned-accesses | 121 ++++++++++++++++++++++++++++++++++
fs/fat/Makefile | 2 +
fs/ubifs/Makefile | 3 +
lib/Makefile | 3 +
9 files changed, 151 insertions(+), 18 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/arch/arm/lib/interrupts.c b/arch/arm/lib/interrupts.c
index 74ff5ce..02124a7 100644
--- a/arch/arm/lib/interrupts.c
+++ b/arch/arm/lib/interrupts.c
@@ -169,7 +169,7 @@ void do_prefetch_abort (struct pt_regs *pt_regs)

void do_data_abort (struct pt_regs *pt_regs)
{
- printf ("data abort\n");
+ printf ("data abort\n\n MAYBE you should read doc/README.arm-unaligned-accesses\n\n");
show_regs (pt_regs);
bad_mode ();
}
diff --git a/board/ti/omap2420h4/sys_info.c b/board/ti/omap2420h4/sys_info.c
index a9f7241..b12011e 100644
--- a/board/ti/omap2420h4/sys_info.c
+++ b/board/ti/omap2420h4/sys_info.c
@@ -237,20 +237,20 @@ 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_s, *db_s, *mem_s, *sec_s;
+ static const char cpu_2420 [] = "2420"; /* cpu type */
+ static const char cpu_2422 [] = "2422";
+ static const char cpu_2423 [] = "2423";
+ static const char db_men [] = "Menelaus"; /* board type */
+ static const char db_ip [] = "IP";
+ static const char mem_sdr [] = "mSDR"; /* memory type */
+ static const char mem_ddr [] = "mDDR";
+ static const char t_tst [] = "TST"; /* security level */
+ static const char t_emu [] = "EMU";
+ static const char t_hs [] = "HS";
+ static const char t_gp [] = "GP";
+ static const char unk [] = "?";
+
+ const char *cpu_s, *db_s, *mem_s, *sec_s;
u32 cpu, rev, sec;

rev = get_cpu_rev();
diff --git a/common/Makefile b/common/Makefile
index 973f05a..a498367 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -232,6 +232,10 @@ $(obj)env_embedded.o: $(src)env_embedded.c $(obj)../tools/envcrc
$(obj)../tools/envcrc:
$(MAKE) -C ../tools

+# SEE README.arm-unaligned-accesses
+$(obj)hush.o: CFLAGS += -mno-unaligned-access
+$(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..08a70c0
--- /dev/null
+++ b/doc/README.arm-unaligned-accesses
@@ -0,0 +1,121 @@
+If you are reading this because of a data abort: the following MIGHT
+be relevant to your abort, if it was caused by an alignment violation.
+In order to determine this, use the PC from the abort dump along with
+an objdump -s -S of the u-boot ELF binary to locate the function where
+the abort happened; then compare this function with the examples below.
+If they match, then you've been hit with a compiler generated unaligned
+access, and you should rewrite your code or add -mno-unaligned-access
+to the command line of the offending file.
+
+Note that the PC shown in the abort message is relocated. In order to
+be able to match it to an address in the ELF binary dump, you will need
+to know the relocation offset. If your target defines CONFIG_CMD_BDI
+and if you can get to the prompt and enter commands before the abort
+happens, then command "bdinfo" will give you the offset. Otherwise you
+will need to try a build with DEBUG set, which will display the offset,
+or use a debugger and set a breakpoint at relocate_code() to see the
+offset (passed as an argument).
+
+*
+
+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 accesses 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 a stricter target, possibly once the
+ bad code is already in mainline.
+
+c) Relax the -munaligned-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 -munaligned-access.
+
+d) Relax the -munaligned-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.
+
+The option retained is d).
+
+Considering that actual occurrences of the issue are rare (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*),
+contributors should not be required to systematically try and detect
+the issue in their patches.
+
+Detecting files susceptible to the issue can be automated through a
+filter installed as a hook in .git which recognizes local char array
+initializations. Automation 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 not prevent committing the patch, but
+shall pre-populate the commit message with a note to the effect that
+this patch contains an initialized local char or 16-bit array and thus
+should be protected from the gcc 4.7 issue.
+
+Upon a positive detection, either option -mno-unaligned-access should
+be applied (if not already) to the affected file(s), or if the array
+is a pseudo 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 a099885..decc5ee 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -83,6 +83,9 @@ 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