Patchwork [U-Boot,V6,RESEND] ARM: prevent misaligned array inits

login
register
mail settings
Submitter Albert ARIBAUD
Date Oct. 9, 2012, 7:28 p.m.
Message ID <1349810895-25809-1-git-send-email-albert.u.boot@aribaud.net>
Download mbox | patch
Permalink /patch/190418/
State Accepted
Delegated to: Tom Rini
Headers show

Comments

Albert ARIBAUD - Oct. 9, 2012, 7:28 p.m.
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>
---
V6: Make sure that gcc does not silently drop -mno-unaligned-access
    if it does not support it.
V5: -mno-unaligned-access was applied to all platforms. Make it apply
    only to armv7.
    Clarified README.arm-unaligned-accesses re how to fix issue.
    Included revert of 'release-only' workaround.
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/cpu/armv7/config.mk         |    6 +-
 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    |  122 ++++++++++++++++++++++++++++++++++
 fs/fat/Makefile                      |    2 +
 fs/ubifs/Makefile                    |    3 +
 lib/Makefile                         |    3 +
 10 files changed, 156 insertions(+), 20 deletions(-)
 create mode 100644 doc/README.arm-unaligned-accesses
Albert ARIBAUD - Oct. 14, 2012, 8:48 a.m.
On Tue,  9 Oct 2012 21:28:15 +0200, Albert ARIBAUD
<albert.u.boot@aribaud.net> wrote:

> 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>
> ---

Tom,

As per our discussion, this patch should be placed in u-boot/next
right above the 2012-10 release commit.

At this time, just after a git fech u-boot, I don't see the patch in
next at all.

Do you want me to prepare an ad hoc branch with this patch then the
current content of u-boot/next, above v2012.10-rc3, so that you just
have to git reset --hard your next on it?

Amicalement,
Tom Rini - Oct. 15, 2012, 5:05 a.m.
On Sun, Oct 14, 2012 at 1:48 AM, Albert ARIBAUD
<albert.u.boot@aribaud.net> wrote:
> On Tue,  9 Oct 2012 21:28:15 +0200, Albert ARIBAUD
> <albert.u.boot@aribaud.net> wrote:
>
>> 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>
>> ---
>
> Tom,
>
> As per our discussion, this patch should be placed in u-boot/next
> right above the 2012-10 release commit.
>
> At this time, just after a git fech u-boot, I don't see the patch in
> next at all.
>
> Do you want me to prepare an ad hoc branch with this patch then the
> current content of u-boot/next, above v2012.10-rc3, so that you just
> have to git reset --hard your next on it?
>
> Amicalement,

As the questions about our usage of rebasing in 'next' have shown,
it's not quite as cut and dry as I had hoped to rebase next at my whim
(and long term, that's a good thing I think).  My plan tomorrow (or
today, depending on your local timezone) is to release v2012.10, apply
this v6 patch and then bring in next to master, see that everything is
still building and go from there.
Albert ARIBAUD - Oct. 15, 2012, 7:17 a.m.
Hi Tom,

On Sun, 14 Oct 2012 22:05:15 -0700, Tom Rini <trini@ti.com> wrote:

> On Sun, Oct 14, 2012 at 1:48 AM, Albert ARIBAUD
> <albert.u.boot@aribaud.net> wrote:
> > On Tue,  9 Oct 2012 21:28:15 +0200, Albert ARIBAUD
> > <albert.u.boot@aribaud.net> wrote:
> >
> >> 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>
> >> ---
> >
> > Tom,
> >
> > As per our discussion, this patch should be placed in u-boot/next
> > right above the 2012-10 release commit.
> >
> > At this time, just after a git fech u-boot, I don't see the patch in
> > next at all.
> >
> > Do you want me to prepare an ad hoc branch with this patch then the
> > current content of u-boot/next, above v2012.10-rc3, so that you just
> > have to git reset --hard your next on it?
> >
> > Amicalement,
> 
> As the questions about our usage of rebasing in 'next' have shown,
> it's not quite as cut and dry as I had hoped to rebase next at my whim
> (and long term, that's a good thing I think).  My plan tomorrow (or
> today, depending on your local timezone) is to release v2012.10, apply
> this v6 patch and then bring in next to master, see that everything is
> still building and go from there.

Fine with me, thanks!

P.S. Until this release, I'd worked under the assumption that next
could be more freely rebased than master is. From now on, I will follow
the same principle for next as for master, i.e. not rebase but merge
(possibly ff) from other and only rollback if required and appropriate.

Amicalement,
Tom Rini - Oct. 15, 2012, 6:48 p.m.
On Tue, Oct 09, 2012 at 09:28:15PM +0200, Albert ARIBAUD wrote:
> 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>
> ---
> V6: Make sure that gcc does not silently drop -mno-unaligned-access
>     if it does not support it.

OK, you were correct.  We need to use cc-option since older compilers
which were not causing issues don't support it either, making this
workable.  I will just fix this in-line rather than make you do a v7.

Patch

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/cpu/armv7/config.mk b/arch/arm/cpu/armv7/config.mk
index 560c084..3c5ca23 100644
--- a/arch/arm/cpu/armv7/config.mk
+++ b/arch/arm/cpu/armv7/config.mk
@@ -26,8 +26,6 @@  PLATFORM_RELFLAGS += -fno-common -ffixed-r8 -msoft-float
 # supported by more tool-chains
 PF_CPPFLAGS_ARMV7 := $(call cc-option, -march=armv7-a, -march=armv5)
 PLATFORM_CPPFLAGS += $(PF_CPPFLAGS_ARMV7)
-PF_CPPFLAGS_NO_UNALIGNED := $(call cc-option, -mno-unaligned-access,)
-PLATFORM_CPPFLAGS += $(PF_CPPFLAGS_NO_UNALIGNED)
 
 # =========================================================================
 #
@@ -36,6 +34,10 @@  PLATFORM_CPPFLAGS += $(PF_CPPFLAGS_NO_UNALIGNED)
 # =========================================================================
 PF_RELFLAGS_SLB_AT := $(call cc-option,-mshort-load-bytes,$(call cc-option,-malignment-traps,))
 PLATFORM_RELFLAGS += $(PF_RELFLAGS_SLB_AT)
+
+# SEE README.arm-unaligned-accesses
+PLATFORM_NO_UNALIGNED := -mno-unaligned-access
+
 ifneq ($(CONFIG_IMX_CONFIG),)
 ALL-y	+= $(obj)u-boot.imx
 endif
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 5442fbb..8a85dec 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -231,6 +231,10 @@  $(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 += $(PLATFORM_NO_UNALIGNED)
+$(obj)fdt_support.o: CFLAGS += $(PLATFORM_NO_UNALIGNED)
+
 #########################################################################
 
 # 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..c37d135
--- /dev/null
+++ b/doc/README.arm-unaligned-accesses
@@ -0,0 +1,122 @@ 
+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 starting at armv7 architectures, the default for
+performing unaligned accesses is to use unaligned native loads and
+stores (-munaligned-access), because the cost of unaligned accesses
+has dropped on armv7 and beyond. 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 and for armv7 architectures and
+   beyond. 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 $(PLATFORM_NO_UNALIGNED) should be
+added to CFLAGS for the affected file(s), or if the array is a pseudo
+const char*, it should be replaced by an actual one.
diff --git a/fs/fat/Makefile b/fs/fat/Makefile
index 9635d36..02e6881 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 += $(PLATFORM_NO_UNALIGNED)
 
 #########################################################################
 
diff --git a/fs/ubifs/Makefile b/fs/ubifs/Makefile
index ccffe85..bfe6874 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 += $(PLATFORM_NO_UNALIGNED)
+
 #########################################################################
 
 # defines $(obj).depend target
diff --git a/lib/Makefile b/lib/Makefile
index a099885..e44e045 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 += $(PLATFORM_NO_UNALIGNED)
+
 #########################################################################
 
 # defines $(obj).depend target