mirror of
https://github.com/AsahiLinux/u-boot
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123 lines
5.6 KiB
Text
123 lines
5.6 KiB
Text
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If you are reading this because of a data abort: the following MIGHT
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be relevant to your abort, if it was caused by an alignment violation.
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In order to determine this, use the PC from the abort dump along with
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an objdump -s -S of the u-boot ELF binary to locate the function where
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the abort happened; then compare this function with the examples below.
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If they match, then you've been hit with a compiler generated unaligned
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access, and you should rewrite your code or add -mno-unaligned-access
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to the command line of the offending file.
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Note that the PC shown in the abort message is relocated. In order to
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be able to match it to an address in the ELF binary dump, you will need
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to know the relocation offset. If your target defines CONFIG_CMD_BDI
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and if you can get to the prompt and enter commands before the abort
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happens, then command "bdinfo" will give you the offset. Otherwise you
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will need to try a build with DEBUG set, which will display the offset,
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or use a debugger and set a breakpoint at relocate_code() to see the
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offset (passed as an argument).
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*
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Since U-Boot runs on a variety of hardware, some only able to perform
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unaligned accesses with a strong penalty, some unable to perform them
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at all, the policy regarding unaligned accesses is to not perform any,
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unless absolutely necessary because of hardware or standards.
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Also, on hardware which permits it, the core is configured to throw
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data abort exceptions on unaligned accesses in order to catch these
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unallowed accesses as early as possible.
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Until version 4.7, the gcc default for performing unaligned accesses
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(-mno-unaligned-access) is to emulate unaligned accesses using aligned
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loads and stores plus shifts and masks. Emulated unaligned accesses
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will not be caught by hardware. These accesses may be costly and may
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be actually unnecessary. In order to catch these accesses and remove
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or optimize them, option -munaligned-access is explicitly set for all
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versions of gcc which support it.
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From gcc 4.7 onward starting at armv7 architectures, the default for
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performing unaligned accesses is to use unaligned native loads and
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stores (-munaligned-access), because the cost of unaligned accesses
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has dropped on armv7 and beyond. This should not affect U-Boot's
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policy of controlling unaligned accesses, however the compiler may
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generate uncontrolled unaligned accesses on its own in at least one
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known case: when declaring a local initialized char array, e.g.
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function foo()
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{
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char buffer[] = "initial value";
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/* or */
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char buffer[] = { 'i', 'n', 'i', 't', 0 };
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...
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}
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Under -munaligned-accesses with optimizations on, this declaration
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causes the compiler to generate native loads from the literal string
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and native stores to the buffer, and the literal string alignment
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cannot be controlled. If it is misaligned, then the core will throw
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a data abort exception.
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Quite probably the same might happen for 16-bit array initializations
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where the constant is aligned on a boundary which is a multiple of 2
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but not of 4:
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function foo()
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{
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u16 buffer[] = { 1, 2, 3 };
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...
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}
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The long term solution to this issue is to add an option to gcc to
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allow controlling the general alignment of data, including constant
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initialization values.
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However this will only apply to the version of gcc which will have such
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an option. For other versions, there are four workarounds:
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a) Enforce as a rule that array initializations as described above
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are forbidden. This is generally not acceptable as they are valid,
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and usual, C constructs. The only case where they could be rejected
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is when they actually equate to a const char* declaration, i.e. the
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array is initialized and never modified in the function's scope.
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b) Drop the requirement on unaligned accesses at least for ARMv7,
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i.e. do not throw a data abort exception upon unaligned accesses.
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But that will allow adding badly aligned code to U-Boot, only for
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it to fail when re-used with a stricter target, possibly once the
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bad code is already in mainline.
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c) Relax the -munaligned-access rule globally. This will prevent native
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unaligned accesses of course, but that will also hide any bug caused
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by a bad unaligned access, making it much harder to diagnose it. It
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is actually what already happens when building ARM targets with a
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pre-4.7 gcc, and it may actually already hide some bugs yet unseen
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until the target gets compiled with -munaligned-access.
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d) Relax the -munaligned-access rule only for for files susceptible to
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the local initialized array issue and for armv7 architectures and
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beyond. This minimizes the quantity of code which can hide unwanted
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misaligned accesses.
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The option retained is d).
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Considering that actual occurrences of the issue are rare (as of this
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writing, 5 files out of 7840 in U-Boot, or .3%, contain an initialized
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local char array which cannot actually be replaced with a const char*),
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contributors should not be required to systematically try and detect
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the issue in their patches.
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Detecting files susceptible to the issue can be automated through a
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filter installed as a hook in .git which recognizes local char array
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initializations. Automation should err on the false positive side, for
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instance flagging non-local arrays as if they were local if they cannot
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be told apart.
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In any case, detection shall not prevent committing the patch, but
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shall pre-populate the commit message with a note to the effect that
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this patch contains an initialized local char or 16-bit array and thus
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should be protected from the gcc 4.7 issue.
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Upon a positive detection, either $(PLATFORM_NO_UNALIGNED) should be
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added to CFLAGS for the affected file(s), or if the array is a pseudo
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const char*, it should be replaced by an actual one.
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