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| 5 | Sparse
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| 6 | ======
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| 7 |
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| 8 | Sparse is a semantic checker for C programs; it can be used to find a
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| 9 | number of potential problems with kernel code. See
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| 10 | https://lwn.net/Articles/689907/ for an overview of sparse; this document
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| 11 | contains some kernel-specific sparse information.
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| 12 | More information on sparse, mainly about its internals, can be found in
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| 13 | its official pages at https://sparse.docs.kernel.org.
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| 14 |
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| 15 |
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| 16 | Using sparse for typechecking
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| 17 | -----------------------------
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| 18 |
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| 19 | "__bitwise" is a type attribute, so you have to do something like this::
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| 20 |
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| 21 | typedef int __bitwise pm_request_t;
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| 22 |
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| 23 | enum pm_request {
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| 24 | PM_SUSPEND = (__force pm_request_t) 1,
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| 25 | PM_RESUME = (__force pm_request_t) 2
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| 26 | };
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| 27 |
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| 28 | which makes PM_SUSPEND and PM_RESUME "bitwise" integers (the "__force" is
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| 29 | there because sparse will complain about casting to/from a bitwise type,
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| 30 | but in this case we really _do_ want to force the conversion). And because
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| 31 | the enum values are all the same type, now "enum pm_request" will be that
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| 32 | type too.
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| 33 |
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| 34 | And with gcc, all the "__bitwise"/"__force stuff" goes away, and it all
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| 35 | ends up looking just like integers to gcc.
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| 36 |
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| 37 | Quite frankly, you don't need the enum there. The above all really just
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| 38 | boils down to one special "int __bitwise" type.
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| 39 |
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| 40 | So the simpler way is to just do::
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| 41 |
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| 42 | typedef int __bitwise pm_request_t;
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| 43 |
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| 44 | #define PM_SUSPEND ((__force pm_request_t) 1)
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| 45 | #define PM_RESUME ((__force pm_request_t) 2)
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| 46 |
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| 47 | and you now have all the infrastructure needed for strict typechecking.
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| 48 |
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| 49 | One small note: the constant integer "0" is special. You can use a
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| 50 | constant zero as a bitwise integer type without sparse ever complaining.
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| 51 | This is because "bitwise" (as the name implies) was designed for making
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| 52 | sure that bitwise types don't get mixed up (little-endian vs big-endian
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| 53 | vs cpu-endian vs whatever), and there the constant "0" really _is_
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| 54 | special.
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| 55 |
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| 56 | Using sparse for lock checking
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| 57 | ------------------------------
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| 58 |
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| 59 | The following macros are undefined for gcc and defined during a sparse
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| 60 | run to use the "context" tracking feature of sparse, applied to
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| 61 | locking. These annotations tell sparse when a lock is held, with
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| 62 | regard to the annotated function's entry and exit.
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| 63 |
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| 64 | __must_hold - The specified lock is held on function entry and exit.
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| 65 |
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| 66 | __acquires - The specified lock is held on function exit, but not entry.
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| 67 |
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| 68 | __releases - The specified lock is held on function entry, but not exit.
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| 69 |
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| 70 | If the function enters and exits without the lock held, acquiring and
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| 71 | releasing the lock inside the function in a balanced way, no
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| 72 | annotation is needed. The three annotations above are for cases where
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| 73 | sparse would otherwise report a context imbalance.
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| 74 |
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| 75 | Getting sparse
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| 76 | --------------
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| 77 |
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| 78 | You can get tarballs of the latest released versions from:
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| 79 | https://www.kernel.org/pub/software/devel/sparse/dist/
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| 80 |
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| 81 | Alternatively, you can get snapshots of the latest development version
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| 82 | of sparse using git to clone::
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| 83 |
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| 84 | git://git.kernel.org/pub/scm/devel/sparse/sparse.git
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| 85 |
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| 86 | Once you have it, just do::
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| 87 |
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| 88 | make
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| 89 | make install
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| 90 |
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| 91 | as a regular user, and it will install sparse in your ~/bin directory.
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| 92 |
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| 93 | Using sparse
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| 94 | ------------
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| 95 |
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| 96 | Do a kernel make with "make C=1" to run sparse on all the C files that get
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| 97 | recompiled, or use "make C=2" to run sparse on the files whether they need to
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| 98 | be recompiled or not. The latter is a fast way to check the whole tree if you
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| 99 | have already built it.
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| 100 |
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| 101 | The optional make variable CF can be used to pass arguments to sparse. The
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| 102 | build system passes -Wbitwise to sparse automatically.
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| 103 |
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| 104 | Note that sparse defines the __CHECKER__ preprocessor symbol.
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