Re: linux kernel warnings/errors

[Randy Dunlap <rdunlap@xenotime.net>]

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On 02/23/10 12:59, Josh Triplett wrote:
> On Tue, Feb 23, 2010 at 11:49:06AM -0800, Randy Dunlap wrote:
>> Hi,
>>
>> You probably know that sparse produces a ton of errors & warnings when
>> run on the Linux kernel tree (a little over 1 MB in my latest 'make C=1'
>> on x86_64 arch.).
>>
>> I'm wondering if all of these are valid.
>>
>> Examples:
>>
>>
>> 1.  Use of the BUILD_BUG_ON() macro causes this error from sparse:
>>
>> arch/x86/kernel/paravirt.c:101:9: error: invalid bitfield width, -1.
>>
>> include/linux/kernel.h:
>>
>> /* Force a compilation error if condition is true */
>> #define BUILD_BUG_ON(condition) ((void)BUILD_BUG_ON_ZERO(condition))
>>
>> /* Force a compilation error if condition is true, but also produce a
>>    result (of value 0 and type size_t), so the expression can be used
>>    e.g. in a structure initializer (or where-ever else comma expressions
>>    aren't permitted). */
>> #define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
> 
> In theory that should only happen if the BUILD_BUG_ON_ZERO actually
> triggers, and thus sparse fails to build just as GCC would.  If sparse
> has this problem and GCC doesn't, could you please provide a minimal
> test case?

Sure, attached.


>> 2.  drivers/block/drbd/drbd_int.h uses __protected_by(var);
>> this seems to be unknown to sparse.
>>
>> #ifdef __CHECKER__
>> # define __protected_by(x)       __attribute__((require_context(x,1,999,"rdwr")))
>>
>> sparse says many times:
>>
>> drivers/block/drbd/drbd_int.h:887:39: error: attribute 'require_context': unknown attribute
> 
> That looks broken.  A few patches went around for new Sparse
> context-tracking features, one of which got reverted before the 0.4.2
> release, but I don't think any of them would have allowed *that*.

Yeah, I recall seeing some of the context patches.

Thanks.
-- 
~Randy

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/* Force a compilation error if condition is true */
#define BUILD_BUG_ON(condition) ((void)BUILD_BUG_ON_ZERO(condition))

/* Force a compilation error if condition is true, but also produce a
   result (of value 0 and type size_t), so the expression can be used
   e.g. in a structure initializer (or where-ever else comma expressions
   aren't permitted). */
#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))

struct branch {
	unsigned char opcode;
	int delta;
} __attribute__((packed));

unsigned paravirt_patch_call(void *insnbuf)
{
	struct branch *b = insnbuf;

	b->opcode = 0xe8; /* call */
	b->delta = 0x1234;
	BUILD_BUG_ON(sizeof(*b) != 5);

	return 5;
}

int main(int argc, char *argv[])
{
}