On Mon, Mar 25, 2019 at 03:06:12PM +0200, Nikolay Borisov wrote:
>
>
> On 25.03.19 г. 15:02 ч., Arnd Bergmann wrote:
> > BUG_ON(1) leads to bogus warnings from clang when
> > CONFIG_PROFILE_ANNOTATED_BRANCHES is set:
> >
> > fs/btrfs/volumes.c:5041:3: error: variable 'max_chunk_size' is used uninitialized whenever 'if' condition is false
> > [-Werror,-Wsometimes-uninitialized]
> > BUG_ON(1);
> > ^~~~~~~~~
> > include/asm-generic/bug.h:61:36: note: expanded from macro 'BUG_ON'
> > #define BUG_ON(condition) do { if (unlikely(condition)) BUG(); } while (0)
> > ^~~~~~~~~~~~~~~~~~~
> > include/linux/compiler.h:48:23: note: expanded from macro 'unlikely'
> > # define unlikely(x) (__branch_check__(x, 0, __builtin_constant_p(x)))
> > ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > fs/btrfs/volumes.c:5046:9: note: uninitialized use occurs here
> > max_chunk_size);
> > ^~~~~~~~~~~~~~
> > include/linux/kernel.h:860:36: note: expanded from macro 'min'
> > #define min(x, y) __careful_cmp(x, y, <)
> > ^
> > include/linux/kernel.h:853:17: note: expanded from macro '__careful_cmp'
> > __cmp_once(x, y, __UNIQUE_ID(__x), __UNIQUE_ID(__y), op))
> > ^
> > include/linux/kernel.h:847:25: note: expanded from macro '__cmp_once'
> > typeof(y) unique_y = (y); \
> > ^
> > fs/btrfs/volumes.c:5041:3: note: remove the 'if' if its condition is always true
> > BUG_ON(1);
> > ^
> > include/asm-generic/bug.h:61:32: note: expanded from macro 'BUG_ON'
> > #define BUG_ON(condition) do { if (unlikely(condition)) BUG(); } while (0)
> > ^
> > fs/btrfs/volumes.c:4993:20: note: initialize the variable 'max_chunk_size' to silence this warning
> > u64 max_chunk_size;
> > ^
> > = 0
> >
> > Change it to BUG() so clang can see that this code path can never
> > continue.
>
> Long-term we have to eliminate to bug_on's but until then:
I think in those places it's "this must never happen", so it's not the
the typical example of BUG_ON misuse we want to eliminate. Fixing the
compiler waring is ok though.
