Re: [PATCH] selftests/mm: Fix BUG_ON checking wrong variable in mremap_dontunmap

[Hongfu Li <hongfu.li@linux.dev>]

> > From: Hongfu Li <lihongfu@kylinos.cn>
> > 
> > In mremap_dontunmap_partial_mapping_overwrite(), mremap() stores its
> > returned remapped address into remapped_mapping, while the BUG_ON
> > incorrectly checks dest_mapping instead.
> > 
> > If mremap() fails, dest_mapping still holds a valid pointer obtained from
> > the prior mmap() call, making this BUG_ON never trigger at all. Correct
> > the check to validate the actual mremap() return value stored in
> > remapped_mapping.
> > 
> > Signed-off-by: Hongfu Li <lihongfu@kylinos.cn>
> > ---
> >  tools/testing/selftests/mm/mremap_dontunmap.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/tools/testing/selftests/mm/mremap_dontunmap.c b/tools/testing/selftests/mm/mremap_dontunmap.c
> > index a4f75d836733..96ba537facf7 100644
> > --- a/tools/testing/selftests/mm/mremap_dontunmap.c
> > +++ b/tools/testing/selftests/mm/mremap_dontunmap.c
> > @@ -313,7 +313,7 @@ static void mremap_dontunmap_partial_mapping_overwrite(void)
> >  	    mremap(source_mapping, 5 * page_size,
> >  		   5 * page_size,
> >  		   MREMAP_DONTUNMAP | MREMAP_MAYMOVE | MREMAP_FIXED, dest_mapping);
> > -	BUG_ON(dest_mapping == MAP_FAILED, "mremap");
> > +	BUG_ON(remapped_mapping == MAP_FAILED, "mremap");
> >  	BUG_ON(dest_mapping != remapped_mapping, "expected to remap to dest_mapping");
> 
> Given that we know from a previous
> 
> 	BUG_ON(dest_mapping == MAP_FAILED, "mmap");
> 
> We can just drop this line completely and rely on the
> 
> 	BUG_ON(dest_mapping != remapped_mapping, "expected to remap to dest_mapping");

The subsequent BUG_ON comparison will implicitly catch the MAP_FAILED
condition, so this check is redundant.

This line will be dropped in the next revision. Thanks a lot for your
review.

Best regards,
Hongfu