[PATCH] mm/khugepaged: Initialize index and nr in collapse_file()

[PATCH] mm/khugepaged: Initialize index and nr in collapse_file()

[Nathan Chancellor]

Clang warns (trimmed for brevity):

  mm/khugepaged.c:1729:7: warning: variable 'index' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
  mm/khugepaged.c:1716:6: warning: variable 'index' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
  mm/khugepaged.c:1729:7: warning: variable 'nr' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
  mm/khugepaged.c:1716:6: warning: variable 'nr' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]

There are two goto statements that will use index and nr before they
have been properly initialized. Zero initialize them so that they can be
safely used by the tracepoint at the end of the function.

Fixes: eae5270d3322 ("mm/khugepaged: add tracepoint to collapse_file()")
Link: https://github.com/ClangBuiltLinux/linux/issues/1749
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
 mm/khugepaged.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 977c0ff82c46..789db2f3fc06 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1702,12 +1702,12 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 {
 	struct address_space *mapping = file->f_mapping;
 	struct page *hpage;
-	pgoff_t index, end = start + HPAGE_PMD_NR;
+	pgoff_t index = 0, end = start + HPAGE_PMD_NR;
 	LIST_HEAD(pagelist);
 	XA_STATE_ORDER(xas, &mapping->i_pages, start, HPAGE_PMD_ORDER);
 	int nr_none = 0, result = SCAN_SUCCEED;
 	bool is_shmem = shmem_file(file);
-	int nr;
+	int nr = 0;
 
 	VM_BUG_ON(!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && !is_shmem);
 	VM_BUG_ON(start & (HPAGE_PMD_NR - 1));

base-commit: ec24a700584c4df869282bcd92b6d88329afe395
-- 
2.38.1

Re: [PATCH] mm/khugepaged: Initialize index and nr in collapse_file()

[Zach O'Keefe]

Thanks for the fix, Nathan,

Best,
Zach

Reviewed-by: Zach O'Keefe <zokeefe@google.com>

Re: [PATCH] mm/khugepaged: Initialize index and nr in collapse_file()

[Andrew Morton]

On Tue, 25 Oct 2022 10:34:07 -0700 Nathan Chancellor <nathan@kernel.org> wrote:

> Clang warns (trimmed for brevity):
> 
>   mm/khugepaged.c:1729:7: warning: variable 'index' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
>   mm/khugepaged.c:1716:6: warning: variable 'index' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
>   mm/khugepaged.c:1729:7: warning: variable 'nr' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
>   mm/khugepaged.c:1716:6: warning: variable 'nr' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
> 
> There are two goto statements that will use index and nr before they
> have been properly initialized. Zero initialize them so that they can be
> safely used by the tracepoint at the end of the function.

Thanks.  I've actually dropped the offending patch - I'd like a resend
which includes a fix such as this and a good reason for making the
change.

Re: [PATCH] mm/khugepaged: Initialize index and nr in collapse_file()

[Nathan Chancellor]

On Tue, Oct 25, 2022 at 06:48:02PM -0700, Andrew Morton wrote:
> On Tue, 25 Oct 2022 10:34:07 -0700 Nathan Chancellor <nathan@kernel.org> wrote:
> 
> > Clang warns (trimmed for brevity):
> > 
> >   mm/khugepaged.c:1729:7: warning: variable 'index' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
> >   mm/khugepaged.c:1716:6: warning: variable 'index' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
> >   mm/khugepaged.c:1729:7: warning: variable 'nr' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
> >   mm/khugepaged.c:1716:6: warning: variable 'nr' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
> > 
> > There are two goto statements that will use index and nr before they
> > have been properly initialized. Zero initialize them so that they can be
> > safely used by the tracepoint at the end of the function.
> 
> Thanks.  I've actually dropped the offending patch - I'd like a resend
> which includes a fix such as this and a good reason for making the
> change.

That certainly seems like a reasonable request. Thanks for letting me
know!

Cheers,
Nathan