Re: [PATCH] powerpc/mm: Fix UBSAN warning reported on hugetlb

Powerpc architecture supports 16GB hugetlb pages with hash translation. For 4K
page size, this is implemented as a hugepage directory entry at PGD level and
for 64K it is implemented as a huge page pte at PUD level

With 16GB hugetlb size, offset within a page is greater than 32 bits. Hence
switch to use unsigned long type when using hugepd_shift.

Inorder to keep things simpler, we make sure we always use unsigned long type
when using hugepd_shift() even though all the hugetlb page size won't require
that.

The walk_hugepd_range change won't have any impact because we don't use that for
the hugetlb walk. That code is needed to support hugepd on init_mm with PPC_8XX
and 8M page. Even though 8M page size won't result in any real issue, we update
that to keep it simpler.

The hugetlb_free_p*d_range changes are all related to nohash usage where we can
have multiple pgd entries pointing to the same hugepd entries. Hence on book3s64
where we can have > 4GB hugetlb page size we will always find more < next even
if we compute the value of more correctly.

Hence there is no functional change in this patch except that it fixes the below
warning.

  UBSAN: shift-out-of-bounds in arch/powerpc/mm/hugetlbpage.c:499:21
  shift exponent 34 is too large for 32-bit type 'int'
  CPU: 39 PID: 1673 Comm: a.out Not tainted 6.0.0-rc2-00327-gee88a56e8517-dirty #1
  Call Trace:
  [c00000002ccb3720] [c000000000cb21e4] dump_stack_lvl+0x98/0xe0 (unreliable)
  [c00000002ccb3760] [c000000000cacf60] ubsan_epilogue+0x18/0x70
  [c00000002ccb37c0] [c000000000cac44c] __ubsan_handle_shift_out_of_bounds+0x1bc/0x390
  [c00000002ccb38c0] [c0000000000d6f78] hugetlb_free_pgd_range+0x5d8/0x600
  [c00000002ccb39f0] [c000000000550e94] free_pgtables+0x114/0x290
  [c00000002ccb3ac0] [c00000000056cbe0] exit_mmap+0x150/0x550
  [c00000002ccb3be0] [c00000000017bf0c] mmput+0xcc/0x210
  [c00000002ccb3c20] [c00000000018f180] do_exit+0x420/0xdd0
  [c00000002ccb3cf0] [c00000000018fcdc] do_group_exit+0x4c/0xd0
  [c00000002ccb3d30] [c00000000018fd84] sys_exit_group+0x24/0x30
  [c00000002ccb3d50] [c00000000003cde0] system_call_exception+0x250/0x600
  [c00000002ccb3e10] [c00000000000c3bc] system_call_common+0xec/0x250

Signed-off-by: Aneesh Kumar K.V <redacted>
---
  arch/powerpc/mm/hugetlbpage.c | 6 +++---
  mm/pagewalk.c                 | 2 +-
  2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/pagewalk.c b/mm/pagewalk.c
index fa7a3d21a751..e210b737658c 100644
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c

@@ -65,7 +65,7 @@ static int walk_hugepd_range(hugepd_t *phpd, unsigned long addr,
  	int err = 0;
  	const struct mm_walk_ops *ops = walk->ops;
  	int shift = hugepd_shift(*phpd);
-	int page_size = 1 << shift;
+	long page_size = 1UL << shift;