[PATCH 07/10] mm: correct sign-extension issue in MMF_* flag masks

Lorenzo Stoakes
2025-08-12T15:48:33Z

There is an issue with the mask declarations in linux/mm_types.h, which
naively do (1 << bit) operations. Unfortunately this results in the 1 being
defaulted as a signed (32-bit) integer.

When the compiler expands the MMF_INIT_MASK bitmask it comes up with:

(((1 << 2) - 1) | (((1 << 9) - 1) << 2) | (1 << 24) | (1 << 28) | (1 << 30)
| (1 << 31))

Which overflows the signed integer to -788,527,105. Implicitly casting this
to an unsigned integer results in sign-expansion, and thus this value
becomes 0xffffffffd10007ff, rather than the intended 0xd10007ff.

While we're limited to a maximum of 32 bits in mm->flags, this isn't an
issue as the remaining bits being masked will always be zero.

However, now we are moving towards having more bits in this flag, this
becomes an issue.

Simply resolve this by using the _BITUL() helper to cast the shifted value
to an unsigned long.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
include/linux/mm_types.h | 19 +++++++++----------
1 file changed, 9 insertions(+), 10 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 46d3fb8935c7..38b3fa927997 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -1756,7 +1756,7 @@ enum {
* the modes are SUID_DUMP_* defined in linux/sched/coredump.h
*/
#define MMF_DUMPABLE_BITS 2
-#define MMF_DUMPABLE_MASK ((1 << MMF_DUMPABLE_BITS) - 1)
+#define MMF_DUMPABLE_MASK (_BITUL(MMF_DUMPABLE_BITS) - 1)
/* coredump filter bits */
#define MMF_DUMP_ANON_PRIVATE 2
#define MMF_DUMP_ANON_SHARED 3
@@ -1771,13 +1771,13 @@ enum {
#define MMF_DUMP_FILTER_SHIFT MMF_DUMPABLE_BITS
#define MMF_DUMP_FILTER_BITS 9
#define MMF_DUMP_FILTER_MASK \
- (((1 << MMF_DUMP_FILTER_BITS) - 1) << MMF_DUMP_FILTER_SHIFT)
+ ((_BITUL(MMF_DUMP_FILTER_BITS) - 1) << MMF_DUMP_FILTER_SHIFT)
#define MMF_DUMP_FILTER_DEFAULT \
- ((1 << MMF_DUMP_ANON_PRIVATE) | (1 << MMF_DUMP_ANON_SHARED) |\
- (1 << MMF_DUMP_HUGETLB_PRIVATE) | MMF_DUMP_MASK_DEFAULT_ELF)
+ (_BITUL(MMF_DUMP_ANON_PRIVATE) | _BITUL(MMF_DUMP_ANON_SHARED) | \
+ _BITUL(MMF_DUMP_HUGETLB_PRIVATE) | MMF_DUMP_MASK_DEFAULT_ELF)

#ifdef CONFIG_CORE_DUMP_DEFAULT_ELF_HEADERS
-# define MMF_DUMP_MASK_DEFAULT_ELF (1 << MMF_DUMP_ELF_HEADERS)
+# define MMF_DUMP_MASK_DEFAULT_ELF _BITUL(MMF_DUMP_ELF_HEADERS)
#else
# define MMF_DUMP_MASK_DEFAULT_ELF 0
#endif
@@ -1797,7 +1797,7 @@ enum {
#define MMF_UNSTABLE 22 /* mm is unstable for copy_from_user */
#define MMF_HUGE_ZERO_FOLIO 23 /* mm has ever used the global huge zero folio */
#define MMF_DISABLE_THP 24 /* disable THP for all VMAs */
-#define MMF_DISABLE_THP_MASK (1 << MMF_DISABLE_THP)
+#define MMF_DISABLE_THP_MASK _BITUL(MMF_DISABLE_THP)
#define MMF_OOM_REAP_QUEUED 25 /* mm was queued for oom_reaper */
#define MMF_MULTIPROCESS 26 /* mm is shared between processes */
/*
@@ -1810,16 +1810,15 @@ enum {
#define MMF_HAS_PINNED 27 /* FOLL_PIN has run, never cleared */

#define MMF_HAS_MDWE 28
-#define MMF_HAS_MDWE_MASK (1 << MMF_HAS_MDWE)
-
+#define MMF_HAS_MDWE_MASK _BITUL(MMF_HAS_MDWE)

#define MMF_HAS_MDWE_NO_INHERIT 29

#define MMF_VM_MERGE_ANY 30
-#define MMF_VM_MERGE_ANY_MASK (1 << MMF_VM_MERGE_ANY)
+#define MMF_VM_MERGE_ANY_MASK _BITUL(MMF_VM_MERGE_ANY)

#define MMF_TOPDOWN 31 /* mm searches top down by default */
-#define MMF_TOPDOWN_MASK (1 << MMF_TOPDOWN)
+#define MMF_TOPDOWN_MASK _BITUL(MMF_TOPDOWN)

#define MMF_INIT_MASK (MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK |\
MMF_DISABLE_THP_MASK | MMF_HAS_MDWE_MASK |\
--
2.50.1