Re: [PATCH] selinux: use explicit 64-bit division

[Paul Moore]

On Tue, Sep 15, 2026 at 3:57 PM Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
>
> From: Arnd Bergmann <arnd@xxxxxxxx>
>
> On 32-bit targets, the division of a 64-bit integer by 33 causes a
> function call:
>
> arm-linux-gnueabi-ld: security/selinux/selinuxfs.o: in function `sel_read_class':
> selinuxfs.c:(.text+0xdcc): undefined reference to `__aeabi_uldivmod'
> arm-linux-gnueabi-ld: security/selinux/selinuxfs.o: in function `sel_read_perm':
> selinuxfs.c:(.text+0xed0): undefined reference to `__aeabi_uldivmod'
>
> Replace this with an explicit call to div_u64() and div_u64_rem() to
> annotate that these are potentially very slow.
>
> Fixes: fc68b6a45f16 ("selinux: convert selinuxfs inode numbers from unsigned long to u64")
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
>  security/selinux/selinuxfs.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)

Thanks Arnd.

As sashiko pointed out[1], since we don't really use the full 64-bit
inode space, what do you think about the following fix instead?

diff --git a/security/selinux/selinuxfs.c b/security/selinux/selinuxfs.c
index 292302eb60f3..d864c314c873 100644
--- a/security/selinux/selinuxfs.c
+++ b/security/selinux/selinuxfs.c
@@ -1574,7 +1574,8 @@ static inline u64 sel_class_to_ino(u16 class)

 static inline u16 sel_ino_to_class(u64 ino)
 {
-       return (ino & SEL_INO_MASK) / (SEL_VEC_MAX + 1);
+       u32 ino_masked = ino & SEL_INO_MASK;
+       return ino_masked / (SEL_VEC_MAX + 1);
 }

 static inline u64 sel_perm_to_ino(u16 class, u32 perm)
@@ -1584,7 +1585,8 @@ static inline u64 sel_perm_to_ino(u16 class, u32 perm)

 static inline u32 sel_ino_to_perm(u64 ino)
 {
-       return (ino & SEL_INO_MASK) % (SEL_VEC_MAX + 1);
+       u32 ino_masked = ino & SEL_INO_MASK;
+       return ino_masked % (SEL_VEC_MAX + 1);
 }

[1] https://lore.kernel.org/selinux/20260915200522.BBC8A1F000FF@xxxxxxxxxxxxxxx/

-- 
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