Re: [PATCH] mm/swap: Avoid undefined behavior in __swapoffset

[Pi-Hsun Shih]

On Thu, Mar 7, 2019 at 8:23 PM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>
> On Thu 07-03-19 17:46:50, Pi-Hsun Shih wrote:
> > Use offsetof to calculate offset of a field to avoid UBSAN warning like:
> >
> > ===================================================================
> > UBSAN: Undefined behaviour in mm/swapfile.c:3010:38
> > member access within null pointer of type 'union swap_header'
> > CPU: 6 PID: 1833 Comm: swapon Tainted: G S                4.19.23 #43
> > Call trace:
> >  dump_backtrace+0x0/0x194
> >  show_stack+0x20/0x2c
> >  __dump_stack+0x20/0x28
> >  dump_stack+0x70/0x94
> >  ubsan_epilogue+0x14/0x44
> >  ubsan_type_mismatch_common+0xf4/0xfc
> >  __ubsan_handle_type_mismatch_v1+0x34/0x54
> >  __se_sys_swapon+0x654/0x1084
> >  __arm64_sys_swapon+0x1c/0x24
> >  el0_svc_common+0xa8/0x150
> >  el0_svc_compat_handler+0x2c/0x38
> >  el0_svc_compat+0x8/0x18
> > ==================================================================
>
> Could you be more specific about what exactly is undefined here and
> why offsetof is any better. AFAIR it uses the same construct unless a
> compiler defines a built in.
>
> I do not object the change itself because it is cleaner to use the
> existing helper but I am wondering why this is fixing ubsan. Is ubsan
> defining the compiler variant and consider it safe?
>

The undefined behavior is from trying to accessing a member of NULL,
even not using it value but only use the address.

Since the compiler variant for offsetof is used for recent compiler
(GCC >= 4 has support for it), ubsan would not warn if the compiler
variant is used. For old compiler, I guess ubsan would complain on all
offsetof uses.