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

[Pi-Hsun Shih]

On Thu, Mar 7, 2019 at 9:23 PM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>
> On Thu 07-03-19 20:47:52, Pi-Hsun Shih wrote:
> > 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.
>
> Hmm, we've been using this trick for ages and I do not remember any
> compiler to complain as there is no real access. I am not sure what the
> C standard has to tell about that but I presume reasonable compilers
> will not abuse the UB here.
>

Some more testing shows that GCC optimize the
((size_t)&((type*)0)->member) to a constant in the result binary, and
never emit any UBSAN checks on the statement.
Clang doesn't optimize it to a constant in -O0, optimize it to a
constant in -O1 or above, and always emit the
__ubsan_handle_type_mismatch check when "-fsanitize=undefined" is
given.
So this UBSAN warning only happens when kernel is compiled by clang, not GCC.

>From what I've found, it's a UB from C standard view point
(https://software.intel.com/en-us/blogs/2015/04/20/null-pointer-dereferencing-causes-undefined-behavior),
but I agree that probably no reasonable compilers would abuse the UB
here.

> > 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.
>
> Is this the case for all compilers? If yes then we might want to drop
> the non-compiler part. Btw.
> $ git grep "#define offsetof"
> drivers/gpu/drm/radeon/mkregtable.c:#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
> drivers/infiniband/core/uverbs_cmd.c:#define offsetof_after(_struct, _member)                                       \
> include/linux/stddef.h:#define offsetof(TYPE, MEMBER)   __compiler_offsetof(TYPE, MEMBER)
> include/linux/stddef.h:#define offsetof(TYPE, MEMBER)   ((size_t)&((TYPE *)0)->MEMBER)
> include/linux/stddef.h:#define offsetofend(TYPE, MEMBER) \
> samples/bpf/cookie_uid_helper_example.c:#define offsetof(type, member)  __builtin_offsetof(type, member)
> scripts/kconfig/list.h:#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
> tools/include/linux/kernel.h:#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
> tools/testing/selftests/bpf/test_select_reuseport_kern.c:#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
> tools/usb/usbip/libsrc/list.h:#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
>
> so there is much more to take care of. Not to mention any open coded
> variants.

Did a "git grep '(0|NULL)\)->'" and there are about 25 of them that
can be changed to use offsetof.

> --
> Michal Hocko
> SUSE Labs