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

[Michal Hocko]

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.

> 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.
-- 
Michal Hocko
SUSE Labs