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

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

[Pi-Hsun Shih]

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
==================================================================

Signed-off-by: Pi-Hsun Shih <pihsun@chromium.org>
---
 include/linux/swap.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index fc50e21b3b88..4bfb5c4ac108 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -157,9 +157,9 @@ struct swap_extent {
 /*
  * Max bad pages in the new format..
  */
-#define __swapoffset(x) ((unsigned long)&((union swap_header *)0)->x)
 #define MAX_SWAP_BADPAGES \
-	((__swapoffset(magic.magic) - __swapoffset(info.badpages)) / sizeof(int))
+	((offsetof(union swap_header, magic.magic) - \
+	  offsetof(union swap_header, info.badpages)) / sizeof(int))
 
 enum {
 	SWP_USED	= (1 << 0),	/* is slot in swap_info[] used? */
-- 
2.21.0.352.gf09ad66450-goog

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

[Michal Hocko]

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?

> 
> Signed-off-by: Pi-Hsun Shih <pihsun@chromium.org>
> ---
>  include/linux/swap.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index fc50e21b3b88..4bfb5c4ac108 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -157,9 +157,9 @@ struct swap_extent {
>  /*
>   * Max bad pages in the new format..
>   */
> -#define __swapoffset(x) ((unsigned long)&((union swap_header *)0)->x)
>  #define MAX_SWAP_BADPAGES \
> -	((__swapoffset(magic.magic) - __swapoffset(info.badpages)) / sizeof(int))
> +	((offsetof(union swap_header, magic.magic) - \
> +	  offsetof(union swap_header, info.badpages)) / sizeof(int))
>  
>  enum {
>  	SWP_USED	= (1 << 0),	/* is slot in swap_info[] used? */
> -- 
> 2.21.0.352.gf09ad66450-goog
> 

-- 
Michal Hocko
SUSE Labs

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

[Pi-Hsun Shih]

On Thu, Mar 7, 2019 at 8:23 PM Michal Hocko <mhocko@kernel.org> 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.

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@kernel.org> 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

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

[Pi-Hsun Shih]

On Thu, Mar 7, 2019 at 9:23 PM Michal Hocko <mhocko@kernel.org> 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@kernel.org> 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

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

[Michal Hocko]

On Tue 12-03-19 15:02:38, Pi-Hsun Shih wrote:
> On Thu, Mar 7, 2019 at 9:23 PM Michal Hocko <mhocko@kernel.org> 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@kernel.org> 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.

I really do not want to go and lawyering about the standard here but
getting an address of an offset based on NULL ptr is not really
dereferencing of a NULL ptr. At least this was not the case for ages
and no compiler can afford to change it because there is quite a lot of
userspace to rely on this construct.

But as I've said using offseoff is nicer so I completely support a patch
that get's read of a custom redefinition of it or open code directly.
But calling it an UB is a bit of stretch.
-- 
Michal Hocko
SUSE Labs

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

[Pi-Hsun Shih]

On Tue, Mar 12, 2019 at 4:07 PM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Tue 12-03-19 15:02:38, Pi-Hsun Shih wrote:
> > On Thu, Mar 7, 2019 at 9:23 PM Michal Hocko <mhocko@kernel.org> 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@kernel.org> 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.
>
> I really do not want to go and lawyering about the standard here but
> getting an address of an offset based on NULL ptr is not really
> dereferencing of a NULL ptr. At least this was not the case for ages
> and no compiler can afford to change it because there is quite a lot of
> userspace to rely on this construct.
>
> But as I've said using offseoff is nicer so I completely support a patch
> that get's read of a custom redefinition of it or open code directly.
> But calling it an UB is a bit of stretch.

Ok I'll send a v2 with a better commit title / message.

> --
> Michal Hocko
> SUSE Labs

[PATCH v2] mm/swap: Use offsetof instead of custom __swapoffset macro.

[Pi-Hsun Shih]

Use offsetof to calculate offset of a field to take advantage of
compiler built-in version when possible, and avoid UBSAN warning when
compiling with Clang:

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

Signed-off-by: Pi-Hsun Shih <pihsun@chromium.org>
---
 include/linux/swap.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index fc50e21b3b88..4bfb5c4ac108 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -157,9 +157,9 @@ struct swap_extent {
 /*
  * Max bad pages in the new format..
  */
-#define __swapoffset(x) ((unsigned long)&((union swap_header *)0)->x)
 #define MAX_SWAP_BADPAGES \
-	((__swapoffset(magic.magic) - __swapoffset(info.badpages)) / sizeof(int))
+	((offsetof(union swap_header, magic.magic) - \
+	  offsetof(union swap_header, info.badpages)) / sizeof(int))
 
 enum {
 	SWP_USED	= (1 << 0),	/* is slot in swap_info[] used? */
-- 
2.21.0.360.g471c308f928-goog

Re: [PATCH v2] mm/swap: Use offsetof instead of custom __swapoffset macro.

[Michal Hocko]

On Tue 12-03-19 16:18:59, Pi-Hsun Shih wrote:
> Use offsetof to calculate offset of a field to take advantage of
> compiler built-in version when possible, and avoid UBSAN warning when
> compiling with Clang:
> 
> 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
> 
> Signed-off-by: Pi-Hsun Shih <pihsun@chromium.org>

Yes this makes more sense. I still think the warning is bogus but
whatever. A cleanup is worth it.

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
>  include/linux/swap.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index fc50e21b3b88..4bfb5c4ac108 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -157,9 +157,9 @@ struct swap_extent {
>  /*
>   * Max bad pages in the new format..
>   */
> -#define __swapoffset(x) ((unsigned long)&((union swap_header *)0)->x)
>  #define MAX_SWAP_BADPAGES \
> -	((__swapoffset(magic.magic) - __swapoffset(info.badpages)) / sizeof(int))
> +	((offsetof(union swap_header, magic.magic) - \
> +	  offsetof(union swap_header, info.badpages)) / sizeof(int))
>  
>  enum {
>  	SWP_USED	= (1 << 0),	/* is slot in swap_info[] used? */
> -- 
> 2.21.0.360.g471c308f928-goog
> 

-- 
Michal Hocko
SUSE Labs