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 <[email protected]>
---
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
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 <[email protected]>
> ---
> 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
On Thu, Mar 7, 2019 at 8:23 PM Michal Hocko <[email protected]> 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.
On Thu 07-03-19 20:47:52, Pi-Hsun Shih wrote:
> On Thu, Mar 7, 2019 at 8:23 PM Michal Hocko <[email protected]> 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
On Thu, Mar 7, 2019 at 9:23 PM Michal Hocko <[email protected]> wrote:
>
> On Thu 07-03-19 20:47:52, Pi-Hsun Shih wrote:
> > On Thu, Mar 7, 2019 at 8:23 PM Michal Hocko <[email protected]> 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
On Tue 12-03-19 15:02:38, Pi-Hsun Shih wrote:
> On Thu, Mar 7, 2019 at 9:23 PM Michal Hocko <[email protected]> wrote:
> >
> > On Thu 07-03-19 20:47:52, Pi-Hsun Shih wrote:
> > > On Thu, Mar 7, 2019 at 8:23 PM Michal Hocko <[email protected]> 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
On Tue, Mar 12, 2019 at 4:07 PM Michal Hocko <[email protected]> wrote:
>
> On Tue 12-03-19 15:02:38, Pi-Hsun Shih wrote:
> > On Thu, Mar 7, 2019 at 9:23 PM Michal Hocko <[email protected]> wrote:
> > >
> > > On Thu 07-03-19 20:47:52, Pi-Hsun Shih wrote:
> > > > On Thu, Mar 7, 2019 at 8:23 PM Michal Hocko <[email protected]> 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
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 <[email protected]>
---
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
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 <[email protected]>
Yes this makes more sense. I still think the warning is bogus but
whatever. A cleanup is worth it.
Acked-by: Michal Hocko <[email protected]>
> ---
> 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