Messages in this thread | | | From | Pi-Hsun Shih <> | Date | Tue, 12 Mar 2019 15:02:38 +0800 | Subject | Re: [PATCH] mm/swap: Avoid undefined behavior in __swapoffset |
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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
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