Messages in this thread | | | Date | Tue, 12 Mar 2019 09:07:46 +0100 | From | Michal Hocko <> | Subject | Re: [PATCH] mm/swap: Avoid undefined behavior in __swapoffset |
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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
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