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
