Messages in this thread | | | From | Pi-Hsun Shih <> | Date | Thu, 7 Mar 2019 20:47:52 +0800 | Subject | Re: [PATCH] mm/swap: Avoid undefined behavior in __swapoffset |
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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.
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