On Tue, Jul 06, 2021 at 12:08:14PM +0100, Matthew Wilcox wrote: > On Mon, Jul 05, 2021 at 11:35:08AM +0800, Leizhen (ThunderTown) wrote: > > > > > > On 2021/7/3 3:56, Matthew Wilcox wrote: > > > On Fri, Jul 02, 2021 at 05:21:09PM +0800, Zhen Lei wrote: > > >> Move the evaluation expression "size - offset" after the "if (offset < 0)" > > >> judgment statement to eliminate a false positive produced by the UBSAN. > > >> > > >> No functional changes. > > >> > > >> ========================================================================== > > >> UBSAN: Undefined behaviour in fs/iomap.c:1435:9 > > >> signed integer overflow: > > >> 0 - -9223372036854775808 cannot be represented in type 'long long int' > > > > > > I don't understand. I thought we defined the behaviour of signed > > > integer overflow in the kernel with whatever-the-gcc-flag-is? > > > > -9223372036854775808 ==> 0x8000000000000000 ==> -0 (actually, this is incorrect. think about how twos-complement arithmetic works. first you negate every bit, so 8000..000 turns into 7fff..fff, then you add one, returning to 8000..000, so -LLONG_MIN == LLONG_MIN) > > I don't fully understand what you mean. This is triggered by explicit error > > injection '-0' at runtime, which should not be detected by compilation options. > > We use -fwrapv on the gcc command line: > > '-fwrapv' > This option instructs the compiler to assume that signed arithmetic > overflow of addition, subtraction and multiplication wraps around > using twos-complement representation. This flag enables some > optimizations and disables others. > > > lseek(r1, 0x8000000000000000, 0x3) > > I'll see about adding this to xfstests ... I have and it doesn't produce the problem. My config: CONFIG_UBSAN=y # CONFIG_UBSAN_TRAP is not set CONFIG_CC_HAS_UBSAN_BOUNDS=y CONFIG_UBSAN_BOUNDS=y CONFIG_UBSAN_ONLY_BOUNDS=y CONFIG_UBSAN_SHIFT=y CONFIG_UBSAN_DIV_ZERO=y CONFIG_UBSAN_BOOL=y CONFIG_UBSAN_ENUM=y # CONFIG_UBSAN_ALIGNMENT is not set CONFIG_UBSAN_SANITIZE_ALL=y # CONFIG_TEST_UBSAN is not set I even went as far as adding printks to be sure I'm hitting it: hole length 0x8000000000000000 data length 0x8000000000000000 Are you compiling with: KBUILD_CFLAGS += -fno-strict-overflow Or have you done something weird? What compiler version are you using?
