On Tue, Apr 20, 2021 at 3:57 PM Josh Poimboeuf <jpoim...@redhat.com> wrote: > > On Tue, Apr 20, 2021 at 01:25:43PM -0700, Sami Tolvanen wrote: > > On Tue, Apr 20, 2021 at 11:14 AM Josh Poimboeuf <jpoim...@redhat.com> wrote: > > > > > > On Fri, Apr 16, 2021 at 01:38:30PM -0700, Sami Tolvanen wrote: > > > > With -ffunction-sections, Clang can generate a jump beyond the end of > > > > a section when the section ends in an unreachable instruction. > > > > > > Why? Can you show an example? > > > > Here's the warning I'm seeing when building allyesconfig + CFI: > > > > vmlinux.o: warning: objtool: > > rockchip_spi_transfer_one.f088382d97b74759d70e27e891fe8f1c()+0x149: > > can't find jump dest instruction at > > .text.rockchip_spi_transfer_one.f088382d97b74759d70e27e891fe8f1c+0x7dc > > > > $ objdump -d -r -j > > .text.rockchip_spi_transfer_one.f088382d97b74759d70e27e891fe8f1c > > vmlinux.o > > 0000000000000000 > > <rockchip_spi_transfer_one.f088382d97b74759d70e27e891fe8f1c>: > > ... > > 149: 0f 85 8d 06 00 00 jne 7dc <.compoundliteral.4> > > ... > > 7d7: e8 00 00 00 00 callq 7dc <.compoundliteral.4> > > 7d8: R_X86_64_PLT32 __stack_chk_fail-0x4 > > Instead of silencing the warning by faking the jump destination, I'd > rather improve the warning to something like > > "warning: rockchip_spi_transfer_one() falls through to the next function" > > which is what we normally do in this type of situation. > > It may be caused by UB, or a compiler bug, but either way we should > figure out the root cause.
We probably want to creduce or cvise this. IIRC we still have outstanding issues with switch statements with user-annotated unreachable branches not getting eliminated. -- Thanks, ~Nick Desaulniers