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* [Bug translation/113066] New: Returning from a function marked noreturn allows execution to fall-through @ 2023-12-18 15:23 luke.geeson at cs dot ucl.ac.uk 2023-12-18 15:34 ` [Bug middle-end/113066] " pinskia at gcc dot gnu.org ` (3 more replies) 0 siblings, 4 replies; 5+ messages in thread From: luke.geeson at cs dot ucl.ac.uk @ 2023-12-18 15:23 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113066 Bug ID: 113066 Summary: Returning from a function marked noreturn allows execution to fall-through Product: gcc Version: 14.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: translation Assignee: unassigned at gcc dot gnu.org Reporter: luke.geeson at cs dot ucl.ac.uk Target Milestone: --- Consider the following program. ``` [[noreturn]] inline void ub_if_reached() { return; } void P0 () { atomic_store_explicit(y,1,memory_order_release); ub_if_reached(); } ``` If compiled to target Armv8-a using `clang -O3` (https://godbolt.org/z/T33Yncjsh), the whole of `P0` is optimised away. This is because `ub_if_reached` is marked as `noreturn` which means this function should not return, however the `return` statement does exactly that, inducing undefined behaviour. Clang warns of this behaviour, and optimises away the code in `P0` before the call, since it assumes a program is UB-free and that `P0` is therefore unreachable. Further if we define an `f` that calls a `g` that calls `P0` from `main` then all functions including `main` are also optimised away (https://godbolt.org/z/qsraYr7o5). Technically the standards and compiler are not required to do anything here as there is UB, however if this program is compiled and linked with a program that calls `P0` then the resultant execution may fallthrough to the code below `P0`. Both GCC and LLVM allow `P0` to fallthrough - this is not good and only functions guaranteed to return should be subject to this optimisation. In practice we can fix this by: - emitting an explicit abort (or undefined instruction that causes a trap) at the end of `noreturn` functions like `ub_if_reached`, or - if the compiler can statically determine that a `noreturn` function does in fact `return` then ignore the noreturn attribute, and emit an abort on return. As far as I can tell, this behaviour arises in clang, but clang has an optional `-mllvm -trap-unreachable` to prevent this behaviour. I was made aware that GCC has `-fsanitize=undefined` `-fsanitize-trap=undefined` for this, but these seem to be general options. It would be good for GCC to have a flag similar to `-mllvm -trap-unreachable`. Disclaimer: The views of the authors expressed in this bug report are my own and are not endorsed by Arm or any other company mentioned. ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug middle-end/113066] Returning from a function marked noreturn allows execution to fall-through 2023-12-18 15:23 [Bug translation/113066] New: Returning from a function marked noreturn allows execution to fall-through luke.geeson at cs dot ucl.ac.uk @ 2023-12-18 15:34 ` pinskia at gcc dot gnu.org 2023-12-18 15:37 ` pinskia at gcc dot gnu.org ` (2 subsequent siblings) 3 siblings, 0 replies; 5+ messages in thread From: pinskia at gcc dot gnu.org @ 2023-12-18 15:34 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113066 --- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> --- GCC does warn not just once but twice: ``` <source>: In function 'ub_if_reached': <source>:2:44: warning: function declared 'noreturn' has a 'return' statement 2 | [[noreturn]] inline void ub_if_reached() { return; } | ^~~~~~ <source>:2:44: warning: 'noreturn' function does return 2 | [[noreturn]] inline void ub_if_reached() { return; } | ^ ``` ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug middle-end/113066] Returning from a function marked noreturn allows execution to fall-through 2023-12-18 15:23 [Bug translation/113066] New: Returning from a function marked noreturn allows execution to fall-through luke.geeson at cs dot ucl.ac.uk 2023-12-18 15:34 ` [Bug middle-end/113066] " pinskia at gcc dot gnu.org @ 2023-12-18 15:37 ` pinskia at gcc dot gnu.org 2023-12-18 15:40 ` pinskia at gcc dot gnu.org 2023-12-18 15:57 ` luke.geeson at cs dot ucl.ac.uk 3 siblings, 0 replies; 5+ messages in thread From: pinskia at gcc dot gnu.org @ 2023-12-18 15:37 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113066 --- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> --- GCC 2.95.3 has the same behavior .... ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug middle-end/113066] Returning from a function marked noreturn allows execution to fall-through 2023-12-18 15:23 [Bug translation/113066] New: Returning from a function marked noreturn allows execution to fall-through luke.geeson at cs dot ucl.ac.uk 2023-12-18 15:34 ` [Bug middle-end/113066] " pinskia at gcc dot gnu.org 2023-12-18 15:37 ` pinskia at gcc dot gnu.org @ 2023-12-18 15:40 ` pinskia at gcc dot gnu.org 2023-12-18 15:57 ` luke.geeson at cs dot ucl.ac.uk 3 siblings, 0 replies; 5+ messages in thread From: pinskia at gcc dot gnu.org @ 2023-12-18 15:40 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113066 --- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> --- Note -funreachable-traps works for what you want it to work at -O1 and above but not at -O0 due to ub_if_reached not being inlined ... ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug middle-end/113066] Returning from a function marked noreturn allows execution to fall-through 2023-12-18 15:23 [Bug translation/113066] New: Returning from a function marked noreturn allows execution to fall-through luke.geeson at cs dot ucl.ac.uk ` (2 preceding siblings ...) 2023-12-18 15:40 ` pinskia at gcc dot gnu.org @ 2023-12-18 15:57 ` luke.geeson at cs dot ucl.ac.uk 3 siblings, 0 replies; 5+ messages in thread From: luke.geeson at cs dot ucl.ac.uk @ 2023-12-18 15:57 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113066 Luke Geeson <luke.geeson at cs dot ucl.ac.uk> changed: What |Removed |Added ---------------------------------------------------------------------------- Status|UNCONFIRMED |RESOLVED Resolution|--- |INVALID --- Comment #4 from Luke Geeson <luke.geeson at cs dot ucl.ac.uk> --- Ah my apologies - I missed that command line flag when I was checking... I'm sorry for wasting your time. I will close and mark it as invalid. Thanks! ^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2023-12-18 15:57 UTC | newest] Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2023-12-18 15:23 [Bug translation/113066] New: Returning from a function marked noreturn allows execution to fall-through luke.geeson at cs dot ucl.ac.uk 2023-12-18 15:34 ` [Bug middle-end/113066] " pinskia at gcc dot gnu.org 2023-12-18 15:37 ` pinskia at gcc dot gnu.org 2023-12-18 15:40 ` pinskia at gcc dot gnu.org 2023-12-18 15:57 ` luke.geeson at cs dot ucl.ac.uk
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