On Tue, Sep 15, 2020 at 4:40 PM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote: > > On 16/09/2020 00:11, Andy Lutomirski wrote: > >> On Sep 15, 2020, at 2:24 PM, Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote: > >> > >> On Tue, Sep 15, 2020 at 1:56 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote: > >>> The old smap_save() code was: > >>> > >>> pushf > >>> pop %0 > >>> > >>> with %0 defined by an "=rm" constraint. This is fine if the > >>> compiler picked the register option, but it was incorrect with an > >>> %rsp-relative memory operand. > >> It is incorrect because ... (I think mentioning the point about the > >> red zone would be good, unless there were additional concerns?) > > This isn’t a red zone issue — it’s a just-plain-wrong issue. The popf is storing the result in the wrong place in memory — it’s RSP-relative, but RSP is whatever the compiler thinks it should be minus 8, because the compiler doesn’t know that pushfq changed RSP. > > It's worse than that. Even when stating that %rsp is modified in the > asm, the generated code sequence is still buggy, for recent Clang and GCC. > > https://godbolt.org/z/ccz9v7 > > It's clearly not safe to ever use memory operands with pushf/popf asm > fragments. > Would this apply to native_save_fl() and native_restore_fl in arch/x86/include/asm/irqflags.h? It was like that two revisions ago, but it was changed (back) to "=rm" with a comment about it being safe. > >> This is something we should fix. Bill, James, and I are discussing > >> this internally. Thank you for filing a bug; I owe you a beer just > >> for that. > > I’m looking forward to the day that beers can be exchanged in person again :) > > +1 to that. > +100 -bw
