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    SubjectRe: [PATCH v6 13/29] arm64/build: Assert for unwanted sections
    On Tue, Oct 27, 2020 at 09:15:17PM +0100, Ard Biesheuvel wrote:
    > On Tue, 27 Oct 2020 at 21:12, Nick Desaulniers <ndesaulniers@google.com> wrote:
    > >
    > > On Tue, Oct 27, 2020 at 12:25 PM Geert Uytterhoeven
    > > <geert@linux-m68k.org> wrote:
    > > >
    > > > Hi Nick,
    > > >
    > > > CC Josh
    > > >
    > > > On Mon, Oct 26, 2020 at 6:49 PM Nick Desaulniers
    > > > <ndesaulniers@google.com> wrote:
    > > > > On Mon, Oct 26, 2020 at 10:44 AM Geert Uytterhoeven
    > > > > <geert@linux-m68k.org> wrote:
    > > > > > On Mon, Oct 26, 2020 at 6:39 PM Ard Biesheuvel <ardb@kernel.org> wrote:
    > > > > > > On Mon, 26 Oct 2020 at 17:01, Geert Uytterhoeven <geert@linux-m68k.org> wrote:
    > > > > > > > On Mon, Oct 26, 2020 at 2:29 PM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
    > > > > > > > > On Mon, Oct 26, 2020 at 1:29 PM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
    > > > > > > > > > I.e. including the ".eh_frame" warning. I have tried bisecting that
    > > > > > > > > > warning (i.e. with be2881824ae9eb92 reverted), but that leads me to
    > > > > > > > > > commit b3e5d80d0c48c0cc ("arm64/build: Warn on orphan section
    > > > > > > > > > placement"), which is another red herring.
    > > > > > > > >
    > > > > > > > > kernel/bpf/core.o is the only file containing an eh_frame section,
    > > > > > > > > causing the warning.
    > > > >
    > > > > When I see .eh_frame, I think -fno-asynchronous-unwind-tables is
    > > > > missing from someone's KBUILD_CFLAGS.
    > > > > But I don't see anything curious in kernel/bpf/Makefile, unless
    > > > > cc-disable-warning is somehow broken.
    > > >
    > > > I tracked it down to kernel/bpf/core.c:___bpf_prog_run() being tagged
    > > > with __no_fgcse aka __attribute__((optimize("-fno-gcse"))).
    > > >
    > > > Even if the function is trivially empty ("return 0;"), a ".eh_frame" section
    > > > is generated. Removing the __no_fgcse tag fixes that.
    > >
    > > That's weird. I feel pretty strongly that unless we're working around
    > > a well understood compiler bug with a comment that links to a
    > > submitted bug report, turning off rando compiler optimizations is a
    > > terrible hack for which one must proceed straight to jail; do not pass
    > > go; do not collect $200. But maybe I'd feel differently for this case
    > > given the context of the change that added it. (Ard mentions
    > > retpolines+orc+objtool; can someone share the relevant SHA if you have
    > > it handy so I don't have to go digging?)
    >
    > commit 3193c0836f203a91bef96d88c64cccf0be090d9c
    > Author: Josh Poimboeuf <jpoimboe@redhat.com>
    > Date: Wed Jul 17 20:36:45 2019 -0500
    >
    > bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()
    >
    > has
    >
    > Fixes: e55a73251da3 ("bpf: Fix ORC unwinding in non-JIT BPF code")

    That commit is broken.
    I had this patch in my queue:
    -#define __no_fgcse __attribute__((optimize("-fno-gcse")))
    +#define __no_fgcse __attribute__((optimize("-fno-gcse,-fno-omit-frame-pointer")))

    Sounds like you want to add -fno-asynchronous-unwind-tables to the above list?

    > and mentions objtool and CONFIG_RETPOLINE.
    >
    > > (I feel the same about there
    > > being an empty asm(); statement in the definition of asm_volatile_goto
    > > for compiler-gcc.h). Might be time to "fix the compiler."
    > >
    > > (It sounds like Arvind is both in agreement with my sentiment, and has
    > > the root cause).
    > >
    >
    > I agree that the __no_fgcse hack is terrible. Does Clang support the
    > following pragmas?
    >
    > #pragma GCC push_options
    > #pragma GCC optimize ("-fno-gcse")
    > #pragma GCC pop_options

    That will work too, but optimize("-fno...,-fno..,-fno..") is imo cleaner.

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