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Date:   Mon, 6 Jul 2020 12:37:08 +0200
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, mbenes@...e.cz
Subject: Re: [Linux v5.8-rc4] objtool warnings with gcc-10

On Mon, Jul 6, 2020 at 11:02 AM Sedat Dilek <sedat.dilek@...il.com> wrote:
>
> On Mon, Jul 6, 2020 at 10:43 AM Sedat Dilek <sedat.dilek@...il.com> wrote:
> >
> > On Mon, Jul 6, 2020 at 10:39 AM Peter Zijlstra <peterz@...radead.org> wrote:
> > >
> > > On Mon, Jul 06, 2020 at 09:09:55AM +0200, Sedat Dilek wrote:
> > > > [ Please CC me I am not subscribed to this mailing-list ]
> > > >
> > > > Hi Josh and Peter,
> > > >
> > > > today I switched over from Linux v5.7.y to Linux v5.8-rc4 and built
> > > > the first time with GCC version 10 on Debian/testing AMD64.
> > > >
> > > > $ cat /proc/version
> > > > Linux version 5.8.0-rc4-1-amd64-gcc10 (sedat.dilek@...il.com@...za)
> > > > (gcc-10 (Debian 10.1.0-4) 10.1.0, GNU ld (GNU Binutils for Debian)
> > > > 2.34) #1~bullseye+dileks1 SMP 2020-07-06
> > > >
> > > > I see these objtool warnings (which are new to me):
> > > >
> > > > $ grep warning: build-log_5.8.0-rc4-1-amd64-gcc10.txt | sort
> > > > arch/x86/kernel/cpu/mce/core.o: warning: objtool: mce_panic()+0x118: unreachable instruction
> > > > drivers/atm/horizon.o: warning: objtool: interrupt_handler()+0x19f: unreachable instruction
> > > > drivers/message/fusion/mptbase.o: warning: objtool: mpt_Soft_Hard_ResetHandler()+0x33a: unreachable instruction
> > > > drivers/scsi/aic7xxx/aic79xx_core.o: warning: objtool: ahd_intr.part.0()+0x10f: unreachable instruction
> > > > drivers/scsi/pcmcia/aha152x_core.o: warning: objtool: run()+0x4c0: unreachable instruction
> > > > fs/btrfs/backref.o: warning: objtool: btrfs_backref_finish_upper_links()+0x309: unreachable instruction
> > > > fs/btrfs/extent_io.o: warning: objtool: __set_extent_bit.cold()+0xc: unreachable instruction
> > > > fs/btrfs/relocation.o: warning: objtool: update_backref_cache.part.0()+0x1de: unreachable instruction
> > > > kernel/exit.o: warning: objtool: __x64_sys_exit_group()+0x14: unreachable instruction
> > > > net/core/skbuff.o: warning: objtool: skb_push.cold()+0x15: unreachable instruction
> > >
> > > That's more CONFIG_LIVEPATCH=y wreckage I expect. The problem is that
> > > GCC -flive-patching= thing wreck the propagation of the noreturn.
> > >
> > > This really is a compiler issue and we've not managed a sensible
> > > work-around in objtool.
> >
> > Thanks for the quick response.
> >
> > I have...
> >
> > CONFIG_LIVEPATCH=y
> >
> > ...and see in my build-log:
> >
> > -flive-patching=inline-clone
> >
> > - Sedat -
>
> OK, I found some discussions in "Re: linux-next: Tree for May 21
> (objtool warnings)" [1].
>
> GCC docs say [2]:
> - BOQ (Begin Of Quote) -
> [ -flive-patching=level ]
>
> The level argument should be one of the following:
>
> [ ‘inline-clone’ ]
>
> Only enable inlining and cloning optimizations, which includes
> inlining, cloning, interprocedural scalar replacement of aggregates
> and partial inlining. As a result, when patching a function, all its
> callers and its clones’ callers are impacted, therefore need to be
> patched as well.
>
> -flive-patching=inline-clone disables the following optimization flags:
>
> -fwhole-program  -fipa-pta  -fipa-reference  -fipa-ra
> -fipa-icf  -fipa-icf-functions  -fipa-icf-variables
> -fipa-bit-cp  -fipa-vrp  -fipa-pure-const  -fipa-reference-addressable
> -fipa-stack-alignment
>
> [ ‘inline-only-static’ ]
>
> Only enable inlining of static functions. As a result, when patching a
> static function, all its callers are impacted and so need to be
> patched as well.
>
> In addition to all the flags that -flive-patching=inline-clone
> disables, -flive-patching=inline-only-static disables the following
> additional optimization flags:
>
> -fipa-cp-clone  -fipa-sra  -fpartial-inlining  -fipa-cp
>
> When -flive-patching is specified without any value, the default value
> is inline-clone.
>
> This flag is disabled by default.
>
> Note that -flive-patching is not supported with link-time optimization (-flto).
>  - EOQ (End of Quote) -
>
> Josh says in [3]:
>
> > The issue here is that with -fno-ipa-pure-const, GCC no longer
> > automatically detects that the static inline function is noreturn, so it
> > emits unreachable instructions after a call to it.
>
> - Sedat -
>
> [1] https://marc.info/?t=159011496400002&r=1&w=2
> [2] https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html
> [3] https://marc.info/?l=linux-next&m=159059217601108&w=2

[ arch/x86/kernel/cpu/mce/core.o ]

After "-flive-patching=inline-clone" I manually added in the make line:

#1: -fno-ipa-pure-const

$ ./tools/objtool/objtool orc generate  --no-fp --retpoline --uaccess
arch/x86/kernel/cpu/mce/core.o
arch/x86/kernel/cpu/mce/core.o: warning: objtool: mce_panic()+0x118:
unreachable instruction

#2: -fipa-pure-const

$ gcc-10 -Wp,-MMD,arch/x86/kernel/cpu/mce/.core.o.d -nostdinc -isystem
/usr/lib/gcc/x86_64-linux-gnu/10/include -I./arch/x86/include
-I./arch/x86/include/generated  -I./include -I./arch/x86/include/uapi
-I./arch/x86/include/generated/uapi -I./include/uapi
-I./include/generated/uapi -include ./include/linux/kconfig.h -include
./include/linux/compiler_types.h -D__KERNEL__ -Wall -Wundef
-Werror=strict-prototypes -Wno-trigraphs -fno-strict-aliasing
-fno-common -fshort-wchar -fno-PIE
-Werror=implicit-function-declaration -Werror=implicit-int
-Wno-format-security -std=gnu89 -mno-sse -mno-mmx -mno-sse2 -mno-3dnow
-mno-avx -m64 -falign-jumps=1 -falign-loops=1 -mno-80387
-mno-fp-ret-in-387 -mpreferred-stack-boundary=3 -mskip-rax-setup
-mtune=generic -mno-red-zone -mcmodel=kernel -Wno-sign-compare
-fno-asynchronous-unwind-tables -mindirect-branch=thunk-extern
-mindirect-branch-register -fno-jump-tables
-fno-delete-null-pointer-checks -Wno-frame-address
-Wno-format-truncation -Wno-format-overflow
-Wno-address-of-packed-member -O2 -fno-allow-store-data-races
-Wframe-larger-than=2048 -fstack-protector-strong
-Wno-unused-but-set-variable -Wimplicit-fallthrough
-Wno-unused-const-variable -fno-var-tracking-assignments -g -gdwarf-4
-gz=zlib -pg -mrecord-mcount -mfentry -DCC_USING_FENTRY
-flive-patching=inline-clone -fipa-pure-const
-Wdeclaration-after-statement -Wvla -Wno-pointer-sign
-Wno-stringop-truncation -Wno-zero-length-bounds -Wno-array-bounds
-Wno-stringop-overflow -Wno-restrict -Wno-maybe-uninitialized
-fno-strict-overflow -fno-merge-all-constants -fmerge-constants
-fno-stack-check -fconserve-stack -Werror=date-time
-Werror=incompatible-pointer-types -Werror=designated-init
-fmacro-prefix-map=./= -fcf-protection=none -Wno-packed-not-aligned
-DKBUILD_MODFILE='"arch/x86/kernel/cpu/mce/core"'
-DKBUILD_BASENAME='"core"' -DKBUILD_MODNAME='"core"' -c -o
arch/x86/kernel/cpu/mce/core.o arch/x86/kernel/cpu/mce/core.c

cc1: error: ‘-fipa-pure-const’ is incompatible with
‘-flive-patching=inline-only-static|inline-clone’

- Sedat -

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