[Bug 207173] New: kvm compiling problem 5.6.x kvm_main.c:2236:42: error: ‘nr_pages_avail’ may be used uninitialized in this function

[Bug 207173] New: kvm compiling problem 5.6.x kvm_main.c:2236:42: error: ‘nr_pages_avail’ may be used uninitialized in this function

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https://bugzilla.kernel.org/show_bug.cgi?id=207173

            Bug ID: 207173
           Summary: kvm compiling problem 5.6.x kvm_main.c:2236:42: error:
                    ‘nr_pages_avail’ may be used uninitialized in this
                    function
           Product: Virtualization
           Version: unspecified
    Kernel Version: linux-5.6.-linux-5.6.3
          Hardware: All
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: blocking
          Priority: P1
         Component: kvm
          Assignee: virtualization_kvm@kernel-bugs.osdl.org
          Reporter: zoran.davidovac@gmail.com
        Regression: No

happens with stock kernel 5.6.x gcc version 9.3.0 (GCC) 


  CC      arch/x86/kvm/../../../virt/kvm/kvm_main.o
arch/x86/kvm/../../../virt/kvm/kvm_main.c: In function
‘__kvm_gfn_to_hva_cache_init’:
arch/x86/kvm/../../../virt/kvm/kvm_main.c:2236:42: error: ‘nr_pages_avail’ may
be used uninitialized in this function [-Werror=maybe-uninitialized]
 2236 |  for ( ; start_gfn <= end_gfn; start_gfn += nr_pages_avail) {
      |                                ~~~~~~~~~~^~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
make[2]: *** [scripts/Makefile.build:268:
arch/x86/kvm/../../../virt/kvm/kvm_main.o] Error 1
make[1]: *** [scripts/Makefile.build:505: arch/x86/kvm] Error 2
make: *** [Makefile:1683: arch/x86] Error 2

Waited till .3 and I am still surprised that nobody noticed :)

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[Bug 207173] kvm compiling problem 5.6.x kvm_main.c:2236:42: error: ‘nr_pages_avail’ may be used uninitialized in this function

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https://bugzilla.kernel.org/show_bug.cgi?id=207173

ticat (ticat_fp@163.com) changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ticat_fp@163.com

--- Comment #1 from ticat (ticat_fp@163.com) ---
I don't think it's a problem or bug.Why?
If program executes this for()-statement,that's start_gfn >= end_gfn.
"start_gfn += nr_pages_avail" first is executed when for-body's end,so for
nr_pages_avail, it's OKAY.

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[Bug 207173] kvm compiling problem 5.6.x kvm_main.c:2236:42: error: ‘nr_pages_avail’ may be used uninitialized in this function

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https://bugzilla.kernel.org/show_bug.cgi?id=207173

Sean Christopherson (sean.j.christopherson@intel.com) changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |sean.j.christopherson@intel
                   |                            |.com

--- Comment #2 from Sean Christopherson (sean.j.christopherson@intel.com) ---
Is this reproducible on all 5.6.x releases?  Just want to double check that
something didn't change between 5.6 and 5.6.3 that somehow broke things.

Can you attach your config?  This has been reported before[*], but only with a
proprietary compiler.  Maybe the issue is dependent on the config more so than
the compiler.

https://lkml.kernel.org/r/20200218184756.242904-1-oupton@google.com

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[Bug 207173] kvm compiling problem 5.6.x kvm_main.c:2236:42: error: ‘nr_pages_avail’ may be used uninitialized in this function

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--- Comment #3 from zoran.davidovac@gmail.com ---
Yes all 5.6.X (up to 4 tested) before it was while loop now there is for loop
and some other changes in kvm code vs 5.5.x kernel that compiles with same
code,
and yes always reproducible.

Standard gcc 9.2 now 9.3

https://drive.google.com/open?id=18NDMqounXy6RwNhRxTJDUizTckm7QBOR
I do not see any other way to add file except paste it encode it ?

# make bzImage
  CALL    scripts/checksyscalls.sh
  CALL    scripts/atomic/check-atomics.sh
  DESCEND  objtool
  CHK     include/generated/compile.h
  CC      arch/x86/kvm/../../../virt/kvm/kvm_main.o
arch/x86/kvm/../../../virt/kvm/kvm_main.c: In function
‘__kvm_gfn_to_hva_cache_init’:
arch/x86/kvm/../../../virt/kvm/kvm_main.c:2236:42: error: ‘nr_pages_avail’ may
be used uninitialized in this function [-Werror=maybe-uninitialized]
 2236 |  for ( ; start_gfn <= end_gfn; start_gfn += nr_pages_avail) {
      |                                ~~~~~~~~~~^~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
make[2]: *** [scripts/Makefile.build:268:
arch/x86/kvm/../../../virt/kvm/kvm_main.o] Error 1
make[1]: *** [scripts/Makefile.build:505: arch/x86/kvm] Error 2
make: *** [Makefile:1683: arch/x86] Error 2
root@host:/usr/src/linux-5.6.4#

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[Bug 207173] kvm compiling problem 5.6.x kvm_main.c:2236:42: error: ‘nr_pages_avail’ may be used uninitialized in this function

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--- Comment #4 from Sean Christopherson (sean.j.christopherson@intel.com) ---
Hrm, still can't reproduce this even with your config and gcc 9.2.  Can you
paste the build command for kvm_main.o?  It should be the first line in
virt/kvm/.kvm_main.o.cmd in your build directory, e.g.

cmd_arch/x86/kvm/../../../virt/kvm/kvm_main.o := /usr/bin/gcc-9
-Wp,-MD,arch/x86/kvm/../../../virt/kvm/.kvm_main.o.d  -nostdinc -isystem
/usr/lib/gcc/x86_64-linux-gnu/9/include -I./arch/x86/include
-I./arch/x86/include/generated -I./include -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 -march=core2
-mno-red-zone -mcmodel=kernel -DCONFIG_AS_CFI=1 -DCONFIG_AS_CFI_SIGNAL_FRAME=1
-DCONFIG_AS_CFI_SECTIONS=1 -DCONFIG_AS_SSSE3=1 -DCONFIG_AS_AVX=1
-DCONFIG_AS_AVX2=1 -DCONFIG_AS_AVX512=1 -DCONFIG_AS_SHA1_NI=1
-DCONFIG_AS_SHA256_NI=1 -DCONFIG_AS_ADX=1 -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 --param=allow-store-data-races=0
-fstack-protector-strong -Wno-unused-but-set-variable -Wimplicit-fallthrough
-Wno-unused-const-variable -fomit-frame-pointer -fno-var-tracking-assignments
-Wdeclaration-after-statement -Wvla -Wno-pointer-sign -Wno-stringop-truncation
-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
-Iarch/x86/kvm -Werror -I ./arch/x86/kvm -I ./arch/x86/kvm   
-DKBUILD_MODFILE='"arch/x86/kvm/kvm"' -DKBUILD_BASENAME='"kvm_main"'
-DKBUILD_MODNAME='"kvm"' -c -o arch/x86/kvm/../../../virt/kvm/kvm_main.o
./arch/x86/kvm/../../../virt/kvm/kvm_main.c

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[Bug 207173] kvm compiling problem 5.6.x kvm_main.c:2236:42: error: ‘nr_pages_avail’ may be used uninitialized in this function

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--- Comment #5 from zoran.davidovac@gmail.com ---
I've back-ported kvm and instead loop changed to while as in 5.4 
and I could compile it.

But I found what the problem is. 

In short I can compile it with -O2 , -O2 -march=native, but NOT with -O3 with
or without -march=native.
Further more in kernel menuconfig optimization I see only O2 an Os(ize) now!?

How to reproduce:
-use my config
-edit Makefile and replace -O2 to -O3 and you will get this error.


I always compile stock kernel with -O3 -march=native for servers 
and my laptop with -Ofast -march=native for ages without any problem :)

On error .kvm_main.o.cmd is not generated, thank you Sean, 
you made me look to something that I would never suspect
as I had same compile problem on server, laptop and virtual server.

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[Bug 207173] kvm compiling problem 5.6.x kvm_main.c:2236:42: error: ‘nr_pages_avail’ may be used uninitialized in this function

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--- Comment #6 from Sean Christopherson (sean.j.christopherson@intel.com) ---
Ah, fun.  So this falls into the "not technically a kernel problem" category. 
O3 isn't supported outside of the ARC architecture (no clue why it "needs" O3),
and AFAICT, has never been a selectable Kconfig option outside of ARC.

The false positives with -Wmaybe-uninitialized and -O3 (and -Os) is a known
issue, e.g. the kernel adds -Wno-maybe-uninitialized when compiling with either
of those options.  Unfortunately, there is no direct way to force
CONFIG_CC_DISABLE_WARN_MAYBE_UNINITIALIZED=y.  You could select
CONFIG_CC_OPTIMIZE_FOR_SIZE and then edit your Makefile, but I think that might
have unwanted side effects.

The easiest way to workaround this issue is to do CONFIG_KVM_WERROR=n.  The
false positive warning will still occur, but it won't get escalated to an error
and break your build.  I'm guessing there's a way to get
-Wno-maybe-uninitialized, but disabling -Werror for KVM is easy and doesn't
really have downsides.

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[Bug 207173] kvm compiling problem 5.6.x kvm_main.c:2236:42: error: ‘nr_pages_avail’ may be used uninitialized in this function

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zoran.davidovac@gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|blocking                    |enhancement

--- Comment #7 from zoran.davidovac@gmail.com ---
Interesting, 

I did not know that -O3 is not supported on x86 or x86_64,
in kernel Doc it does not say it is not supported and
It worked very well until now :)

So this happen also to size optimized kernel?

But now I also know why O3 is disabled in menuconfig :)

KBUILD_CFLAGS += -O2
else ifdef CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3
KBUILD_CFLAGS += -O3
else ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
KBUILD_CFLAGS += -Os
endif

I've read somewhere that on arm -O3 is related to implicit parallelism 

Changed category to enhancement, let me know if I need to change anything else?

thx.

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[Bug 207173] kvm compiling problem 5.6.x kvm_main.c:2236:42: error: ‘nr_pages_avail’ may be used uninitialized in this function

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cqwrteur (euloanty@live.com) changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |euloanty@live.com

--- Comment #8 from cqwrteur (euloanty@live.com) ---
I have found out a solution:

change line 2160:

gfn_t nr_pages_avail;

to line 2160.

Just initialize it.

gfn_t nr_pages_avail=0;

done

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[Bug 207173] kvm compiling problem 5.6.x kvm_main.c:2236:42: error: ‘nr_pages_avail’ may be used uninitialized in this function

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--- Comment #9 from Sean Christopherson (sean.j.christopherson@intel.com) ---
Zeroing nr_pages_avail was proposed upstream[*] and effectively rejected
because it's unnecessary, i.e. there is no KVM bug.

[*] https://lkml.kernel.org/r/20200218184756.242904-1-oupton@google.com

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[Bug 207173] kvm compiling problem 5.6.x kvm_main.c:2236:42: error: ‘nr_pages_avail’ may be used uninitialized in this function

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Tony Cook (tony-cook@bigpond.com) changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |tony-cook@bigpond.com

--- Comment #10 from Tony Cook (tony-cook@bigpond.com) ---
arch/x86/kvm/../../../virt/kvm/kvm_main.c: In function
‘__kvm_gfn_to_hva_cache_init’:
arch/x86/kvm/../../../virt/kvm/kvm_main.c:2236:42: error: ‘nr_pages_avail’ may
be used uninitialized in this function [-Werror=maybe-uninitialized]
 2236 |  for ( ; start_gfn <= end_gfn; start_gfn += nr_pages_avail) {
cc1: all warnings being treated as errors
make[2]: *** [scripts/Makefile.build:267:
arch/x86/kvm/../../../virt/kvm/kvm_main.o] Error 1
make[1]: *** [scripts/Makefile.build:505: arch/x86/kvm] Error 2
make[1]: *** Waiting for unfinished jobs....

Nothing non-standard here, just trying to build the latest kernel 5.6.11 with #
Compiler: gcc (GCC) 10.0.1 20200430 (Red Hat 10.0.1-0.14)

I agree that there is in fact no error here as the nr_pages_avail is set before
it is used to increment start_gfn, or at any rate one can infer that it might
be set by the procedure call that passes it by reference. Nonetheless it is
more than just annoying when this breaks my build. Either change the default
setting and disable werror for this module or make the uneccessary assignment
just for my pleasure.

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[Bug 207173] kvm compiling problem 5.6.x kvm_main.c:2236:42: error: ‘nr_pages_avail’ may be used uninitialized in this function

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--- Comment #11 from Mike Auty (mike.auty@gmail.com) ---
I'm also experiencing this, but not on clang, on gcc-10.1.0.  The initially
proposed resolution:

gfn_t nr_pages_avail = 0; 

succeeded.  The alternative resolution:

gfn uninitialized_var(nr_pages_avail);

Needed to be changed to 

gfn_t uninitialized_var(nr_pages_avail);

but also allowed compilation.

The suggestion to mark __gfn_to_hva_many and gfn_to_hva_many as always inline
failed to resolve the problem though.

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--- Comment #12 from Sean Christopherson (sean.j.christopherson@intel.com) ---
This is "fixed" in upstream (for kernel 5.7) by commit 78a5255ffb6a1 ("Stop the
ad-hoc games with -Wno-maybe-initialized").

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