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From:   Paul Burton <paul.burton@mips.com>
To:     Nathan Chancellor <natechancellor@gmail.com>
CC:     Ralf Baechle <ralf@linux-mips.org>,
        Paul Burton <pburton@wavecomp.com>,
        James Hogan <jhogan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        "linux-mips@vger.kernel.org" <linux-mips@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "clang-built-linux@googlegroups.com" 
        <clang-built-linux@googlegroups.com>,
        Nathan Chancellor <natechancellor@gmail.com>,
        "linux-mips@vger.kernel.org" <linux-mips@vger.kernel.org>
Subject: Re: [PATCH 1/5] MIPS: Don't use bc_false uninitialized in
  __mm_isBranchInstr
Thread-Topic: [PATCH 1/5] MIPS: Don't use bc_false uninitialized in
  __mm_isBranchInstr
Thread-Index: AQHVUMkDqKjwYqiFLUa9pBXZ514zBw==
Date:   Mon, 12 Aug 2019 04:47:15 +0000
Message-ID: <MWHPR2201MB127785F4CEC464CB6F11F3DAC1D30@MWHPR2201MB1277.namprd22.prod.outlook.com>
References: <20190812033120.43013-2-natechancellor@gmail.com>
In-Reply-To: <20190812033120.43013-2-natechancellor@gmail.com>
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Hello,

Nathan Chancellor wrote:
> clang warns:
>=20
> arch/mips/kernel/branch.c:148:8: error: variable 'bc_false' is used
> uninitialized whenever switch case is taken
> [-Werror,-Wsometimes-uninitialized]
>                 case mm_bc2t_op:
>                      ^~~~~~~~~~
> arch/mips/kernel/branch.c:157:8: note: uninitialized use occurs here
>                         if (bc_false)
>                             ^~~~~~~~
> arch/mips/kernel/branch.c:149:8: error: variable 'bc_false' is used
> uninitialized whenever switch case is taken
> [-Werror,-Wsometimes-uninitialized]
>                 case mm_bc1t_op:
>                      ^~~~~~~~~~
> arch/mips/kernel/branch.c:157:8: note: uninitialized use occurs here
>                         if (bc_false)
>                             ^~~~~~~~
> arch/mips/kernel/branch.c:142:4: note: variable 'bc_false' is declared
> here
>                         int bc_false =3D 0;
>                         ^
> 2 errors generated.
>=20
> When mm_bc1t_op and mm_bc2t_op are taken, the bc_false initialization
> does not happen, which leads to a garbage value upon use, as illustrated
> below with a small sample program.
>=20
> $ mipsel-linux-gnu-gcc --version | head -n1
> mipsel-linux-gnu-gcc (Debian 8.3.0-2) 8.3.0
>=20
> $ clang --version | head -n1
> ClangBuiltLinux clang version 9.0.0 (git://github.com/llvm/llvm-project
> 544315b4197034a3be8acd12cba56a75fb1f08dc) (based on LLVM 9.0.0svn)
>=20
> $ cat test.c
>  #include <stdio.h>
>=20
>  static void switch_scoped(int opcode)
>  {
> 	 switch (opcode) {
> 	 case 1:
> 	 case 2: {
> 		 int bc_false =3D 0;
>=20
> 		 bc_false =3D 4;
> 	 case 3:
> 	 case 4:
> 		 printf("\t* switch scoped bc_false =3D %d\n", bc_false);
> 	 }
> 	 }
>  }
>=20
>  static void function_scoped(int opcode)
>  {
> 	 int bc_false =3D 0;
>=20
> 	 switch (opcode) {
> 	 case 1:
> 	 case 2: {
> 		 bc_false =3D 4;
> 	 case 3:
> 	 case 4:
> 		 printf("\t* function scoped bc_false =3D %d\n", bc_false);
> 	 }
> 	 }
>  }
>=20
>  int main(void)
>  {
> 	 int opcode;
>=20
> 	 for (opcode =3D 1; opcode < 5; opcode++) {
> 		 printf("opcode =3D %d:\n", opcode);
> 		 switch_scoped(opcode);
> 		 function_scoped(opcode);
> 		 printf("\n");
> 	 }
>=20
> 	 return 0;
>  }
>=20
> $ mipsel-linux-gnu-gcc -std=3Dgnu89 -static test.c && \
>   qemu-mipsel a.out
> opcode =3D 1:
>         * switch scoped bc_false =3D 4
>         * function scoped bc_false =3D 4
>=20
> opcode =3D 2:
>         * switch scoped bc_false =3D 4
>         * function scoped bc_false =3D 4
>=20
> opcode =3D 3:
>         * switch scoped bc_false =3D 2147483004
>         * function scoped bc_false =3D 0
>=20
> opcode =3D 4:
>         * switch scoped bc_false =3D 2147483004
>         * function scoped bc_false =3D 0
>=20
> $ clang -std=3Dgnu89 --target=3Dmipsel-linux-gnu -m32 -static test.c && \
>   qemu-mipsel a.out
> opcode =3D 1:
>         * switch scoped bc_false =3D 4
>         * function scoped bc_false =3D 4
>=20
> opcode =3D 2:
>         * switch scoped bc_false =3D 4
>         * function scoped bc_false =3D 4
>=20
> opcode =3D 3:
>         * switch scoped bc_false =3D 2147483004
>         * function scoped bc_false =3D 0
>=20
> opcode =3D 4:
>         * switch scoped bc_false =3D 2147483004
>         * function scoped bc_false =3D 0
>=20
> Move the definition up so that we get the right behavior and mark it
> __maybe_unused as it will not be used when CONFIG_MIPS_FP_SUPPORT
> isn't enabled.

Applied to mips-next.

> commit c2869aafe719
> https://git.kernel.org/mips/c/c2869aafe719
>=20
> Fixes: 6a1cc218b9cc ("MIPS: branch: Remove FP branch handling when CONFIG=
_MIPS_FP_SUPPORT=3Dn")
> Link: https://github.com/ClangBuiltLinux/linux/issues/603
> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
> Signed-off-by: Paul Burton <paul.burton@mips.com>

Thanks,
    Paul

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