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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
Date: Mon, 12 Aug 2019 04:47:15 +0000
Message-ID: <MWHPR2201MB127785F4CEC464CB6F11F3DAC1D30@MWHPR2201MB1277.namprd22.prod.outlook.com> (raw)
In-Reply-To: <20190812033120.43013-2-natechancellor@gmail.com>

Hello,

Nathan Chancellor wrote:
> clang warns:
> 
> 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 = 0;
>                         ^
> 2 errors generated.
> 
> 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.
> 
> $ mipsel-linux-gnu-gcc --version | head -n1
> mipsel-linux-gnu-gcc (Debian 8.3.0-2) 8.3.0
> 
> $ clang --version | head -n1
> ClangBuiltLinux clang version 9.0.0 (git://github.com/llvm/llvm-project
> 544315b4197034a3be8acd12cba56a75fb1f08dc) (based on LLVM 9.0.0svn)
> 
> $ cat test.c
>  #include <stdio.h>
> 
>  static void switch_scoped(int opcode)
>  {
> 	 switch (opcode) {
> 	 case 1:
> 	 case 2: {
> 		 int bc_false = 0;
> 
> 		 bc_false = 4;
> 	 case 3:
> 	 case 4:
> 		 printf("\t* switch scoped bc_false = %d\n", bc_false);
> 	 }
> 	 }
>  }
> 
>  static void function_scoped(int opcode)
>  {
> 	 int bc_false = 0;
> 
> 	 switch (opcode) {
> 	 case 1:
> 	 case 2: {
> 		 bc_false = 4;
> 	 case 3:
> 	 case 4:
> 		 printf("\t* function scoped bc_false = %d\n", bc_false);
> 	 }
> 	 }
>  }
> 
>  int main(void)
>  {
> 	 int opcode;
> 
> 	 for (opcode = 1; opcode < 5; opcode++) {
> 		 printf("opcode = %d:\n", opcode);
> 		 switch_scoped(opcode);
> 		 function_scoped(opcode);
> 		 printf("\n");
> 	 }
> 
> 	 return 0;
>  }
> 
> $ mipsel-linux-gnu-gcc -std=gnu89 -static test.c && \
>   qemu-mipsel a.out
> opcode = 1:
>         * switch scoped bc_false = 4
>         * function scoped bc_false = 4
> 
> opcode = 2:
>         * switch scoped bc_false = 4
>         * function scoped bc_false = 4
> 
> opcode = 3:
>         * switch scoped bc_false = 2147483004
>         * function scoped bc_false = 0
> 
> opcode = 4:
>         * switch scoped bc_false = 2147483004
>         * function scoped bc_false = 0
> 
> $ clang -std=gnu89 --target=mipsel-linux-gnu -m32 -static test.c && \
>   qemu-mipsel a.out
> opcode = 1:
>         * switch scoped bc_false = 4
>         * function scoped bc_false = 4
> 
> opcode = 2:
>         * switch scoped bc_false = 4
>         * function scoped bc_false = 4
> 
> opcode = 3:
>         * switch scoped bc_false = 2147483004
>         * function scoped bc_false = 0
> 
> opcode = 4:
>         * switch scoped bc_false = 2147483004
>         * function scoped bc_false = 0
> 
> 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
> 
> Fixes: 6a1cc218b9cc ("MIPS: branch: Remove FP branch handling when CONFIG_MIPS_FP_SUPPORT=n")
> 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

[ This message was auto-generated; if you believe anything is incorrect
  then please email paul.burton@mips.com to report it. ]

  reply index

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-12  3:31 [PATCH 0/5] Clang build fixes for MIPS Nathan Chancellor
2019-08-12  3:31 ` [PATCH 1/5] MIPS: Don't use bc_false uninitialized in __mm_isBranchInstr Nathan Chancellor
2019-08-12  4:47   ` Paul Burton [this message]
2019-08-12  3:31 ` [PATCH 2/5] MIPS/ptrace: Update mips_get_syscall_arg's return type Nathan Chancellor
2019-08-12  4:47   ` Paul Burton
2019-08-12  3:31 ` [PATCH 3/5] lib/mpi: Fix for building for MIPS32 with Clang Nathan Chancellor
2019-08-12  5:23   ` Nathan Chancellor
2019-08-12  5:26     ` Nathan Chancellor
2019-08-12 12:28       ` Herbert Xu
2019-08-12 17:58     ` Paul Burton
2019-08-12  7:35   ` Jussi Kivilinna
2019-08-12 17:14     ` Nathan Chancellor
2019-08-12 19:40       ` Jussi Kivilinna
2019-08-12 19:45         ` Nathan Chancellor
2019-08-12  3:31 ` [PATCH 4/5] lib/mpi: Fix for building for MIPS64 " Nathan Chancellor
2019-08-12  5:25   ` Nathan Chancellor
2019-08-12 17:42   ` Nick Desaulniers
2019-08-12 17:45     ` Nathan Chancellor
2019-08-12  3:31 ` [PATCH 5/5] MIPS: tlbex: Explicitly cast _PAGE_NO_EXEC to a boolean Nathan Chancellor
2019-08-12  4:47   ` Paul Burton

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