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From: Nathan Chancellor <natechancellor@gmail.com>
To: Ralf Baechle <ralf@linux-mips.org>,
	Paul Burton <paul.burton@mips.com>,
	James Hogan <jhogan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>,
	linux-mips@vger.kernel.org, linux-kernel@vger.kernel.org,
	clang-built-linux@googlegroups.com,
	Nathan Chancellor <natechancellor@gmail.com>
Subject: [PATCH 1/5] MIPS: Don't use bc_false uninitialized in __mm_isBranchInstr
Date: Sun, 11 Aug 2019 20:31:16 -0700
Message-ID: <20190812033120.43013-2-natechancellor@gmail.com> (raw)
In-Reply-To: <20190812033120.43013-1-natechancellor@gmail.com>

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.

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>
---
 arch/mips/kernel/branch.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/mips/kernel/branch.c b/arch/mips/kernel/branch.c
index 1db29957a931..2c38f75d87ff 100644
--- a/arch/mips/kernel/branch.c
+++ b/arch/mips/kernel/branch.c
@@ -58,6 +58,7 @@ int __mm_isBranchInstr(struct pt_regs *regs, struct mm_decoded_insn dec_insn,
 		       unsigned long *contpc)
 {
 	union mips_instruction insn = (union mips_instruction)dec_insn.insn;
+	int __maybe_unused bc_false = 0;
 
 	if (!cpu_has_mmips)
 		return 0;
@@ -139,7 +140,6 @@ int __mm_isBranchInstr(struct pt_regs *regs, struct mm_decoded_insn dec_insn,
 #ifdef CONFIG_MIPS_FP_SUPPORT
 		case mm_bc2f_op:
 		case mm_bc1f_op: {
-			int bc_false = 0;
 			unsigned int fcr31;
 			unsigned int bit;
 
-- 
2.23.0.rc2


  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 ` Nathan Chancellor [this message]
2019-08-12  4:47   ` [PATCH 1/5] MIPS: Don't use bc_false uninitialized in __mm_isBranchInstr Paul Burton
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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